python-kitchen(1)

Using kitchen to write good code

Kitchen's functions won't automatically make you a better programmer. You have to learn when and how to use them as well. This section of the documentation is intended to show you some of the ways that you can apply kitchen's functions to problems that may have arisen in your life. The goal of this section is to give you enough information to understand what the kitchen API can do for you and where in the Kitchen API docs to look for something that can help you with your next issue. Along the way, you might pick up the knack for identifying issues with your code before you publish it. And that will make you a better coder.

Overcoming frustration: Correctly using unicode in python2

In python-2.x, there's two types that deal with text.

1.: str is for strings of bytes. These are very similar in nature to how strings are handled in C.

2.: unicode is for strings of unicode code points.

NOTE:

Just what the dickens is "Unicode"?

One mistake that people encountering this issue for the first time make is confusing the unicode type and the encodings of unicode stored in the str type. In python, the unicode type stores an abstract sequence of code points. Each code point represents a grapheme. By contrast, byte str stores a sequence of bytes which can then be mapped to a sequence of code points. Each unicode encoding (UTF-8, UTF-7, UTF-16, UTF-32, etc) maps different sequences of bytes to the unicode code points.

What does that mean to you as a programmer? When you're dealing with text manipulations (finding the number of characters in a string or cutting a string on word boundaries) you should be dealing with unicode strings as they abstract characters in a manner that's appropriate for thinking of them as a sequence of letters that you will see on a page. When dealing with I/O, reading to and from the disk, printing to a terminal, sending something over a network link, etc, you should be dealing with byte str as those devices are going to need to deal with concrete implementations of what bytes represent your abstract characters.

In the python2 world many APIs use these two classes interchangably but there are several important APIs where only one or the other will do the right thing. When you give the wrong type of string to an API that wants the other type, you may end up with an exception being raised (UnicodeDecodeError or UnicodeEncodeError). However, these exceptions aren't always raised because python implicitly converts between types... sometimes.

Frustration #1: Inconsistent Errors

Although converting when possible seems like the right thing to do, it's actually the first source of frustration. A programmer can test out their program with a string like: The quick brown fox jumped over the lazy dog and not encounter any issues. But when they release their software into the wild, someone enters the string: I sat down for coffee at the café and suddenly an exception is thrown. The reason? The mechanism that converts between the two types is only able to deal with ASCII characters. Once you throw non-ASCII characters into your strings, you have to start dealing with the conversion manually.

So, if I manually convert everything to either byte str or unicode strings, will I be okay? The answer is.... sometimes.

Frustration #2: Inconsistent APIs

The problem you run into when converting everything to byte str or unicode strings is that you'll be using someone else's API quite often (this includes the APIs in the python standard library) and find that the API will only accept byte str or only accept unicode strings. Or worse, that the code will accept either when you're dealing with strings that consist solely of ASCII but throw an error when you give it a string that's got non-ASCII characters. When you encounter these APIs you first need to identify which type will work better and then you have to convert your values to the correct type for that code. Thus the programmer that wants to proactively fix all unicode errors in their code needs to do two things:

1.: You must keep track of what type your sequences of text are. Does my_sentence contain unicode or str? If you don't know that then you're going to be in for a world of hurt.

2.: Anytime you call a function you need to evaluate whether that function will do the right thing with str or unicode values. Sending the wrong value here will lead to a UnicodeError being thrown when the string contains non-ASCII characters.

NOTE:

There is one mitigating factor here. The python community has been standardizing on using unicode in all its APIs. Although there are some APIs that you need to send byte str to in order to be safe, (including things as ubiquitous as print() as we'll see in the next section), it's getting easier and easier to use unicode strings with most APIs.

Frustration #3: Inconsistent treatment of output

Alright, since the python community is moving to using unicode strings everywhere, we might as well convert everything to unicode strings and use that by default, right? Sounds good most of the time but there's at least one huge caveat to be aware of. Anytime you output text to the terminal or to a file, the text has to be converted into a byte str. Python will try to implicitly convert from unicode to byte str... but it will throw an exception if the bytes are non-ASCII:

>>> string = unicode(raw_input(), 'utf8')
café
>>> log = open('/var/tmp/debug.log', 'w')
>>> log.write(string)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 3: ordinal not in range(128)

Okay, this is simple enough to solve: Just convert to a byte str and we're all set:

>>> string = unicode(raw_input(), 'utf8')
café
>>> string_for_output = string.encode('utf8', 'replace')
>>> log = open('/var/tmp/debug.log', 'w')
>>> log.write(string_for_output)
>>>

So that was simple, right? Well... there's one gotcha that makes things a bit harder to debug sometimes. When you attempt to write non-ASCII unicode strings to a file-like object you get a traceback everytime. But what happens when you use print()? The terminal is a file-like object so it should raise an exception right? The answer to that is.... sometimes:

$ python
>>> print u'café'
café

No exception. Okay, we're fine then?

We are until someone does one of the following:

•: Runs the script in a different locale:

$ LC_ALL=C python >>> # Note: if you're using a good terminal program when running in the C locale >>> # The terminal program will prevent you from entering non-ASCII characters >>> # python will still recognize them if you use the codepoint instead: >>> print u'caf\xe9' Traceback (most recent call last): File "<stdin>", line 1, in <module> UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 3: ordinal not in range(128)

•: Redirects output to a file:

$ cat test.py #!/usr/bin/python -tt # -*- coding: utf-8 -*- print u'café' $ ./test.py >t Traceback (most recent call last): File "./test.py", line 4, in <module> print u'café' UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 3: ordinal not in range(128)

Okay, the locale thing is a pain but understandable: the C locale doesn't understand any characters outside of ASCII so naturally attempting to display those won't work. Now why does redirecting to a file cause problems? It's because print() in python2 is treated specially. Whereas the other file-like objects in python always convert to ASCII unless you set them up differently, using print() to output to the terminal will use the user's locale to convert before sending the output to the terminal. When print() is not outputting to the terminal (being redirected to a file, for instance), print() decides that it doesn't know what locale to use for that file and so it tries to convert to ASCII instead.

So what does this mean for you, as a programmer? Unless you have the luxury of controlling how your users use your code, you should always, always, always convert to a byte str before outputting strings to the terminal or to a file. Python even provides you with a facility to do just this. If you know that every unicode string you send to a particular file-like object (for instance, stdout) should be converted to a particular encoding you can use a codecs.StreamWriter object to convert from a unicode string into a byte str. In particular, codecs.getwriter() will return a StreamWriter class that will help you to wrap a file-like object for output. Using our print() example:

$ cat test.py
#!/usr/bin/python -tt
# -*- coding: utf-8 -*-
import codecs
import sys
UTF8Writer = codecs.getwriter('utf8')
sys.stdout = UTF8Writer(sys.stdout)
print u'café'
$ ./test.py  >t
$ cat t
café

Frustrations #4 and #5 -- The other shoes

In English, there's a saying "waiting for the other shoe to drop". It means that when one event (usually bad) happens, you come to expect another event (usually worse) to come after. In this case we have two other shoes.

Frustration #4: Now it doesn't take byte strings?!

If you wrap sys.stdout using codecs.getwriter() and think you are now safe to print any variable without checking its type I am afraid I must inform you that you're not paying enough attention to Murphy's Law. The StreamWriter that codecs.getwriter() provides will take unicode strings and transform them into byte str before they get to sys.stdout. The problem is if you give it something that's already a byte str it tries to transform that as well. To do that it tries to turn the byte str you give it into unicode and then transform that back into a byte str... and since it uses the ASCII codec to perform those conversions, chances are that it'll blow up when making them:

>>> import codecs
>>> import sys
>>> UTF8Writer = codecs.getwriter('utf8')
>>> sys.stdout = UTF8Writer(sys.stdout)
>>> print 'café'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/codecs.py", line 351, in write
    data, consumed = self.encode(object, self.errors)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 3: ordinal not in range(128)

To work around this, kitchen provides an alternate version of codecs.getwriter() that can deal with both byte str and unicode strings. Use kitchen.text.converters.getwriter() in place of the codecs version like this:

>>> import sys
>>> from kitchen.text.converters import getwriter
>>> UTF8Writer = getwriter('utf8')
>>> sys.stdout = UTF8Writer(sys.stdout)
>>> print u'café'
café
>>> print 'café'
café

Frustration #5: Inconsistent APIs Part deux

Sometimes you do everything right in your code but other people's code fails you. With unicode issues this happens more often than we want. A glaring example of this is when you get values back from a function that aren't consistently unicode string or byte str.

An example from the python standard library is gettext. The gettext functions are used to help translate messages that you display to users in the users' native languages. Since most languages contain letters outside of the ASCII range, the values that are returned contain unicode characters. gettext provides you with ugettext() and ungettext() to return these translations as unicode strings and gettext(), ngettext(), lgettext(), and lngettext() to return them as encoded byte str. Unfortunately, even though they're documented to return only one type of string or the other, the implementation has corner cases where the wrong type can be returned.

This means that even if you separate your unicode string and byte str correctly before you pass your strings to a gettext function, afterwards, you might have to check that you have the right sort of string type again.

NOTE:

kitchen.i18n provides alternate gettext translation objects that return only byte str or only unicode string.

A few solutions

Now that we've identified the issues, can we define a comprehensive strategy for dealing with them?

Convert text at the border

If you get some piece of text from a library, read from a file, etc, turn it into a unicode string immediately. Since python is moving in the direction of unicode strings everywhere it's going to be easier to work with unicode strings within your code.

If your code is heavily involved with using things that are bytes, you can do the opposite and convert all text into byte str at the border and only convert to unicode when you need it for passing to another library or performing string operations on it.

In either case, the important thing is to pick a default type for strings and stick with it throughout your code. When you mix the types it becomes much easier to operate on a string with a function that can only use the other type by mistake.

NOTE:

In python3, the abstract unicode type becomes much more prominent. The type named str is the equivalent of python2's unicode and python3's bytes type replaces python2's str. Most APIs deal in the unicode type of string with just some pieces that are low level dealing with bytes. The implicit conversions between bytes and unicode is removed and whenever you want to make the conversion you need to do so explicitly.

When the data needs to be treated as bytes (or unicode) use a naming convention

Sometimes you're converting nearly all of your data to unicode strings but you have one or two values where you have to keep byte str around. This is often the case when you need to use the value verbatim with some external resource. For instance, filenames or key values in a database. When you do this, use a naming convention for the data you're working with so you (and others reading your code later) don't get confused about what's being stored in the value.

If you need both a textual string to present to the user and a byte value for an exact match, consider keeping both versions around. You can either use two variables for this or a dict whose key is the byte value.

NOTE:

You can use the naming convention used in kitchen as a guide for implementing your own naming convention. It prefixes byte str variables of unknown encoding with b_ and byte str of known encoding with the encoding name like: utf8_. If the default was to handle str and only keep a few unicode values, those variables would be prefixed with u_.

When outputting data, convert back into bytes

When you go to send your data back outside of your program (to the filesystem, over the network, displaying to the user, etc) turn the data back into a byte str. How you do this will depend on the expected output format of the data. For displaying to the user, you can use the user's default encoding using locale.getpreferredencoding(). For entering into a file, you're best bet is to pick a single encoding and stick with it.

WARNING:

When using the encoding that the user has set (for instance, using locale.getpreferredencoding(), remember that they may have their encoding set to something that can't display every single unicode character. That means when you convert from unicode to a byte str you need to decide what should happen if the byte value is not valid in the user's encoding. For purposes of displaying messages to the user, it's usually okay to use the replace encoding error handler to replace the invalid characters with a question mark or other symbol meaning the character couldn't be displayed.

You can use kitchen.text.converters.getwriter() to do this automatically for sys.stdout. When creating exception messages be sure to convert to bytes manually.

When writing unittests, include non-ASCII values and both unicode and str type

Unless you know that a specific portion of your code will only deal with ASCII, be sure to include non-ASCII values in your unittests. Including a few characters from several different scripts is highly advised as well because some code may have special cased accented roman characters but not know how to handle characters used in Asian alphabets.

Similarly, unless you know that that portion of your code will only be given unicode strings or only byte str be sure to try variables of both types in your unittests. When doing this, make sure that the variables are also non-ASCII as python's implicit conversion will mask problems with pure ASCII data. In many cases, it makes sense to check what happens if byte str and unicode strings that won't decode in the present locale are given.

Be vigilant about spotting poor APIs

Make sure that the libraries you use return only unicode strings or byte str. Unittests can help you spot issues here by running many variations of data through your functions and checking that you're still getting the types of string that you expect.

Example: Putting this all together with kitchen

The kitchen library provides a wide array of functions to help you deal with byte str and unicode strings in your program. Here's a short example that uses many kitchen functions to do its work:

#!/usr/bin/python -tt
# -*- coding: utf-8 -*-
import locale
import os
import sys
import unicodedata
from kitchen.text.converters import getwriter, to_bytes, to_unicode
from kitchen.i18n import get_translation_object
if __name__ == '__main__':
    # Setup gettext driven translations but use the kitchen functions so
    # we don't have the mismatched bytes-unicode issues.
    translations = get_translation_object('example')
    # We use _() for marking strings that we operate on as unicode
    # This is pretty much everything
    _ = translations.ugettext
    # And b_() for marking strings that we operate on as bytes.
    # This is limited to exceptions
    b_ = translations.lgettext
    # Setup stdout
    encoding = locale.getpreferredencoding()
    Writer = getwriter(encoding)
    sys.stdout = Writer(sys.stdout)
    # Load data.  Format is filename\0description
    # description should be utf-8 but filename can be any legal filename
    # on the filesystem
    # Sample datafile.txt:
    #   /etc/shells\x00Shells available on caf\xc3\xa9.lan
    #   /var/tmp/file\xff\x00File with non-utf8 data in the filename
    #
    # And to create /var/tmp/file\xff (under bash or zsh) do:
    #   echo 'Some data' > /var/tmp/file$'\377'
    datafile = open('datafile.txt', 'r')
    data = {}
    for line in datafile:
        # We're going to keep filename as bytes because we will need the
        # exact bytes to access files on a POSIX operating system.
        # description, we'll immediately transform into unicode type.
        b_filename, description = line.split('\0', 1)
        # to_unicode defaults to decoding output from utf-8 and replacing
        # any problematic bytes with the unicode replacement character
        # We accept mangling of the description here knowing that our file
        # format is supposed to use utf-8 in that field and that the
        # description will only be displayed to the user, not used as
        # a key value.
        description = to_unicode(description, 'utf-8').strip()
        data[b_filename] = description
    datafile.close()
    # We're going to add a pair of extra fields onto our data to show the
    # length of the description and the filesize.  We put those between
    # the filename and description because we haven't checked that the
    # description is free of NULLs.
    datafile = open('newdatafile.txt', 'w')
    # Name filename with a b_ prefix to denote byte string of unknown encoding
    for b_filename in data:
        # Since we have the byte representation of filename, we can read any
        # filename
        if os.access(b_filename, os.F_OK):
            size = os.path.getsize(b_filename)
        else:
            size = 0
        # Because the description is unicode type,  we know the number of
        # characters corresponds to the length of the normalized unicode
        # string.
        length = len(unicodedata.normalize('NFC', description))
        # Print a summary to the screen
        # Note that we do not let implici type conversion from str to
        # unicode transform b_filename into a unicode string.  That might
        # fail as python would use the ASCII filename.  Instead we use
        # to_unicode() to explictly transform in a way that we know will
        # not traceback.
        print _(u'filename: %s') % to_unicode(b_filename)
        print _(u'file size: %s') % size
        print _(u'desc length: %s') % length
        print _(u'description: %s') % data[b_filename]
        # First combine the unicode portion
        line = u'%s\0%s\0%s' % (size, length, data[b_filename])
        # Since the filenames are bytes, turn everything else to bytes before combining
        # Turning into unicode first would be wrong as the bytes in b_filename
        # might not convert
        b_line = '%s\0%s\n' % (b_filename, to_bytes(line))
        # Just to demonstrate that getwriter will pass bytes through fine
        print b_('Wrote: %s') % b_line
        datafile.write(b_line)
    datafile.close()
    # And just to show how to properly deal with an exception.
    # Note two things about this:
    # 1) We use the b_() function to translate the string.  This returns a
    #    byte string instead of a unicode string
    # 2) We're using the b_() function returned by kitchen.  If we had
    #    used the one from gettext we would need to convert the message to
    #    a byte str first
    message = u'Demonstrate the proper way to raise exceptions.  Sincerely,  \u3068\u3057\u304a'
    raise Exception(b_(message))

SEE ALSO:

kitchen.text.converters

Designing Unicode Aware APIs

APIs that deal with byte str and unicode strings are difficult to get right. Here are a few strategies with pros and cons of each.

Take either bytes or unicode, output only unicode

In this strategy, you allow the user to enter either unicode strings or byte str but what you give back is always unicode. This strategy is easy for novice endusers to start using immediately as they will be able to feed either type of string into the function and get back a string that they can use in other places.

However, it does lead to the novice writing code that functions correctly when testing it with ASCII-only data but fails when given data that contains non-ASCII characters. Worse, if your API is not designed to be flexible, the consumer of your code won't be able to easily correct those problems once they find them.

Here's a good API that uses this strategy:

from kitchen.text.converters import to_unicode
def truncate(msg, max_length, encoding='utf8', errors='replace'):
    msg = to_unicode(msg, encoding, errors)
    return msg[:max_length]

The call to truncate() starts with the essential parameters for performing the task. It ends with two optional keyword arguments that define the encoding to use to transform from a byte str to unicode and the strategy to use if undecodable bytes are encountered. The defaults may vary depending on the use cases you have in mind. When the output is generally going to be printed for the user to see, errors='replace' is a good default. If you are constructing keys to a database, raisng an exception (with errors='strict') may be a better default. In either case, having both parameters allows the person using your API to choose how they want to handle any problems. Having the values is also a clue to them that a conversion from byte str to unicode string is going to occur.

NOTE:

If you're targeting python-3.1 and above, errors='surrogateescape' may be a better default than errors='strict'. You need to be mindful of a few things when using surrogateescape though:

•: surrogateescape will cause issues if a non-ASCII compatible encoding is used (for instance, UTF-16 and UTF-32.) That makes it unhelpful in situations where a true general purpose method of encoding must be found. PEP 383 mentions that surrogateescape was specifically designed with the limitations of translating using system locales (where ASCII compatibility is generally seen as inescapable) so you should keep that in mind.

•: If you use surrogateescape to decode from bytes to unicode you will need to use an error handler other than strict to encode as the lone surrogate that this error handler creates makes for invalid unicode that must be handled when encoding. In Python-3.1.2 or less, a bug in the encoder error handlers mean that you can only use surrogateescape to encode; anything else will throw an error.

Evaluate your usages of the variables in question to see what makes sense.

Here's a bad example of using this strategy:

from kitchen.text.converters import to_unicode
def truncate(msg, max_length):
    msg = to_unicode(msg)
    return msg[:max_length]

In this example, we don't have the optional keyword arguments for encoding and errors. A user who uses this function is more likely to miss the fact that a conversion from byte str to unicode is going to occur. And once an error is reported, they will have to look through their backtrace and think harder about where they want to transform their data into unicode strings instead of having the opportunity to control how the conversion takes place in the function itself. Note that the user does have the ability to make this work by making the transformation to unicode themselves:

from kitchen.text.converters import to_unicode
msg = to_unicode(msg, encoding='euc_jp', errors='ignore')
new_msg = truncate(msg, 5)

Take either bytes or unicode, output the same type

This strategy is sometimes called polymorphic because the type of data that is returned is dependent on the type of data that is received. The concept is that when you are given a byte str to process, you return a byte str in your output. When you are given unicode strings to process, you return unicode strings in your output.

This can work well for end users as the ones that know about the difference between the two string types will already have transformed the strings to their desired type before giving it to this function. The ones that don't can remain blissfully ignorant (at least, as far as your function is concerned) as the function does not change the type.

In cases where the encoding of the byte str is known or can be discovered based on the input data this works well. If you can't figure out the input encoding, however, this strategy can fail in any of the following cases:

1.: It needs to do an internal conversion between byte str and unicode string.

2.: It cannot return the same data as either a unicode string or byte str.

3.: You may need to deal with byte strings that are not byte-compatible with ASCII

First, a couple examples of using this strategy in a good way:

def translate(msg, table):
    replacements = table.keys()
    new_msg = []
    for index, char in enumerate(msg):
        if char in replacements:
            new_msg.append(table[char])
        else:
            new_msg.append(char)
    return ''.join(new_msg)

In this example, all of the strings that we use (except the empty string which is okay because it doesn't have any characters to encode) come from outside of the function. Due to that, the user is responsible for making sure that the msg, and the keys and values in table all match in terms of type (unicode vs str) and encoding (You can do some error checking to make sure the user gave all the same type but you can't do the same for the user giving different encodings). You do not need to make changes to the string that require you to know the encoding or type of the string; everything is a simple replacement of one element in the array of characters in message with the character in table.

import json
from kitchen.text.converters import to_unicode, to_bytes
def first_field_from_json_data(json_string):
    '''Return the first field in a json data structure.
    The format of the json data is a simple list of strings.
    '["one", "two", "three"]'
    '''
    if isinstance(json_string, unicode):
        # On all python versions, json.loads() returns unicode if given
        # a unicode string
        return json.loads(json_string)[0]
    # Byte str: figure out which encoding we're dealing with
    if '\x00' not in json_data[:2]
        encoding = 'utf8'
    elif '\x00\x00\x00' == json_data[:3]:
        encoding = 'utf-32-be'
    elif '\x00\x00\x00' == json_data[1:4]:
        encoding = 'utf-32-le'
    elif '\x00' == json_data[0] and '\x00' == json_data[2]:
        encoding = 'utf-16-be'
    else:
        encoding = 'utf-16-le'
    data = json.loads(unicode(json_string, encoding))
    return data[0].encode(encoding)

In this example the function takes either a byte str type or a unicode string that has a list in json format and returns the first field from it as the type of the input string. The first section of code is very straightforward; we receive a unicode string, parse it with a function, and then return the first field from our parsed data (which our function returned to us as json data).

The second portion that deals with byte str is not so straightforward. Before we can parse the string we have to determine what characters the bytes in the string map to. If we didn't do that, we wouldn't be able to properly find which characters are present in the string. In order to do that we have to figure out the encoding of the byte str. Luckily, the json specification states that all strings are unicode and encoded with one of UTF32be, UTF32le, UTF16be, UTF16le, or UTF-8. It further defines the format such that the first two characters are always ASCII. Each of these has a different sequence of NULLs when they encode an ASCII character. We can use that to detect which encoding was used to create the byte str.

Finally, we return the byte str by encoding the unicode back to a byte str.

As you can see, in this example we have to convert from byte str to unicode and back. But we know from the json specification that byte str has to be one of a limited number of encodings that we are able to detect. That ability makes this strategy work.

Now for some examples of using this strategy in ways that fail:

import unicodedata
def first_char(msg):
    '''Return the first character in a string'''
    if not isinstance(msg, unicode):
        try:
            msg = unicode(msg, 'utf8')
        except UnicodeError:
            msg = unicode(msg, 'latin1')
    msg = unicodedata.normalize('NFC', msg)
    return msg[0]

If you look at that code and think that there's something fragile and prone to breaking in the try: except: block you are correct in being suspicious. This code will fail on multi-byte character sets that aren't UTF-8. It can also fail on data where the sequence of bytes is valid UTF-8 but the bytes are actually of a different encoding. The reasons this code fails is that we don't know what encoding the bytes are in and the code must convert from a byte str to a unicode string in order to function.

In order to make this code robust we must know the encoding of msg. The only way to know that is to ask the user so the API must do that:

import unicodedata
def number_of_chars(msg, encoding='utf8', errors='strict'):
    if not isinstance(msg, unicode):
        msg = unicode(msg, encoding, errors)
    msg = unicodedata.normalize('NFC', msg)
    return len(msg)

Another example of failure:

import os
def listdir(directory):
    files = os.listdir(directory)
    if isinstance(directory, str):
        return files
    # files could contain both bytes and unicode
    new_files = []
    for filename in files:
        if not isinstance(filename, unicode):
            # What to do here?
            continue
        new_files.appen(filename)
    return new_files

This function illustrates the second failure mode. Here, not all of the possible values can be represented as unicode without knowing more about the encoding of each of the filenames involved. Since each filename could have a different encoding there's a few different options to pursue. We could make this function always return byte str since that can accurately represent anything that could be returned. If we want to return unicode we need to at least allow the user to specify what to do in case of an error decoding the bytes to unicode. We can also let the user specify the encoding to use for doing the decoding but that won't help in all cases since not all files will be in the same encoding (or even necessarily in any encoding):

import locale
import os
def listdir(directory, encoding=locale.getpreferredencoding(), errors='strict'):
    # Note: In python-3.1+, surrogateescape may be a better default
    files = os.listdir(directory)
    if isinstance(directory, str):
        return files
    new_files = []
    for filename in files:
        if not isinstance(filename, unicode):
            filename = unicode(filename, encoding=encoding, errors=errors)
        new_files.append(filename)
    return new_files

Note that although we use errors in this example as what to pass to the codec that decodes to unicode we could also have an errors argument that decides other things to do like skip a filename entirely, return a placeholder (Nondisplayable filename), or raise an exception.

This leaves us with one last failure to describe:

def first_field(csv_string):
    '''Return the first field in a comma separated values string.'''
    try:
        return csv_string[:csv_string.index(',')]
    except ValueError:
        return csv_string

This code looks simple enough. The hidden error here is that we are searching for a comma character in a byte str but not all encodings will use the same sequence of bytes to represent the comma. If you use an encoding that's not ASCII compatible on the byte level, then the literal comma ',' in the above code will match inappropriate bytes. Some examples of how it can fail:

•: Will find the byte representing an ASCII comma in another character

•: Will find the comma but leave trailing garbage bytes on the end of the string

•: Will not match the character that represents the comma in this encoding

There are two ways to solve this. You can either take the encoding value from the user or you can take the separator value from the user. Of the two, taking the encoding is the better option for two reasons:

1.: Taking a separator argument doesn't clearly document for the API user that the reason they must give it is to properly match the encoding of the csv_string. They're just as likely to think that it's simply a way to specify an alternate character (like ":" or "|") for the separator.

2.

It's possible for a variable width encoding to reuse the same byte sequence for different characters in multiple sequences.

NOTE:

UTF-8 is resistant to this as any character's sequence of bytes will never be a subset of another character's sequence of bytes.

With that in mind, here's how to improve the API:

def first_field(csv_string, encoding='utf-8', errors='replace'):
    if not isinstance(csv_string, unicode):
        u_string = unicode(csv_string, encoding, errors)
        is_unicode = False
    else:
        u_string = csv_string
    try:
        field = u_string[:U_string.index(u',')]
    except ValueError:
        return csv_string
    if not is_unicode:
        field = field.encode(encoding, errors)
    return field

NOTE:

If you decide you'll never encounter a variable width encoding that reuses byte sequences you can use this code instead:

def first_field(csv_string, encoding='utf-8'):
    try:
        return csv_string[:csv_string.index(','.encode(encoding))]
    except ValueError:
        return csv_string

Separate functions

Sometimes you want to be able to take either byte str or unicode strings, perform similar operations on either one and then return data in the same format as was given. Probably the easiest way to do that is to have separate functions for each and adopt a naming convention to show that one is for working with byte str and the other is for working with unicode strings:

def translate_b(msg, table):
    '''Replace values in str with other byte values like unicode.translate'''
    if not isinstance(msg, str):
        raise TypeError('msg must be of type str')
    str_table = [chr(s) for s in xrange(0,256)]
    delete_chars = []
    for chr_val in (k for k in table.keys() if isinstance(k, int)):
        if chr_val > 255:
            raise ValueError('Keys in table must not exceed 255)')
        if table[chr_val] == None:
            delete_chars.append(chr(chr_val))
        elif isinstance(table[chr_val], int):
            if table[chr_val] > 255:
                raise TypeError('table values cannot be more than 255 or less than 0')
            str_table[chr_val] = chr(table[chr_val])
        else:
            if not isinstance(table[chr_val], str):
                raise TypeError('character mapping must return integer, None or str')
            str_table[chr_val] = table[chr_val]
    str_table = ''.join(str_table)
    delete_chars = ''.join(delete_chars)
    return msg.translate(str_table, delete_chars)
def translate(msg, table):
    '''Replace values in a unicode string with other values'''
    if not isinstance(msg, unicode):
        raise TypeError('msg must be of type unicode')
    return msg.translate(table)

There's several things that we have to do in this API:

•: Because the function names might not be enough of a clue to the user of the functions of the value types that are expected, we have to check that the types are correct.

•: We keep the behaviour of the two functions as close to the same as possible, just with byte str and unicode strings substituted for each other.

Deciding whether to take str or unicode when no value is returned

Not all functions have a return value. Sometimes a function is there to interact with something external to python, for instance, writing a file out to disk or a method exists to update the internal state of a data structure. One of the main questions with these APIs is whether to take byte str, unicode string, or both. The answer depends on your use case but I'll give some examples here.

Writing to external data

When your information is going to an external data source like writing to a file you need to decide whether to take in unicode strings or byte str. Remember that most external data sources are not going to be dealing with unicode directly. Instead, they're going to be dealing with a sequence of bytes that may be interpreted as unicode. With that in mind, you either need to have the user give you a byte str or convert to a byte str inside the function.

Next you need to think about the type of data that you're receiving. If it's textual data, (for instance, this is a chat client and the user is typing messages that they expect to be read by another person) it probably makes sense to take in unicode strings and do the conversion inside your function. On the other hand, if this is a lower level function that's passing data into a network socket, it probably should be taking byte str instead.

Just as noted in the API notes above, you should specify an encoding and errors argument if you need to transform from unicode string to byte str and you are unable to guess the encoding from the data itself.

Updating data structures

Sometimes your API is just going to update a data structure and not immediately output that data anywhere. Just as when writing external data, you should think about both what your function is going to do with the data eventually and what the caller of your function is thinking that they're giving you. Most of the time, you'll want to take unicode strings and enter them into the data structure as unicode when the data is textual in nature. You'll want to take byte str and enter them into the data structure as byte str when the data is not text. Use a naming convention so the user knows what's expected.

APIs to Avoid

There are a few APIs that are just wrong. If you catch yourself making an API that does one of these things, change it before anyone sees your code.

Returning unicode unless a conversion fails

This type of API usually deals with byte str at some point and converts it to unicode because it's usually thought to be text. However, there are times when the bytes fail to convert to a unicode string. When that happens, this API returns the raw byte str instead of a unicode string. One example of this is present in the python standard library: python2's os.listdir():

>>> import os
>>> import locale
>>> locale.getpreferredencoding()
'UTF-8'
>>> os.mkdir('/tmp/mine')
>>> os.chdir('/tmp/mine')
>>> open('nonsense_char_\xff', 'w').close()
>>> open('all_ascii', 'w').close()
>>> os.listdir(u'.')
[u'all_ascii', 'nonsense_char_\xff']

The problem with APIs like this is that they cause failures that are hard to debug because they don't happen where the variables are set. For instance, let's say you take the filenames from os.listdir() and give it to this function:

def normalize_filename(filename):
    '''Change spaces and dashes into underscores'''
    return filename.translate({ord(u' '):u'_', ord(u' '):u'_'})

When you test this, you use filenames that all are decodable in your preferred encoding and everything seems to work. But when this code is run on a machine that has filenames in multiple encodings the filenames returned by os.listdir() suddenly include byte str. And byte str has a different string.translate() function that takes different values. So the code raises an exception where it's not immediately obvious that os.listdir() is at fault.

Ignoring values with no chance of recovery

An early version of python3 attempted to fix the os.listdir() problem pointed out in the last section by returning all values that were decodable to unicode and omitting the filenames that were not. This lead to the following output:

>>> import os
>>> import locale
>>> locale.getpreferredencoding()
'UTF-8'
>>> os.mkdir('/tmp/mine')
>>> os.chdir('/tmp/mine')
>>> open(b'nonsense_char_\xff', 'w').close()
>>> open('all_ascii', 'w').close()
>>> os.listdir('.')
['all_ascii']

The issue with this type of code is that it is silently doing something surprising. The caller expects to get a full list of files back from os.listdir(). Instead, it silently ignores some of the files, returning only a subset. This leads to code that doesn't do what is expected that may go unnoticed until the code is in production and someone notices that something important is being missed.

Raising a UnicodeException with no chance of recovery

Believe it or not, a few libraries exist that make it impossible to deal with unicode text without raising a UnicodeError. What seems to occur in these libraries is that the library has functions that expect to receive a unicode string. However, internally, those functions call other functions that expect to receive a byte str. The programmer of the API was smart enough to convert from a unicode string to a byte str but they did not give the user the chance to specify the encodings to use or how to deal with errors. This results in exceptions when the user passes in a byte str because the initial function wants a unicode string and exceptions when the user passes in a unicode string because the function can't convert the string to bytes in the encoding that it's selected.

Do not put the user in the position of not being able to use your API without raising a UnicodeError with certain values. If you can only safely take unicode strings, document that byte str is not allowed and vice versa. If you have to convert internally, make sure to give the caller of your function parameters to control the encoding and how to treat errors that may occur during the encoding/decoding process. If your code will raise a UnicodeError with non-ASCII values no matter what, you should probably rethink your API.

Knowing your data

If you've read all the way down to this section without skipping you've seen several admonitions about the type of data you are processing affecting the viability of the various API choices.

Here's a few things to consider in your data:

Do you need to operate on both bytes and unicode?

Much of the data in libraries, programs, and the general environment outside of python is written where strings are sequences of bytes. So when we interact with data that comes from outside of python or data that is about to leave python it may make sense to only operate on the data as a byte str. There's two times when this may make sense:

1.: The user is intended to hand the data to the function and then the function takes care of sending the data outside of python (to the filesystem, over the network, etc).

2.: The data is not representable as text. For instance, writing a binary file format.

Even when your code is operating in this area you still need to think a little more about your data. For instance, it might make sense for the person using your API to pass in unicode strings and let the function convert that into the byte str that it then sends over the wire.

There are also times when it might make sense to operate only on unicode strings. unicode represents text so anytime that you are working on textual data that isn't going to leave python it has the potential to be a unicode-only API. However, there's two things that you should consider when designing a unicode-only API:

1.: As your API gains popularity, people are going to use your API in places that you may not have thought of. Corner cases in these other places may mean that processing bytes is desirable.

2.: In python2, byte str and unicode are often used interchangably with each other. That means that people programming against your API may have received str from some other API and it would be most convenient for their code if your API accepted it.

NOTE:

In python3, the separation between the text type and the byte type are more clear. So in python3, there's less need to have all APIs take both unicode and bytes.

Can you restrict the encodings?

If you determine that you have to deal with byte str you should realize that not all encodings are created equal. Each has different properties that may make it possible to provide a simpler API provided that you can reasonably tell the users of your API that they cannot use certain classes of encodings.

As one example, if you are required to find a comma (,) in a byte str you have different choices based on what encodings are allowed. If you can reasonably restrict your API users to only giving ASCII compatible encodings you can do this simply by searching for the literal comma character because that character will be represented by the same byte sequence in all ASCII compatible encodings.

The following are some classes of encodings to be aware of as you decide how generic your code needs to be.

Single byte encodings

Single byte encodings can only represent 256 total characters. They encode the code points for a character to the equivalent number in a single byte.

Most single byte encodings are ASCII compatible. ASCII compatible encodings are the most likely to be usable without changes to code so this is good news. A notable exception to this is the EBDIC family of encodings.

Multibyte encodings

Multibyte encodings use more than one byte to encode some characters.

Fixed width

Fixed width encodings have a set number of bytes to represent all of the characters in the character set. UTF-32 is an example of a fixed width encoding that uses four bytes per character and can express every unicode characters. There are a number of problems with writing APIs that need to operate on fixed width, multibyte characters. To go back to our earlier example of finding a comma in a string, we have to realize that even in UTF-32 where the code point for ASCII characters is the same as in ASCII, the byte sequence for them is different. So you cannot search for the literal byte character as it may pick up false positives and may break a byte sequence in an odd place.

Variable Width

ASCII compatible

UTF-8 and the EUC family of encodings are examples of ASCII compatible multi-byte encodings. They achieve this by adhering to two principles:

•: All of the ASCII characters are represented by the byte that they are in the ASCII encoding.

•: None of the ASCII byte sequences are reused in any other byte sequence for a different character.

Escaped

Some multibyte encodings work by using only bytes from the ASCII encoding but when a particular sequence of those byes is found, they are interpreted as meaning something other than their ASCII values. UTF-7 is one such encoding that can encode all of the unicode code points. For instance, here's a some Japanese characters encoded as UTF-7:

>>> a = u'\u304f\u3089\u3068\u307f'
>>> print a
くらとみ
>>> print a.encode('utf-7')
+ME8wiTBoMH8-

These encodings can be used when you need to encode unicode data that may contain non-ASCII characters for inclusion in an ASCII only transport medium or file.

However, they are not ASCII compatible in the sense that we used earlier as the bytes that represent a ASCII character are being reused as part of other characters. If you were to search for a literal plus sign in this encoded string, you would run across many false positives, for instance.

Other

There are many other popular variable width encodings, for instance UTF-16 and shift-JIS. Many of these are not ASCII compatible so you cannot search for a literal ASCII character without danger of false positives or false negatives.

Kitchen API

Kitchen is structured as a collection of modules. In its current configuration, Kitchen ships with the following modules. Other addon modules that may drag in more dependencies can be found on the project webpage

Kitchen.i18n Module

I18N is an important piece of any modern program. Unfortunately, setting up i18n in your program is often a confusing process. The functions provided here aim to make the programming side of that a little easier.

Most projects will be able to do something like this when they startup:

# myprogram/__init__.py:
import os
import sys
from kitchen.i18n import easy_gettext_setup
_, N_  = easy_gettext_setup('myprogram', localedirs=(
        os.path.join(os.path.realpath(os.path.dirname(__file__)), 'locale'),
        os.path.join(sys.prefix, 'lib', 'locale')
        ))

Then, in other files that have strings that need translating:

# myprogram/commands.py:
from myprogram import _, N_
def print_usage():
    print _(u"""available commands are:
    --help              Display help
    --version           Display version of this program
    --bake-me-a-cake    as fast as you can
        """)
def print_invitations(age):
    print _('Please come to my party.')
    print N_('I will be turning %(age)s year old',
        'I will be turning %(age)s years old', age) % {'age': age}

See the documentation of easy_gettext_setup() and get_translation_object() for more details.

SEE ALSO:

gettext: for details of how the python gettext facilities work

babel: The babel module for in depth information on gettext, message catalogs, and translating your app. babel provides some nice features for i18n on top of gettext

Functions

easy_gettext_setup() should satisfy the needs of most users. get_translation_object() is designed to ease the way for anyone that needs more control.

kitchen.i18n.easy_gettext_setup(domain, localedirs=(), use_unicode=True)

Setup translation functions for an application

Parameters

•: domain -- Name of the message domain. This should be a unique name that can be used to lookup the message catalog for this app.

•: localedirs -- Iterator of directories to look for message catalogs under. The first directory to exist is used regardless of whether messages for this domain are present. If none of the directories exist, fallback on sys.prefix + /share/locale Default: No directories to search so we just use the fallback.

•: use_unicode -- If True return the gettext functions for str strings else return the functions for byte bytes for the translations. Default is True.

Returns: tuple of the gettext function and gettext function for plurals

Setting up gettext can be a little tricky because of lack of documentation. This function will setup gettext using the Class-based API for you. For the simple case, you can use the default arguments and call it like this:

_, N_ = easy_gettext_setup()

This will get you two functions, _() and N_() that you can use to mark strings in your code for translation. _() is used to mark strings that don't need to worry about plural forms no matter what the value of the variable is. N_() is used to mark strings that do need to have a different form if a variable in the string is plural.

SEE ALSO:

Kitchen.i18n Module: This module's documentation has examples of using _() and N_()

get_translation_object(): for information on how to use localedirs to get the proper message catalogs both when in development and when installed to FHS compliant directories on Linux.

NOTE:

The gettext functions returned from this function should be superior to the ones returned from gettext. The traits that make them better are described in the DummyTranslations and NewGNUTranslations documentation.

Changed in version kitchen-0.2.4: ; API kitchen.i18n 2.0.0 Changed easy_gettext_setup() to return the lgettext functions instead of gettext functions when use_unicode=False.

kitchen.i18n.get_translation_object(domain, localedirs=(), languages=None, class_=None, fallback=True, codeset=None, python2_api=True)

Get a translation object bound to the message catalogs

Parameters

•: domain -- Name of the message domain. This should be a unique name that can be used to lookup the message catalog for this app or library.

•: localedirs -- Iterator of directories to look for message catalogs under. The directories are searched in order for message catalogs. For each of the directories searched, we check for message catalogs in any language specified in:attr:languages. The message catalogs are used to create the Translation object that we return. The Translation object will attempt to lookup the msgid in the first catalog that we found. If it's not in there, it will go through each subsequent catalog looking for a match. For this reason, the order in which you specify the localedirs may be important. If no message catalogs are found, either return a DummyTranslations object or raise an IOError depending on the value of fallback. Rhe default localedir from gettext which is os.path.join(sys.prefix, 'share', 'locale') on Unix is implicitly appended to the localedirs, making it the last directory searched.

•

languages --

Iterator of language codes to check for message catalogs. If unspecified, the user's locale settings will be used.

SEE ALSO:

gettext.find() for information on what environment variables are used.

•: class -- The class to use to extract translations from the message catalogs. Defaults to NewGNUTranslations.

•: fallback -- If set to data:False, raise an IOError if no message catalogs are found. If True, the default, return a DummyTranslations object.

•: codeset -- Set the character encoding to use when returning byte bytes objects. This is equivalent to calling output_charset() on the Translations object that is returned from this function.

•: python2_api -- When data:True (default), return Translation objects that use the python2 gettext api (gettext() and lgettext() return byte bytes. ugettext() exists and returns str strings). When False, return Translation objects that use the python3 gettext api (gettext returns str strings and lgettext returns byte bytes. ugettext does not exist.)

Returns: Translation object to get gettext methods from

If you need more flexibility than easy_gettext_setup(), use this function. It sets up a gettext Translation object and returns it to you. Then you can access any of the methods of the object that you need directly. For instance, if you specifically need to access lgettext():

translations = get_translation_object('foo')
translations.lgettext('My Message')

This function is similar to the python standard library gettext.translation() but makes it better in two ways

1.

It returns NewGNUTranslations or DummyTranslations: objects by default. These are superior to the gettext.GNUTranslations and gettext.NullTranslations objects because they are consistent in the string type they return and they fix several issues that can causethe python standard library objects to throw UnicodeError.

2.

This function takes multiple directories to search for: message catalogs.

The latter is important when setting up gettext in a portable manner. There is not a common directory for translations across operating systems so one needs to look in multiple directories for the translations. get_translation_object() is able to handle that if you give it a list of directories to search for catalogs:

translations = get_translation_object('foo', localedirs=(
     os.path.join(os.path.realpath(os.path.dirname(__file__)), 'locale'),
     os.path.join(sys.prefix, 'lib', 'locale')))

This will search for several different directories:

1.: A directory named locale in the same directory as the module that called get_translation_object(),

2.: In /usr/lib/locale

3.: In /usr/share/locale (the fallback directory)

This allows gettext to work on Windows and in development (where the message catalogs are typically in the toplevel module directory) and also when installed under Linux (where the message catalogs are installed in /usr/share/locale). You (or the system packager) just need to install the message catalogs in /usr/share/locale and remove the locale directory from the module to make this work. ie:

In development:
    ~/foo   # Toplevel module directory
    ~/foo/__init__.py
    ~/foo/locale    # With message catalogs below here:
    ~/foo/locale/es/LC_MESSAGES/foo.mo
Installed on Linux:
    /usr/lib/python2.7/site-packages/foo
    /usr/lib/python2.7/site-packages/foo/__init__.py
    /usr/share/locale/  # With message catalogs below here:
    /usr/share/locale/es/LC_MESSAGES/foo.mo

NOTE:

This function will setup Translation objects that attempt to lookup msgids in all of the found message catalogs. This means if you have several versions of the message catalogs installed in different directories that the function searches, you need to make sure that localedirs specifies the directories so that newer message catalogs are searched first. It also means that if a newer catalog does not contain a translation for a msgid but an older one that's in localedirs does, the translation from that older catalog will be returned.

Changed in version kitchen-1.1.0: ; API kitchen.i18n 2.1.0 Add more parameters to get_translation_object() so it can more easily be used as a replacement for gettext.translation(). Also change the way we use localedirs. We cycle through them until we find a suitable locale file rather than simply cycling through until we find a directory that exists. The new code is based heavily on the python standard library gettext.translation() function.

Changed in version kitchen-1.2.0: ; API kitchen.i18n 2.2.0 Add python2_api parameter

Translation Objects

The standard translation objects from the gettext module suffer from several problems:

•: They can throw UnicodeError

•: They can't find translations for non-ASCII byte str messages

•: They may return either unicode string or byte str from the same function even though the functions say they will only return unicode or only return byte str.

DummyTranslations and NewGNUTranslations were written to fix these issues.

class kitchen.i18n.DummyTranslations(fp=None, python2_api=True)

Safer version of gettext.NullTranslations

This Translations class doesn't translate the strings and is intended to be used as a fallback when there were errors setting up a real Translations object. It's safer than gettext.NullTranslations in its handling of byte bytes vs str strings.

Unlike NullTranslations, this Translation class will never throw a UnicodeError. The code that you have around a call to DummyTranslations might throw a UnicodeError but at least that will be in code you control and can fix. Also, unlike NullTranslations all of this Translation object's methods guarantee to return byte bytes except for ugettext() and ungettext() which guarantee to return str strings.

When byte bytes are returned, the strings will be encoded according to this algorithm:

1.: If a fallback has been added, the fallback will be called first. You'll need to consult the fallback to see whether it performs any encoding changes.

2.: If a byte bytes was given, the same byte bytes will be returned.

3.: If a str string was given and set_output_charset() has been called then we encode the string using the output_charset

4.: If a str string was given and this is gettext() or ngettext() and _charset was set output in that charset.

5.: If a str string was given and this is gettext() or ngettext() we encode it using 'utf-8'.

6.: If a str string was given and this is lgettext() or lngettext() we encode using the value of locale.getpreferredencoding()

For ugettext() and ungettext(), we go through the same set of steps with the following differences:

•: We transform byte bytes into str strings for these methods.

•: The encoding used to decode the byte bytes is taken from input_charset if it's set, otherwise we decode using UTF-8.

input_charset: is an extension to the python standard library gettext that specifies what charset a message is encoded in when decoding a message to str. This is used for two purposes:

1.: If the message string is a byte bytes, this is used to decode the string to a str string before looking it up in the message catalog.

2.: In ugettext() and ungettext() methods, if a byte bytes is given as the message and is untranslated this is used as the encoding when decoding to str. This is different from _charset which may be set when a message catalog is loaded because input_charset is used to describe an encoding used in a python source file while _charset describes the encoding used in the message catalog file.

Any characters that aren't able to be transformed from a byte bytes to str string or vice versa will be replaced with a replacement character (ie: u'�' in unicode based encodings, '?' in other ASCII compatible encodings).

SEE ALSO:

gettext.NullTranslations: For information about what methods are available and what they do.

Changed in version kitchen-1.1.0: ; API kitchen.i18n 2.1.0 * Although we had adapted gettext(), ngettext(),
lgettext(), and lngettext() to always return byte
bytes, we hadn't forced those byte bytes to always be
in a specified charset. We now make sure that gettext() and
ngettext() return byte bytes encoded using
output_charset if set, otherwise charset and if
neither of those, UTF-8. With lgettext() and
lngettext() output_charset if set, otherwise
locale.getpreferredencoding(). * Make setting input_charset and output_charset also
set those attributes on any fallback translation objects.

Changed in version kitchen-1.2.0: ; API kitchen.i18n 2.2.0 Add python2_api parameter to __init__()

output_charset(): Compatibility for python2.3 which doesn't have output_charset

set_output_charset(charset)

Set the output charset

This serves two purposes. The normal gettext.NullTranslations.set_output_charset() does not set the output on fallback objects. On python-2.3, gettext.NullTranslations objects don't contain this method.

class kitchen.i18n.NewGNUTranslations(fp=None, python2_api=True)

Safer version of gettext.GNUTranslations

gettext.GNUTranslations suffers from two problems that this class fixes.

1.: gettext.GNUTranslations can throw a UnicodeError in gettext.GNUTranslations.ugettext() if the message being translated has non-ASCII characters and there is no translation for it.

2.: gettext.GNUTranslations can return byte bytes from gettext.GNUTranslations.ugettext() and str strings from the other gettext() methods if the message being translated is the wrong type

When byte bytes are returned, the strings will be encoded according to this algorithm:

1.: If a fallback has been added, the fallback will be called first. You'll need to consult the fallback to see whether it performs any encoding changes.

2.: If a byte bytes was given, the same byte bytes will be returned.

3.: If a str string was given and set_output_charset() has been called then we encode the string using the output_charset

4.: If a str string was given and this is gettext() or ngettext() and a charset was detected when parsing the message catalog, output in that charset.

5.: If a str string was given and this is gettext() or ngettext() we encode it using UTF-8.

6.: If a str string was given and this is lgettext() or lngettext() we encode using the value of locale.getpreferredencoding()

For ugettext() and ungettext(), we go through the same set of steps with the following differences:

•: We transform byte bytes into str strings for these methods.

•: The encoding used to decode the byte bytes is taken from input_charset if it's set, otherwise we decode using UTF-8

input_charset: an extension to the python standard library gettext that specifies what charset a message is encoded in when decoding a message to str. This is used for two purposes:

1.: If the message string is a byte bytes, this is used to decode the string to a str string before looking it up in the message catalog.

2.: In ugettext() and ungettext() methods, if a byte bytes is given as the message and is untranslated his is used as the encoding when decoding to str. This is different from the _charset parameter that may be set when a message catalog is loaded because input_charset is used to describe an encoding used in a python source file while _charset describes the encoding used in the message catalog file.

Any characters that aren't able to be transformed from a byte bytes to str string or vice versa will be replaced with a replacement character (ie: u'�' in unicode based encodings, '?' in other ASCII compatible encodings).

SEE ALSO:

gettext.GNUTranslations.gettext: For information about what methods this class has and what they do

Changed in version kitchen-1.1.0: ; API kitchen.i18n 2.1.0 Although we had adapted gettext(), ngettext(), lgettext(), and lngettext() to always return byte bytes, we hadn't forced those byte bytes to always be in a specified charset. We now make sure that gettext() and ngettext() return byte bytes encoded using output_charset if set, otherwise charset and if neither of those, UTF-8. With lgettext() and lngettext() output_charset if set, otherwise locale.getpreferredencoding().

Kitchen.text: unicode and utf8 and xml oh my!

The kitchen.text module contains functions that deal with text manipulation.

Kitchen.text.converters

Functions to handle conversion of byte bytes and str strings.

Changed in version kitchen: 0.2a2 ; API kitchen.text 2.0.0 Added getwriter()

Changed in version kitchen: 0.2.2 ; API kitchen.text 2.1.0 Added exception_to_unicode(), exception_to_bytes(), EXCEPTION_CONVERTERS, and BYTE_EXCEPTION_CONVERTERS

Changed in version kitchen: 1.0.1 ; API kitchen.text 2.1.1 Deprecated BYTE_EXCEPTION_CONVERTERS as we've simplified exception_to_unicode() and exception_to_bytes() to make it unnecessary

Byte Strings and Unicode in Python2

Python2 has two string types, str and unicode. unicode represents an abstract sequence of text characters. It can hold any character that is present in the unicode standard. str can hold any byte of data. The operating system and python work together to display these bytes as characters in many cases but you should always keep in mind that the information is really a sequence of bytes, not a sequence of characters. In python2 these types are interchangeable a large amount of the time. They are one of the few pairs of types that automatically convert when used in equality:

>>> # string is converted to unicode and then compared
>>> "I am a string" == u"I am a string"
True
>>> # Other types, like int, don't have this special treatment
>>> 5 == "5"
False

However, this automatic conversion tends to lull people into a false sense of security. As long as you're dealing with ASCII characters the automatic conversion will save you from seeing any differences. Once you start using characters that are not in ASCII, you will start getting UnicodeError and UnicodeWarning as the automatic conversions between the types fail:

>>> "I am an ñ" == u"I am an ñ"
__main__:1: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
False

Why do these conversions fail? The reason is that the python2 unicode type represents an abstract sequence of unicode text known as code points. str, on the other hand, really represents a sequence of bytes. Those bytes are converted by your operating system to appear as characters on your screen using a particular encoding (usually with a default defined by the operating system and customizable by the individual user.) Although ASCII characters are fairly standard in what bytes represent each character, the bytes outside of the ASCII range are not. In general, each encoding will map a different character to a particular byte. Newer encodings map individual characters to multiple bytes (which the older encodings will instead treat as multiple characters). In the face of these differences, python refuses to guess at an encoding and instead issues a warning or exception and refuses to convert.

SEE ALSO:

Overcoming frustration: Correctly using unicode in python2: For a longer introduction on this subject.

Strategy for Explicit Conversion

So what is the best method of dealing with this weltering babble of incoherent encodings? The basic strategy is to explicitly turn everything into unicode when it first enters your program. Then, when you send it to output, you can transform the unicode back into bytes. Doing this allows you to control the encodings that are used and avoid getting tracebacks due to UnicodeError. Using the functions defined in this module, that looks something like this:

>>> from kitchen.text.converters import to_unicode, to_bytes
>>> name = raw_input('Enter your name: ')
Enter your name: Toshio くらとみ
>>> name
'Toshio \xe3\x81\x8f\xe3\x82\x89\xe3\x81\xa8\xe3\x81\xbf'
>>> type(name)
<type 'str'>
>>> unicode_name = to_unicode(name)
>>> type(unicode_name)
<type 'unicode'>
>>> unicode_name
u'Toshio \u304f\u3089\u3068\u307f'
>>> # Do a lot of other things before needing to save/output again:
>>> output = open('datafile', 'w')
>>> output.write(to_bytes(u'Name: %s\\n' % unicode_name))

A few notes:

Looking at line 6, you'll notice that the input we took from the user was a byte str. In general, anytime we're getting a value from outside of python (The filesystem, reading data from the network, interacting with an external command, reading values from the environment) we are interacting with something that will want to give us a byte str. Some python standard library modules and third party libraries will automatically attempt to convert a byte str to unicode strings for you. This is both a boon and a curse. If the library can guess correctly about the encoding that the data is in, it will return unicode objects to you without you having to convert. However, if it can't guess correctly, you may end up with one of several problems:

UnicodeError: The library attempted to decode a byte str into a unicode, string failed, and raises an exception.

Garbled data: If the library returns the data after decoding it with the wrong encoding, the characters you see in the unicode string won't be the ones that you expect.

A byte str instead of unicode string: Some libraries will return a unicode string when they're able to decode the data and a byte str when they can't. This is generally the hardest problem to debug when it occurs. Avoid it in your own code and try to avoid or open bugs against upstreams that do this. See Designing Unicode Aware APIs for strategies to do this properly.

On line 8, we convert from a byte str to a unicode string. to_unicode() does this for us. It has some error handling and sane defaults that make this a nicer function to use than calling str.decode() directly:

•: Instead of defaulting to the ASCII encoding which fails with all but the simple American English characters, it defaults to UTF-8.

•: Instead of raising an error if it cannot decode a value, it will replace the value with the unicode "Replacement character" symbol (�).

•: If you happen to call this method with something that is not a str or unicode, it will return an empty unicode string.

All three of these can be overridden using different keyword arguments to the function. See the to_unicode() documentation for more information.

On line 15 we push the data back out to a file. Two things you should note here:

1.: We deal with the strings as unicode until the last instant. The string format that we're using is unicode and the variable also holds unicode. People sometimes get into trouble when they mix a byte str format with a variable that holds a unicode string (or vice versa) at this stage.

2.: to_bytes(), does the reverse of to_unicode(). In this case, we're using the default values which turn unicode into a byte str using UTF-8. Any errors are replaced with a � and sending nonstring objects yield empty unicode strings. Just like to_unicode(), you can look at the documentation for to_bytes() to find out how to override any of these defaults.

When to use an alternate strategy

The default strategy of decoding to unicode strings when you take data in and encoding to a byte str when you send the data back out works great for most problems but there are a few times when you shouldn't:

•: The values aren't meant to be read as text

•: The values need to be byte-for-byte when you send them back out -- for instance if they are database keys or filenames.

•: You are transferring the data between several libraries that all expect byte str.

In each of these instances, there is a reason to keep around the byte str version of a value. Here's a few hints to keep your sanity in these situations:

1.: Keep your unicode and str values separate. Just like the pain caused when you have to use someone else's library that returns both unicode and str you can cause yourself pain if you have functions that can return both types or variables that could hold either type of value.

2.: Name your variables so that you can tell whether you're storing byte str or unicode string. One of the first things you end up having to do when debugging is determine what type of string you have in a variable and what type of string you are expecting. Naming your variables consistently so that you can tell which type they are supposed to hold will save you from at least one of those steps.

3.

When you get values initially, make sure that you're dealing with the type of value that you expect as you save it. You can use isinstance() or to_bytes() since to_bytes() doesn't do any modifications of the string if it's already a str. When using to_bytes() for this purpose you might want to use:

try:
    b_input = to_bytes(input_should_be_bytes_already, errors='strict', nonstring='strict')
except:
    handle_errors_somehow()

The reason is that the default of to_bytes() will take characters that are illegal in the chosen encoding and transform them to replacement characters. Since the point of keeping this data as a byte str is to keep the exact same bytes when you send it outside of your code, changing things to replacement characters should be rasing red flags that something is wrong. Setting errors to strict will raise an exception which gives you an opportunity to fail gracefully.

4.: Sometimes you will want to print out the values that you have in your byte str. When you do this you will need to make sure that you transform unicode to str before combining them. Also be sure that any other function calls (including gettext) are going to give you strings that are the same type. For instance:

print to_bytes(_('Username: %(user)s'), 'utf-8') % {'user': b_username}

Gotchas and how to avoid them

Even when you have a good conceptual understanding of how python2 treats unicode and str there are still some things that can surprise you. In most cases this is because, as noted earlier, python or one of the python libraries you depend on is trying to convert a value automatically and failing. Explicit conversion at the appropriate place usually solves that.

str(obj)

One common idiom for getting a simple, string representation of an object is to use:

str(obj)

Unfortunately, this is not safe. Sometimes str(obj) will return unicode. Sometimes it will return a byte str. Sometimes, it will attempt to convert from a unicode string to a byte str, fail, and throw a UnicodeError. To be safe from all of these, first decide whether you need unicode or str to be returned. Then use to_unicode() or to_bytes() to get the simple representation like this:

u_representation = to_unicode(obj, nonstring='simplerepr')
b_representation = to_bytes(obj, nonstring='simplerepr')

print

python has a builtin print() statement that outputs strings to the terminal. This originated in a time when python only dealt with byte str. When unicode strings came about, some enhancements were made to the print() statement so that it could print those as well. The enhancements make print() work most of the time. However, the times when it doesn't work tend to make for cryptic debugging.

The basic issue is that print() has to figure out what encoding to use when it prints a unicode string to the terminal. When python is attached to your terminal (ie, you're running the interpreter or running a script that prints to the screen) python is able to take the encoding value from your locale settings LC_ALL or LC_CTYPE and print the characters allowed by that encoding. On most modern Unix systems, the encoding is utf-8 which means that you can print any unicode character without problem.

There are two common cases of things going wrong:

1.

Someone has a locale set that does not accept all valid unicode characters. For instance:

$ LC_ALL=C python
>>> print u'\ufffd'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\ufffd' in position 0: ordinal not in range(128)

This often happens when a script that you've written and debugged from the terminal is run from an automated environment like cron. It also occurs when you have written a script using a utf-8 aware locale and released it for consumption by people all over the internet. Inevitably, someone is running with a locale that can't handle all unicode characters and you get a traceback reported.

2.

You redirect output to a file. Python isn't using the values in LC_ALL unconditionally to decide what encoding to use. Instead it is using the encoding set for the terminal you are printing to which is set to accept different encodings by LC_ALL. If you redirect to a file, you are no longer printing to the terminal so LC_ALL won't have any effect. At this point, python will decide it can't find an encoding and fallback to ASCII which will likely lead to UnicodeError being raised. You can see this in a short script:

#! /usr/bin/python -tt
print u'\ufffd'

And then look at the difference between running it normally and redirecting to a file:

$ ./test.py
�
$ ./test.py > t
Traceback (most recent call last):
  File "test.py", line 3, in <module>
      print u'\ufffd'
UnicodeEncodeError: 'ascii' codec can't encode character u'\ufffd' in position 0: ordinal not in range(128)

The short answer to dealing with this is to always use bytes when writing output. You can do this by explicitly converting to bytes like this:

from kitchen.text.converters import to_bytes
u_string = u'\ufffd'
print to_bytes(u_string)

or you can wrap stdout and stderr with a StreamWriter. A StreamWriter is convenient in that you can assign it to encode for sys.stdout or sys.stderr and then have output automatically converted but it has the drawback of still being able to throw UnicodeError if the writer can't encode all possible unicode codepoints. Kitchen provides an alternate version which can be retrieved with kitchen.text.converters.getwriter() which will not traceback in its standard configuration.

Unicode, str, and dict keys

The hash() of the ASCII characters is the same for unicode and byte str. When you use them in dict keys, they evaluate to the same dictionary slot:

>>> u_string = u'a'
>>> b_string = 'a'
>>> hash(u_string), hash(b_string)
(12416037344, 12416037344)
>>> d = {}
>>> d[u_string] = 'unicode'
>>> d[b_string] = 'bytes'
>>> d
{u'a': 'bytes'}

When you deal with key values outside of ASCII, unicode and byte str evaluate unequally no matter what their character content or hash value:

>>> u_string = u'ñ'
>>> b_string = u_string.encode('utf-8')
>>> print u_string
ñ
>>> print b_string
ñ
>>> d = {}
>>> d[u_string] = 'unicode'
>>> d[b_string] = 'bytes'
>>> d
{u'\\xf1': 'unicode', '\\xc3\\xb1': 'bytes'}
>>> b_string2 = '\\xf1'
>>> hash(u_string), hash(b_string2)
(30848092528, 30848092528)
>>> d = {}
>>> d[u_string] = 'unicode'
>>> d[b_string2] = 'bytes'
{u'\\xf1': 'unicode', '\\xf1': 'bytes'}

How do you work with this one? Remember rule #1: Keep your unicode and byte str values separate. That goes for keys in a dictionary just like anything else.

•: For any given dictionary, make sure that all your keys are either unicode or str. Do not mix the two. If you're being given both unicode and str but you don't need to preserve separate keys for each, I recommend using to_unicode() or to_bytes() to convert all keys to one type or the other like this:

>>> from kitchen.text.converters import to_unicode >>> u_string = u'one' >>> b_string = 'two' >>> d = {} >>> d[to_unicode(u_string)] = 1 >>> d[to_unicode(b_string)] = 2 >>> d {u'two': 2, u'one': 1}

•: These issues also apply to using dicts with tuple keys that contain a mixture of unicode and str. Once again the best fix is to standardise on either str or unicode.

•: If you absolutely need to store values in a dictionary where the keys could be either unicode or str you can use StrictDict which has separate entries for all unicode and byte str and deals correctly with any tuple containing mixed unicode and byte str.

Functions

Unicode and byte str conversion

kitchen.text.converters.to_unicode(obj, encoding='utf-8', errors='replace', nonstring=None, non_string=None)

Convert an object into a str string

Parameters

•: obj -- Object to convert to a str string. This should normally be a byte bytes

•: encoding -- What encoding to try converting the byte bytes as. Defaults to utf-8

•: errors -- If errors are found while decoding, perform this action. Defaults to replace which replaces the invalid bytes with a character that means the bytes were unable to be decoded. Other values are the same as the error handling schemes in the codec base classes. For instance strict which raises an exception and ignore which simply omits the non-decodable characters.

•

nonstring --

How to treat nonstring values. Possible values are:

simplerepr: Attempt to call the object's "simple representation" method and return that value. Python-2.3+ has two methods that try to return a simple representation: object.__unicode__() and object.__str__(). We first try to get a usable value from object.__unicode__(). If that fails we try the same with object.__str__().

empty: Return an empty str string

strict: Raise a TypeError

passthru: Return the object unchanged

repr: Attempt to return a str string of the repr of the object

Default is simplerepr

•: non_string -- Deprecated Use nonstring instead

Raises

•: TypeError -- if nonstring is strict and a non-basestring object is passed in or if nonstring is set to an unknown value

•: UnicodeDecodeError -- if errors is strict and obj is not decodable using the given encoding

Returns: str string or the original object depending on the value of nonstring.

Usually this should be used on a byte bytes but it can take both byte bytes and str strings intelligently. Nonstring objects are handled in different ways depending on the setting of the nonstring parameter.

The default values of this function are set so as to always return a str string and never raise an error when converting from a byte bytes to a str string. However, when you do not pass validly encoded text (or a nonstring object), you may end up with output that you don't expect. Be sure you understand the requirements of your data, not just ignore errors by passing it through this function.

Changed in version 0.2.1a2: Deprecated non_string in favor of nonstring parameter and changed default value to simplerepr

kitchen.text.converters.to_bytes(obj, encoding='utf-8', errors='replace', nonstring=None, non_string=None)

Convert an object into a byte bytes

Parameters

•: obj -- Object to convert to a byte bytes. This should normally be a str string.

•: encoding -- Encoding to use to convert the str string into a byte bytes. Defaults to utf-8.

•

errors --

If errors are found while encoding, perform this action. Defaults to replace which replaces the invalid bytes with a character that means the bytes were unable to be encoded. Other values are the same as the error handling schemes in the codec base classes. For instance strict which raises an exception and ignore which simply omits the non-encodable characters.

•

nonstring --

How to treat nonstring values. Possible values are:

simplerepr: Attempt to call the object's "simple representation" method and return that value. Python-2.3+ has two methods that try to return a simple representation: object.__unicode__() and object.__str__(). We first try to get a usable value from object.__str__(). If that fails we try the same with object.__unicode__().

empty: Return an empty byte bytes

strict: Raise a TypeError

passthru: Return the object unchanged

repr: Attempt to return a byte bytes of the repr() of the object

Default is simplerepr.

•: non_string -- Deprecated Use nonstring instead.

Raises

•: TypeError -- if nonstring is strict and a non-basestring object is passed in or if nonstring is set to an unknown value.

•: UnicodeEncodeError -- if errors is strict and all of the bytes of obj are unable to be encoded using encoding.

Returns: byte bytes or the original object depending on the value of nonstring.

WARNING:

If you pass a byte bytes into this function the byte bytes is returned unmodified. It is not re-encoded with the specified encoding. The easiest way to achieve that is:

to_bytes(to_unicode(text), encoding='utf-8')

The initial to_unicode() call will ensure text is a str string. Then, to_bytes() will turn that into a byte bytes with the specified encoding.

Usually, this should be used on a str string but it can take either a byte bytes or a str string intelligently. Nonstring objects are handled in different ways depending on the setting of the nonstring parameter.

The default values of this function are set so as to always return a byte bytes and never raise an error when converting from unicode to bytes. However, when you do not pass an encoding that can validly encode the object (or a non-string object), you may end up with output that you don't expect. Be sure you understand the requirements of your data, not just ignore errors by passing it through this function.

Changed in version 0.2.1a2: Deprecated non_string in favor of nonstring parameter and changed default value to simplerepr

kitchen.text.converters.getwriter(encoding)

Return a codecs.StreamWriter that resists tracing back.

Parameters: encoding -- Encoding to use for transforming str strings into byte bytes.

Return type: codecs.StreamWriter

Returns: StreamWriter that you can instantiate to wrap output streams to automatically translate str strings into encoding.

This is a reimplemetation of codecs.getwriter() that returns a StreamWriter that resists issuing tracebacks. The StreamWriter that is returned uses kitchen.text.converters.to_bytes() to convert str strings into byte bytes. The departures from codecs.getwriter() are:

1.: The StreamWriter that is returned will take byte bytes as well as str strings. Any byte bytes will be passed through unmodified.

2.: The default error handler for unknown bytes is to replace the bytes with the unknown character (? in most ascii-based encodings, � in the utf encodings) whereas codecs.getwriter() defaults to strict. Like codecs.StreamWriter, the returned StreamWriter can have its error handler changed in code by setting stream.errors = 'new_handler_name'

Example usage:

$ LC_ALL=C python
>>> import sys
>>> from kitchen.text.converters import getwriter
>>> UTF8Writer = getwriter('utf-8')
>>> unwrapped_stdout = sys.stdout
>>> sys.stdout = UTF8Writer(unwrapped_stdout)
>>> print 'caf\xc3\xa9'
café
>>> print u'caf\xe9'
café
>>> ASCIIWriter = getwriter('ascii')
>>> sys.stdout = ASCIIWriter(unwrapped_stdout)
>>> print 'caf\xc3\xa9'
café
>>> print u'caf\xe9'
caf?

SEE ALSO:

API docs for codecs.StreamWriter and codecs.getwriter() and Print Fails on the python wiki.

Added in version kitchen: 0.2a2, API: kitchen.text 1.1.0

kitchen.text.converters.to_str(obj)

Deprecated

This function converts something to a byte bytes if it isn't one. It's used to call str() or unicode() on the object to get its simple representation without danger of getting a UnicodeError. You should be using to_unicode() or to_bytes() explicitly instead.

If you need str strings:

to_unicode(obj, nonstring='simplerepr')

If you need byte bytes:

to_bytes(obj, nonstring='simplerepr')

kitchen.text.converters.to_utf8(obj, errors='replace', non_string='passthru')

Deprecated

Convert str to an encoded utf-8 byte bytes. You should be using to_bytes() instead:

to_bytes(obj, encoding='utf-8', non_string='passthru')

Transformation to XML

kitchen.text.converters.unicode_to_xml(string, encoding='utf-8', attrib=False, control_chars='replace')

Take a str string and turn it into a byte bytes suitable for xml

Parameters

•: string -- str string to encode into an XML compatible byte bytes

•: encoding -- encoding to use for the returned byte bytes. Default is to encode to UTF-8. If some of the characters in string are not encodable in this encoding, the unknown characters will be entered into the output string using xml character references.

•: attrib -- If True, quote the string for use in an xml attribute. If False (default), quote for use in an xml text field.

•

control_chars --

control characters are not allowed in XML documents. When we encounter those we need to know what to do. Valid options are:

replace: (default) Replace the control characters with ?

ignore: Remove the characters altogether from the output

strict: Raise an XmlEncodeError when we encounter a control character

Raises

•: kitchen.text.exceptions.XmlEncodeError -- If control_chars is set to strict and the string to be made suitable for output to xml contains control characters or if string is not a str string then we raise this exception.

•: ValueError -- If control_chars is set to something other than replace, ignore, or strict.

Return type: byte bytes

Returns: representation of the str string as a valid XML byte bytes

XML files consist mainly of text encoded using a particular charset. XML also denies the use of certain bytes in the encoded text (example: ASCII Null). There are also special characters that must be escaped if they are present in the input (example: <). This function takes care of all of those issues for you.

There are a few different ways to use this function depending on your needs. The simplest invocation is like this:

unicode_to_xml(u'String with non-ASCII characters: <"á と">')

This will return the following to you, encoded in utf-8:

'String with non-ASCII characters: &lt;"á と"&gt;'

Pretty straightforward. Now, what if you need to encode your document in something other than utf-8? For instance, latin-1? Let's see:

unicode_to_xml(u'String with non-ASCII characters: <"á と">', encoding='latin-1')
'String with non-ASCII characters: &lt;"á &#12392;"&gt;'

Because the と character is not available in the latin-1 charset, it is replaced with と in our output. This is an xml character reference which represents the character at unicode codepoint 12392, the と character.

When you want to reverse this, use xml_to_unicode() which will turn a byte bytes into a str string and replace the xml character references with the unicode characters.

XML also has the quirk of not allowing control characters in its output. The control_chars parameter allows us to specify what to do with those. For use cases that don't need absolute character by character fidelity (example: holding strings that will just be used for display in a GUI app later), the default value of replace works well:

unicode_to_xml(u'String with disallowed control chars: \u0000\u0007')
'String with disallowed control chars: ??'

If you do need to be able to reproduce all of the characters at a later date (examples: if the string is a key value in a database or a path on a filesystem) you have many choices. Here are a few that rely on utf-7, a verbose encoding that encodes control characters (as well as non-ASCII unicode values) to characters from within the ASCII printable characters. The good thing about doing this is that the code is pretty simple. You just need to use utf-7 both when encoding the field for xml and when decoding it for use in your python program:

unicode_to_xml(u'String with unicode: と and control char: ', encoding='utf7')
'String with unicode: +MGg and control char: +AAc-'
# [...]
xml_to_unicode('String with unicode: +MGg and control char: +AAc-', encoding='utf7')
u'String with unicode: と and control char: '

As you can see, the utf-7 encoding will transform even characters that would be representable in utf-8. This can be a drawback if you want unicode characters in the file to be readable without being decoded first. You can work around this with increased complexity in your application code:

encoding = 'utf-8'
u_string = u'String with unicode: と and control char: '
try:
    # First attempt to encode to utf8
    data = unicode_to_xml(u_string, encoding=encoding, errors='strict')
except XmlEncodeError:
    # Fallback to utf-7
    encoding = 'utf-7'
    data = unicode_to_xml(u_string, encoding=encoding, errors='strict')
write_tag('<mytag encoding=%s>%s</mytag>' % (encoding, data))
# [...]
encoding = tag.attributes.encoding
u_string = xml_to_unicode(u_string, encoding=encoding)

Using code similar to that, you can have some fields encoded using your default encoding and fallback to utf-7 if there are control characters present.

NOTE:

If your goal is to preserve the control characters you cannot save the entire file as utf-7 and set the xml encoding parameter to utf-7 if your goal is to preserve the control characters. Because XML doesn't allow control characters, you have to encode those separate from any encoding work that the XML parser itself knows about.

SEE ALSO:

bytes_to_xml(): if you're dealing with bytes that are non-text or of an unknown encoding that you must preserve on a byte for byte level.

guess_encoding_to_xml(): if you're dealing with strings in unknown encodings that you don't need to save with char-for-char fidelity.

kitchen.text.converters.xml_to_unicode(byte_string, encoding='utf-8', errors='replace')

Transform a byte bytes from an xml file into a str string

Parameters

•: byte_string -- byte bytes to decode

•: encoding -- encoding that the byte bytes is in

•: errors -- What to do if not every character is valid in encoding. See the to_unicode() documentation for legal values.

Return type: str string

Returns: string decoded from byte_string

This function attempts to reverse what unicode_to_xml() does. It takes a byte bytes (presumably read in from an xml file) and expands all the html entities into unicode characters and decodes the byte bytes into a str string. One thing it cannot do is restore any control characters that were removed prior to inserting into the file. If you need to keep such characters you need to use xml_to_bytes() and bytes_to_xml() or use on of the strategies documented in unicode_to_xml() instead.

kitchen.text.converters.byte_string_to_xml(byte_string, input_encoding='utf-8', errors='replace', output_encoding='utf-8', attrib=False, control_chars='replace')

Make sure a byte bytes is validly encoded for xml output

Parameters

•: byte_string -- Byte bytes to turn into valid xml output

•: input_encoding -- Encoding of byte_string. Default utf-8

•

errors --

How to handle errors encountered while decoding the byte_string into str at the beginning of the process. Values are:

replace: (default) Replace the invalid bytes with a ?

ignore: Remove the characters altogether from the output

strict: Raise an UnicodeDecodeError when we encounter a non-decodable character

•: output_encoding -- Encoding for the xml file that this string will go into. Default is utf-8. If all the characters in byte_string are not encodable in this encoding, the unknown characters will be entered into the output string using xml character references.

•: attrib -- If True, quote the string for use in an xml attribute. If False (default), quote for use in an xml text field.

•

control_chars --

XML does not allow control characters. When we encounter those we need to know what to do. Valid options are:

replace: (default) Replace the control characters with ?

ignore: Remove the characters altogether from the output

strict: Raise an error when we encounter a control character

Raises

•: XmlEncodeError -- If control_chars is set to strict and the string to be made suitable for output to xml contains control characters then we raise this exception.

•: UnicodeDecodeError -- If errors is set to strict and the byte_string contains bytes that are not decodable using input_encoding, this error is raised

Return type: byte bytes

Returns: representation of the byte bytes in the output encoding with any bytes that aren't available in xml taken care of.

Use this when you have a byte bytes representing text that you need to make suitable for output to xml. There are several cases where this is the case. For instance, if you need to transform some strings encoded in latin-1 to utf-8 for output:

utf8_string = byte_string_to_xml(latin1_string, input_encoding='latin-1')

If you already have strings in the proper encoding you may still want to use this function to remove control characters:

cleaned_string = byte_string_to_xml(string, input_encoding='utf-8', output_encoding='utf-8')

SEE ALSO:

unicode_to_xml(): for other ideas on using this function

kitchen.text.converters.xml_to_byte_string(byte_string, input_encoding='utf-8', errors='replace', output_encoding='utf-8')

Transform a byte bytes from an xml file into str string

Parameters

•: byte_string -- byte bytes to decode

•: input_encoding -- encoding that the byte bytes is in

•: errors -- What to do if not every character is valid in encoding. See the to_unicode() docstring for legal values.

•: output_encoding -- Encoding for the output byte bytes

Returns: str string decoded from byte_string

This function attempts to reverse what unicode_to_xml() does. It takes a byte bytes (presumably read in from an xml file) and expands all the html entities into unicode characters and decodes the byte bytes into a str string. One thing it cannot do is restore any control characters that were removed prior to inserting into the file. If you need to keep such characters you need to use xml_to_bytes() and bytes_to_xml() or use one of the strategies documented in unicode_to_xml() instead.

kitchen.text.converters.bytes_to_xml(byte_string, *args, **kwargs)

Return a byte bytes encoded so it is valid inside of any xml file

Parameters

•: byte_string -- byte bytes to transform

•

**kwargs (*args,) --

extra arguments to this function are passed on to the function actually implementing the encoding. You can use this to tweak the output in some cases but, as a general rule, you shouldn't because the underlying encoding function is not guaranteed to remain the same.

Return type: byte bytes consisting of all ASCII characters

Returns: byte bytes representation of the input. This will be encoded using base64.

This function is made especially to put binary information into xml documents.

This function is intended for encoding things that must be preserved byte-for-byte. If you want to encode a byte string that's text and don't mind losing the actual bytes you probably want to try byte_string_to_xml() or guess_encoding_to_xml() instead.

NOTE:

Although the current implementation uses base64.b64encode() and there's no plans to change it, that isn't guaranteed. If you want to make sure that you can encode and decode these messages it's best to use xml_to_bytes() if you use this function to encode.

kitchen.text.converters.xml_to_bytes(byte_string, *args, **kwargs)

Decode a string encoded using bytes_to_xml()

Parameters

•: byte_string -- byte bytes to transform. This should be a base64 encoded sequence of bytes originally generated by bytes_to_xml().

•

**kwargs (*args,) --

extra arguments to this function are passed on to the function actually implementing the encoding. You can use this to tweak the output in some cases but, as a general rule, you shouldn't because the underlying encoding function is not guaranteed to remain the same.

Return type: byte bytes

Returns: byte bytes that's the decoded input

If you've got fields in an xml document that were encoded with bytes_to_xml() then you want to use this function to undecode them. It converts a base64 encoded string into a byte bytes.

NOTE:

Although the current implementation uses base64.b64decode() and there's no plans to change it, that isn't guaranteed. If you want to make sure that you can encode and decode these messages it's best to use bytes_to_xml() if you use this function to decode.

kitchen.text.converters.guess_encoding_to_xml(string, output_encoding='utf-8', attrib=False, control_chars='replace')

Return a byte bytes suitable for inclusion in xml

Parameters

•: string -- str or byte bytes to be transformed into a byte bytes suitable for inclusion in xml. If string is a byte bytes we attempt to guess the encoding. If we cannot guess, we fallback to latin-1.

•: output_encoding -- Output encoding for the byte bytes. This should match the encoding of your xml file.

•: attrib -- If True, escape the item for use in an xml attribute. If False (default) escape the item for use in a text node.

Returns: utf-8 encoded byte bytes

kitchen.text.converters.to_xml(string, encoding='utf-8', attrib=False, control_chars='ignore'): Deprecated: Use guess_encoding_to_xml() instead

Working with exception messages

kitchen.text.converters.EXCEPTION_CONVERTERS = (<function <lambda>>, <function <lambda>>)

Tuple of functions to try to use to convert an exception into a string

representation. Its main use is to extract a string (str or bytes) from an exception object in exception_to_unicode() and exception_to_bytes(). The functions here will try the exception's args[0] and the exception itself (roughly equivalent to str(exception)) to extract the message. This is only a default and can be easily overridden when calling those functions. There are several reasons you might wish to do that. If you have exceptions where the best string representing the exception is not returned by the default functions, you can add another function to extract from a different field:

from kitchen.text.converters import (EXCEPTION_CONVERTERS,
        exception_to_unicode)
class MyError(Exception):
    def __init__(self, message):
        self.value = message
c = [lambda e: e.value]
c.extend(EXCEPTION_CONVERTERS)
try:
    raise MyError('An Exception message')
except MyError, e:
    print exception_to_unicode(e, converters=c)

Another reason would be if you're converting to a byte bytes and you know the bytes needs to be a non-utf-8 encoding. exception_to_bytes() defaults to utf-8 but if you convert into a byte bytes explicitly using a converter then you can choose a different encoding:

from kitchen.text.converters import (EXCEPTION_CONVERTERS,
        exception_to_bytes, to_bytes)
c = [lambda e: to_bytes(e.args[0], encoding='euc_jp'),
        lambda e: to_bytes(e, encoding='euc_jp')]
c.extend(EXCEPTION_CONVERTERS)
try:
    do_something()
except Exception, e:
    log = open('logfile.euc_jp', 'a')
    log.write('%s

' % exception_to_bytes(e, converters=c)

log.close()

Each function in this list should take the exception as its sole argument and return a string containing the message representing the exception. The functions may return the message as a :byte class:bytes, a str string, or even an object if you trust the object to return a decent string representation. The exception_to_unicode() and exception_to_bytes() functions will make sure to convert the string to the proper type before returning.

Added in version 0.2.2.

kitchen.text.converters.BYTE_EXCEPTION_CONVERTERS = (<function <lambda>>, <function to_bytes>)

Deprecated: Use EXCEPTION_CONVERTERS instead.

Tuple of functions to try to use to convert an exception into a string representation. This tuple is similar to the one in EXCEPTION_CONVERTERS but it's used with exception_to_bytes() instead. Ideally, these functions should do their best to return the data as a byte bytes but the results will be run through to_bytes() before being returned.

Added in version 0.2.2.

Changed in version 1.0.1: Deprecated as simplifications allow EXCEPTION_CONVERTERS to perform the same function.

kitchen.text.converters.exception_to_unicode(exc, converters=(<function <lambda>>, <function <lambda>>))

Convert an exception object into a unicode representation

Parameters

•: exc -- Exception object to convert

•: converters -- List of functions to use to convert the exception into a string. See EXCEPTION_CONVERTERS for the default value and an example of adding other converters to the defaults. The functions in the list are tried one at a time to see if they can extract a string from the exception. The first one to do so without raising an exception is used.

Returns: str string representation of the exception. The value extracted by the converters will be converted into str before being returned using the utf-8 encoding. If you know you need to use an alternate encoding add a function that does that to the list of functions in converters)

Added in version 0.2.2.

kitchen.text.converters.exception_to_bytes(exc, converters=(<function <lambda>>, <function <lambda>>))

Convert an exception object into a str representation

Parameters

•: exc -- Exception object to convert

•: converters -- List of functions to use to convert the exception into a string. See EXCEPTION_CONVERTERS for the default value and an example of adding other converters to the defaults. The functions in the list are tried one at a time to see if they can extract a string from the exception. The first one to do so without raising an exception is used.

Returns: byte bytes representation of the exception. The value extracted by the converters will be converted into bytes before being returned using the utf-8 encoding. If you know you need to use an alternate encoding add a function that does that to the list of functions in converters)

Added in version 0.2.2.

Changed in version 1.0.1: Code simplification allowed us to switch to using EXCEPTION_CONVERTERS as the default value of converters.

Format Text for Display

Functions related to displaying unicode text. Unicode characters don't all have the same width so we need helper functions for displaying them.

Added in version 0.2: kitchen.display API 1.0.0

kitchen.text.display.textual_width(msg, control_chars='guess', encoding='utf-8', errors='replace')

Get the textual width of a string

Parameters

•: msg -- str string or byte bytes to get the width of

•

control_chars --

specify how to deal with control characters. Possible values are:

guess: (default) will take a guess for control character widths. Most codes will return zero width. backspace, delete, and clear delete return -1. escape currently returns -1 as well but this is not guaranteed as it's not always correct

strict: will raise kitchen.text.exceptions.ControlCharError if a control character is encountered

•: encoding -- If we are given a byte bytes this is used to decode it into str string. Any characters that are not decodable in this encoding will get a value dependent on the errors parameter.

•: errors -- How to treat errors encoding the byte bytes to str string. Legal values are the same as for kitchen.text.converters.to_unicode(). The default value of replace will cause undecodable byte sequences to have a width of one. ignore will have a width of zero.

Raises: ControlCharError -- if msg contains a control character and control_chars is strict.

Returns: Textual width of the msg. This is the amount of space that the string will consume on a monospace display. It's measured in the number of cell positions or columns it will take up on a monospace display. This is not the number of glyphs that are in the string.

NOTE:

This function can be wrong sometimes because Unicode does not specify a strict width value for all of the code points. In particular, we've found that some Tamil characters take up to four character cells but we return a lesser amount.

kitchen.text.display.textual_width_chop(msg, chop, encoding='utf-8', errors='replace')

Given a string, return it chopped to a given textual width

Parameters

•: msg -- str string or byte bytes to chop

•: chop -- Chop msg if it exceeds this textual width

•: encoding -- If we are given a byte bytes, this is used to decode it into a str string. Any characters that are not decodable in this encoding will be assigned a width of one.

•: errors -- How to treat errors encoding the byte bytes to str. Legal values are the same as for kitchen.text.converters.to_unicode()

Return type: str string

Returns: str string of the msg chopped at the given textual width

This is what you want to use instead of %.*s, as it does the "right" thing with regard to UTF-8 sequences, control characters, and characters that take more than one cell position. Eg:

>>> # Wrong: only displays 8 characters because it is operating on bytes
>>> print "%.*s" % (10, 'café ñunru!')
café ñun
>>> # Properly operates on graphemes
>>> '%s' % (textual_width_chop('café ñunru!', 10))
café ñunru
>>> # takes too many columns because the kanji need two cell positions
>>> print '1234567890\n%.*s' % (10, u'一二三四五六七八九十')
1234567890
一二三四五六七八九十
>>> # Properly chops at 10 columns
>>> print '1234567890\n%s' % (textual_width_chop(u'一二三四五六七八九十', 10))
1234567890
一二三四五

kitchen.text.display.textual_width_fill(msg, fill, chop=None, left=True, prefix='', suffix='')

Expand a str string to a specified textual width or chop to same

Parameters

•: msg -- str string to format

•: fill -- pad string until the textual width of the string is this length

•: chop -- before doing anything else, chop the string to this length. Default: Don't chop the string at all

•: left -- If True (default) left justify the string and put the padding on the right. If False, pad on the left side.

•: prefix -- Attach this string before the field we're filling

•: suffix -- Append this string to the end of the field we're filling

Return type: str string

Returns: msg formatted to fill the specified width. If no chop is specified, the string could exceed the fill length when completed. If prefix or suffix are printable characters, the string could be longer than the fill width.

NOTE:

prefix and suffix should be used for "invisible" characters like highlighting, color changing escape codes, etc. The fill characters are appended outside of any prefix or suffix elements. This allows you to only highlight msg inside of the field you're filling.

WARNING:

msg, prefix, and suffix should all be representable as unicode characters. In particular, any escape sequences in prefix and suffix need to be convertible to str. If you need to use byte sequences here rather than unicode characters, use byte_string_textual_width_fill() instead.

This function expands a string to fill a field of a particular textual width. Use it instead of %*.*s, as it does the "right" thing with regard to UTF-8 sequences, control characters, and characters that take more than one cell position in a display. Example usage:

>>> msg = u'一二三四五六七八九十'
>>> # Wrong: This uses 10 characters instead of 10 cells:
>>> u":%-*.*s:" % (10, 10, msg[:9])
:一二三四五六七八九 :
>>> # This uses 10 cells like we really want:
>>> u":%s:" % (textual_width_fill(msg[:9], 10, 10))
:一二三四五:
>>> # Wrong: Right aligned in the field, but too many cells
>>> u"%20.10s" % (msg)
          一二三四五六七八九十
>>> # Correct: Right aligned with proper number of cells
>>> u"%s" % (textual_width_fill(msg, 20, 10, left=False))
          一二三四五
>>> # Wrong: Adding some escape characters to highlight the line but too many cells
>>> u"%s%20.10s%s" % (prefix, msg, suffix)
u'[7m          一二三四五六七八九十[0m'
>>> # Correct highlight of the line
>>> u"%s%s%s" % (prefix, display.textual_width_fill(msg, 20, 10, left=False), suffix)
u'[7m          一二三四五[0m'
>>> # Correct way to not highlight the fill
>>> u"%s" % (display.textual_width_fill(msg, 20, 10, left=False, prefix=prefix, suffix=suffix))
u'          [7m一二三四五[0m'

kitchen.text.display.wrap(text, width=70, initial_indent='', subsequent_indent='', encoding='utf-8', errors='replace')

Works like we want textwrap.wrap() to work,

Parameters

•: text -- str string or byte bytes to wrap

•: width -- textual width at which to wrap. Default: 70

•: initial_indent -- string to use to indent the first line. Default: do not indent.

•: subsequent_indent -- string to use to wrap subsequent lines. Default: do not indent

•: encoding -- Encoding to use if text is a byte bytes

•: errors -- error handler to use if text is a byte bytes and contains some undecodable characters.

Return type: list of str strings

Returns: list of lines that have been text wrapped and indented.

textwrap.wrap() from the python standard library has two drawbacks that this attempts to fix:

1.: It does not handle textual width. It only operates on bytes or characters which are both inadequate (due to multi-byte and double width characters).

2.: It malforms lists and blocks.

kitchen.text.display.fill(text, *args, **kwargs)

Works like we want textwrap.fill() to work

Parameters: text -- str string or byte bytes to process

Returns: str string with each line separated by a newline

SEE ALSO:

kitchen.text.display.wrap(): for other parameters that you can give this command.

This function is a light wrapper around kitchen.text.display.wrap(). Where that function returns a list of lines, this function returns one string with each line separated by a newline.

kitchen.text.display.byte_string_textual_width_fill(msg, fill, chop=None, left=True, prefix='', suffix='', encoding='utf-8', errors='replace')

Expand a byte bytes to a specified textual width or chop to same

Parameters

•: msg -- byte bytes encoded in UTF-8 that we want formatted

•: fill -- pad msg until the textual width is this long

•: chop -- before doing anything else, chop the string to this length. Default: Don't chop the string at all

•: left -- If True (default) left justify the string and put the padding on the right. If False, pad on the left side.

•: prefix -- Attach this byte bytes before the field we're filling

•: suffix -- Append this byte bytes to the end of the field we're filling

Return type: byte bytes

Returns: msg formatted to fill the specified textual width. If no chop is specified, the string could exceed the fill length when completed. If prefix or suffix are printable characters, the string could be longer than fill width.

NOTE:

prefix and suffix should be used for "invisible" characters like highlighting, color changing escape codes, etc. The fill characters are appended outside of any prefix or suffix elements. This allows you to only highlight msg inside of the field you're filling.

SEE ALSO:

textual_width_fill()

For example usage. This function has only two differences.

1.: it takes byte bytes for prefix and suffix so you can pass in arbitrary sequences of bytes, not just unicode characters.

2.: it returns a byte bytes instead of a str string.

Internal Data

There are a few internal functions and variables in this module. Code outside of kitchen shouldn't use them but people coding on kitchen itself may find them useful.

kitchen.text.display._COMBINING = ((768, 879), (1155, 1161), (1425, 1469), (1471, 1471), (1473, 1474), (1476, 1477), (1479, 1479), (1536, 1539), (1552, 1562), (1611, 1631), (1648, 1648), (1750, 1764), (1767, 1768), (1770, 1773), (1807, 1807), (1809, 1809), (1840, 1866), (1958, 1968), (2027, 2035), (2045, 2045), (2070, 2073), (2075, 2083), (2085, 2087), (2089, 2093), (2137, 2139), (2259, 2273), (2275, 2303), (2305, 2306), (2364, 2364), (2369, 2376), (2381, 2381), (2385, 2388), (2402, 2403), (2433, 2433), (2492, 2492), (2497, 2500), (2509, 2509), (2530, 2531), (2558, 2558), (2561, 2562), (2620, 2620), (2625, 2626), (2631, 2632), (2635, 2637), (2672, 2673), (2689, 2690), (2748, 2748), (2753, 2757), (2759, 2760), (2765, 2765), (2786, 2787), (2817, 2817), (2876, 2876), (2879, 2879), (2881, 2883), (2893, 2893), (2902, 2902), (2946, 2946), (3008, 3008), (3021, 3021), (3134, 3136), (3142, 3144), (3146, 3149), (3157, 3158), (3260, 3260), (3263, 3263), (3270, 3270), (3276, 3277), (3298, 3299), (3387, 3388), (3393, 3395), (3405, 3405), (3530, 3530), (3538, 3540), (3542, 3542), (3633, 3633), (3636, 3642), (3655, 3662), (3761, 3761), (3764, 3772), (3784, 3789), (3864, 3865), (3893, 3893), (3895, 3895), (3897, 3897), (3953, 3966), (3968, 3972), (3974, 3975), (3984, 3991), (3993, 4028), (4038, 4038), (4141, 4144), (4146, 4146), (4150, 4151), (4153, 4154), (4184, 4185), (4237, 4237), (4448, 4607), (4957, 4959), (5906, 5908), (5938, 5940), (5970, 5971), (6002, 6003), (6068, 6069), (6071, 6077), (6086, 6086), (6089, 6099), (6109, 6109), (6155, 6157), (6313, 6313), (6432, 6434), (6439, 6440), (6450, 6450), (6457, 6459), (6679, 6680), (6752, 6752), (6773, 6780), (6783, 6783), (6832, 6845), (6912, 6915), (6964, 6964), (6966, 6970), (6972, 6972), (6978, 6978), (6980, 6980), (7019, 7027), (7082, 7083), (7142, 7142), (7154, 7155), (7223, 7223), (7376, 7378), (7380, 7392), (7394, 7400), (7405, 7405), (7412, 7412), (7416, 7417), (7616, 7673), (7675, 7679), (8203, 8207), (8234, 8238), (8288, 8291), (8298, 8303), (8400, 8432), (11503, 11505), (11647, 11647), (11744, 11775), (12330, 12335), (12441, 12442), (42607, 42607), (42612, 42621), (42654, 42655), (42736, 42737), (43014, 43014), (43019, 43019), (43045, 43046), (43204, 43204), (43232, 43249), (43307, 43309), (43347, 43347), (43443, 43443), (43456, 43456), (43696, 43696), (43698, 43700), (43703, 43704), (43710, 43711), (43713, 43713), (43766, 43766), (44013, 44013), (64286, 64286), (65024, 65039), (65056, 65071), (65279, 65279), (65529, 65531), (66045, 66045), (66272, 66272), (66422, 66426), (68097, 68099), (68101, 68102), (68108, 68111), (68152, 68154), (68159, 68159), (68325, 68326), (68900, 68903), (69446, 69456), (69702, 69702), (69759, 69759), (69817, 69818), (69888, 69890), (69939, 69940), (70003, 70003), (70080, 70080), (70090, 70090), (70197, 70198), (70377, 70378), (70459, 70460), (70477, 70477), (70502, 70508), (70512, 70516), (70722, 70722), (70726, 70726), (70750, 70750), (70850, 70851), (71103, 71104), (71231, 71231), (71350, 71351), (71467, 71467), (71737, 71738), (72160, 72160), (72244, 72244), (72263, 72263), (72345, 72345), (72767, 72767), (73026, 73026), (73028, 73029), (73111, 73111), (92912, 92916), (92976, 92982), (113822, 113822), (119141, 119145), (119149, 119170), (119173, 119179), (119210, 119213), (119362, 119364), (122880, 122886), (122888, 122904), (122907, 122913), (122915, 122916), (122918, 122922), (123184, 123190), (123628, 123631), (125136, 125142), (125252, 125258), (917505, 917505), (917536, 917631), (917760, 917999))

Internal table, provided by this module to list code points which combine with other characters and therefore should have no textual width. This is a sorted tuple of non-overlapping intervals. Each interval is a tuple listing a starting code point and ending code point. Every code point between the two end points is a combining character.

SEE ALSO:

_generate_combining_table(): for how this table is generated

This table was last regenerated on python-3.8.0a3 with unicodedata.unidata_version 12.0.0

kitchen.text.display._generate_combining_table()

Combine Markus Kuhn's data with unicodedata to make combining char list

Return type: tuple of tuples

Returns: tuple of intervals of code points that are combining character. Each interval is a 2-tuple of the starting code point and the ending code point for the combining characters.

In normal use, this function serves to tell how we're generating the combining char list. For speed reasons, we use this to generate a static list and just use that later.

Markus Kuhn's list of combining characters is more complete than what's in the python unicodedata library but the python unicodedata is synced against later versions of the unicode database

This is used to generate the _COMBINING table.

kitchen.text.display._print_combining_table()

Print out a new _COMBINING table

This will print a new _COMBINING table in the format used in kitchen/text/display.py. It's useful for updating the _COMBINING table with updated data from a new python as the format won't change from what's already in the file.

kitchen.text.display._interval_bisearch(value, table)

Binary search in an interval table.

Parameters

•: value -- numeric value to search for

•: table -- Ordered list of intervals. This is a list of two-tuples. The elements of the two-tuple define an interval's start and end points.

Returns: If value is found within an interval in the table return True. Otherwise, False

This function checks whether a numeric value is present within a table of intervals. It checks using a binary search algorithm, dividing the list of values in half and checking against the values until it determines whether the value is in the table.

kitchen.text.display._ucp_width(ucs, control_chars='guess')

Get the textual width of a ucs character

Parameters

•: ucs -- integer representing a single unicode code point

•

control_chars --

specify how to deal with control characters. Possible values are:

guess: (default) will take a guess for control character widths. Most codes will return zero width. backspace, delete, and clear delete return -1. escape currently returns -1 as well but this is not guaranteed as it's not always correct

strict: will raise ControlCharError if a control character is encountered

Raises: ControlCharError -- if the code point is a unicode control character and control_chars is set to 'strict'

Returns: textual width of the character.

NOTE:

It's important to remember this is textual width and not the number of characters or bytes.

kitchen.text.display._textual_width_le(width, *args)

Optimize the common case when deciding which textual width is larger

Parameters

•: width -- textual width to compare against.

•: *args -- str strings to check the total textual width of

Returns: True if the total length of args are less than or equal to width. Otherwise False.

We often want to know "does X fit in Y". It takes a while to use textual_width() to calculate this. However, we know that the number of canonically composed str characters is always going to have 1 or 2 for the textual width per character. With this we can take the following shortcuts:

1.: If the number of canonically composed characters is more than width, the true textual width cannot be less than width.

2.: If the number of canonically composed characters * 2 is less than the width then the textual width must be ok.

textual width of a canonically composed str string will always be greater than or equal to the the number of str characters. So we can first check if the number of composed str characters is less than the asked for width. If it is we can return True immediately. If not, then we must do a full textual width lookup.

Miscellaneous functions for manipulating text

Collection of text functions that don't fit in another category.

Changed in version kitchen: 1.2.0, API: kitchen.text 2.2.0 Added isbasestring(), isbytestring(), and isunicodestring() to help tell which string type is which on python2 and python3

kitchen.text.misc.byte_string_valid_encoding(byte_string, encoding='utf-8')

Detect if a byte bytes is valid in a specific encoding

Parameters

•: byte_string -- Byte bytes to test for bytes not valid in this encoding

•: encoding -- encoding to test against. Defaults to UTF-8.

Returns: True if there are no invalid UTF-8 characters. False if an invalid character is detected.

NOTE:

This function checks whether the byte bytes is valid in the specified encoding. It does not detect whether the byte bytes actually was encoded in that encoding. If you want that sort of functionality, you probably want to use guess_encoding() instead.

kitchen.text.misc.byte_string_valid_xml(byte_string, encoding='utf-8')

Check that a byte bytes would be valid in xml

Parameters

•: byte_string -- Byte bytes to check

•: encoding -- Encoding of the xml file. Default: UTF-8

Returns: True if the string is valid. False if it would be invalid in the xml file

In some cases you'll have a whole bunch of byte strings and rather than transforming them to str and back to byte bytes for output to xml, you will just want to make sure they work with the xml file you're constructing. This function will help you do that. Example:

ARRAY_OF_MOSTLY_UTF8_STRINGS = [...]
processed_array = []
for string in ARRAY_OF_MOSTLY_UTF8_STRINGS:
    if byte_string_valid_xml(string, 'utf-8'):
        processed_array.append(string)
    else:
        processed_array.append(guess_bytes_to_xml(string, encoding='utf-8'))
output_xml(processed_array)

kitchen.text.misc.guess_encoding(byte_string, disable_chardet=False)

Try to guess the encoding of a byte bytes

Parameters

•: byte_string -- byte bytes to guess the encoding of

•: disable_chardet -- If this is True, we never attempt to use chardet to guess the encoding. This is useful if you need to have reproducibility whether chardet is installed or not. Default: False.

Raises: TypeError -- if byte_string is not a byte bytes type

Returns: string containing a guess at the encoding of byte_string. This is appropriate to pass as the encoding argument when encoding and decoding unicode strings.

We start by attempting to decode the byte bytes as UTF-8. If this succeeds we tell the world it's UTF-8 text. If it doesn't and chardet is installed on the system and disable_chardet is False this function will use it to try detecting the encoding of byte_string. If it is not installed or chardet cannot determine the encoding with a high enough confidence then we rather arbitrarily claim that it is latin-1. Since latin-1 will encode to every byte, decoding from latin-1 to str will not cause UnicodeErrors although the output might be mangled.

kitchen.text.misc.html_entities_unescape(string)

Substitute unicode characters for HTML entities

Parameters: string -- str string to substitute out html entities

Raises: TypeError -- if something other than a str string is given

Return type: str string

Returns: The plain text without html entities

kitchen.text.misc.isbasestring(obj)

Determine if obj is a byte bytes or str string

In python2 this is eqiuvalent to isinstance(obj, basestring). In python3 it checks whether the object is an instance of str, bytes, or bytearray. This is an aid to porting code that needed to test whether an object was derived from basestring in python2 (commonly used in unicode-bytes conversion functions)

Parameters: obj -- Object to test

Returns: True if the object is a basestring. Otherwise False.

Added in version Kitchen:: 1.2.0, API kitchen.text 2.2.0

kitchen.text.misc.isbytestring(obj)

Determine if obj is a byte bytes

In python2 this is equivalent to isinstance(obj, str). In python3 it checks whether the object is an instance of bytes or bytearray.

Parameters: obj -- Object to test

Returns: True if the object is a byte bytes. Otherwise, False.

Added in version Kitchen:: 1.2.0, API kitchen.text 2.2.0

kitchen.text.misc.isunicodestring(obj)

Determine if obj is a str string

In python2 this is equivalent to isinstance(obj, unicode). In python3 it checks whether the object is an instance of bytes.

Parameters: obj -- Object to test

Returns: True if the object is a str string. Otherwise, False.

Added in version Kitchen:: 1.2.0, API kitchen.text 2.2.0

kitchen.text.misc.process_control_chars(string, strategy='replace')

Look for and transform control characters in a string

Parameters

•: string -- string to search for and transform control characters within

•

strategy --

XML does not allow ASCII control characters. When we encounter those we need to know what to do. Valid options are:

replace: (default) Replace the control characters with "?"

ignore: Remove the characters altogether from the output

strict: Raise a ControlCharError when we encounter a control character

Raises

•: TypeError -- if string is not a unicode string.

•: ValueError -- if the strategy is not one of replace, ignore, or strict.

•: kitchen.text.exceptions.ControlCharError -- if the strategy is strict and a control character is present in the string

Returns: str string with no control characters in it.

Changed in version kitchen: 1.2.0, API: kitchen.text 2.2.0 Strip out the C1 control characters in addition to the C0 control characters.

kitchen.text.misc.str_eq(str1, str2, encoding='utf-8', errors='replace')

Compare two strings, converting to byte bytes if one is str

Parameters

•: str1 -- First string to compare

•: str2 -- Second string to compare

•: encoding -- If we need to convert one string into a byte bytes to compare, the encoding to use. Default is utf-8.

•: errors -- What to do if we encounter errors when encoding the string. See the kitchen.text.converters.to_bytes() documentation for possible values. The default is replace.

This function prevents UnicodeError (python-2.4 or less) and UnicodeWarning (python 2.5 and higher) when we compare a str string to a byte bytes. The errors normally arise because the conversion is done to ASCII. This function lets you convert to utf-8 or another encoding instead.

NOTE:

When we need to convert one of the strings from str in order to compare them we convert the str string into a byte bytes. That means that strings can compare differently if you use different encodings for each.

Note that str1 == str2 is faster than this function if you can accept the following limitations:

•: Limited to python-2.5+ (otherwise a UnicodeDecodeError may be thrown)

•: Will generate a UnicodeWarning if non-ASCII byte bytes is compared to str string.

UTF-8

Functions for operating on byte bytes encoded as UTF-8

NOTE:

In many cases, it is better to convert to str, operate on the strings, then convert back to UTF-8. str type can handle many of these functions itself. For those that it doesn't (removing control characters from length calculations, for instance) the code to do so with a str type is often simpler.

WARNING:

All of the functions in this module are deprecated. Most of them have been replaced with functions that operate on unicode values in kitchen.text.display. kitchen.text.utf8.utf8_valid() has been replaced with a function in kitchen.text.misc.

kitchen.text.utf8.utf8_text_fill(text, *args, **kwargs)

Deprecated Similar to textwrap.fill() but understands utf-8 strings and doesn't screw up lists/blocks/etc.

Use kitchen.text.display.fill() instead.

kitchen.text.utf8.utf8_text_wrap(text, width=70, initial_indent='', subsequent_indent='')

Deprecated Similar to textwrap.wrap() but understands utf-8 data and doesn't screw up lists/blocks/etc

Use kitchen.text.display.wrap() instead

kitchen.text.utf8.utf8_valid(msg)

Deprecated Detect if a string is valid utf-8

Use kitchen.text.misc.byte_string_valid_encoding() instead.

kitchen.text.utf8.utf8_width(msg)

Deprecated Get the textual width of a utf-8 string

Use kitchen.text.display.textual_width() instead.

kitchen.text.utf8.utf8_width_chop(msg, chop=None)

Deprecated Return a string chopped to a given textual width

Use textual_width_chop() and textual_width() instead:

>>> msg = 'く ku ら ra と to み mi'
>>> # Old way:
>>> utf8_width_chop(msg, 5)
(5, 'く ku')
>>> # New way
>>> from kitchen.text.converters import to_bytes
>>> from kitchen.text.display import textual_width, textual_width_chop
>>> (textual_width(msg), to_bytes(textual_width_chop(msg, 5)))
(5, 'く ku')

kitchen.text.utf8.utf8_width_fill(msg, fill, chop=None, left=True, prefix='', suffix='')

Deprecated Pad a utf-8 string to fill a specified width

Use byte_string_textual_width_fill() instead

converters: deals with converting text for different encodings and to and from XML

display: deals with issues with printing text to a screen

misc: is a catchall for text manipulation functions that don't seem to fit elsewhere

utf8: contains deprecated functions to manipulate utf8 byte strings

Kitchen.collections

StrictDict

kitchen.collections.StrictDict provides a dictionary that treats bytes and str as distinct key values.

kitchen.collections.strictdict.StrictDict: alias of defaultdict

Kitchen.iterutils Module

Functions to manipulate iterables

Added in version Kitchen:: 0.2.1a1

Module author: Toshio Kuratomi <[email protected]>

Module author: Luke Macken <[email protected]>

kitchen.iterutils.isiterable(obj, include_string=False)

Check whether an object is an iterable

Parameters

•: obj -- Object to test whether it is an iterable

•: include_string -- If True and obj is a byte bytes or str string this function will return True. If set to False, byte bytes and str strings will cause this function to return False. Default False.

Returns: True if obj is iterable, otherwise False.

kitchen.iterutils.iterate(obj, include_string=False)

Generator that can be used to iterate over anything

Parameters

•: obj -- The object to iterate over

•: include_string -- if True, treat strings as iterables. Otherwise treat them as a single scalar value. Default False

This function will create an iterator out of any scalar or iterable. It is useful for making a value given to you an iterable before operating on it. Iterables have their items returned. scalars are transformed into iterables. A string is treated as a scalar value unless the include_string parameter is set to True. Example usage:

>>> list(iterate(None))
[None]
>>> list(iterate([None]))
[None]
>>> list(iterate([1, 2, 3]))
[1, 2, 3]
>>> list(iterate(set([1, 2, 3])))
[1, 2, 3]
>>> list(iterate(dict(a='1', b='2')))
['a', 'b']
>>> list(iterate(1))
[1]
>>> list(iterate(iter([1, 2, 3])))
[1, 2, 3]
>>> list(iterate('abc'))
['abc']
>>> list(iterate('abc', include_string=True))
['a', 'b', 'c']

Helpers for versioning software

PEP-386 compliant versioning

PEP 386 defines a standard format for version strings. This module contains a function for creating strings in that format.

kitchen.versioning.version_tuple_to_string(version_info)

Return a PEP 386 version string from a PEP 386 style version tuple

Parameters: version_info -- Nested set of tuples that describes the version. See below for an example.

Returns: a version string

This function implements just enough of PEP 386 to satisfy our needs. PEP 386 defines a standard format for version strings and refers to a function that will be merged into the python standard library that transforms a tuple of version information into a standard version string. This function is an implementation of that function. Once that function becomes available in the python standard library we will start using it and deprecate this function.

version_info takes the form that PEP 386's NormalizedVersion.from_parts() uses:

((Major, Minor, [Micros]), [(Alpha/Beta/rc marker, version)],
    [(post/dev marker, version)])
Ex: ((1, 0, 0), ('a', 2), ('dev', 3456))

It generates a PEP 386 compliant version string:

N.N[.N]+[{a|b|c|rc}N[.N]+][.postN][.devN]
Ex: 1.0.0a2.dev3456

WARNING:

This function does next to no error checking. It's up to the person defining the version tuple to make sure that the values make sense. If the PEP 386 compliant version parser doesn't get released soon we'll look at making this function check that the version tuple makes sense before transforming it into a string.

It's recommended that you use this function to keep a __version_info__ tuple and __version__ string in your modules. Why do we need both a tuple and a string? The string is often useful for putting into human readable locations like release announcements, version strings in tarballs, etc. Meanwhile the tuple is very easy for a computer to compare. For example, kitchen sets up its version information like this:

from kitchen.versioning import version_tuple_to_string
__version_info__ = ((0, 2, 1),)
__version__ = version_tuple_to_string(__version_info__)

Other programs that depend on a kitchen version between 0.2.1 and 0.3.0 can find whether the present version is okay with code like this:

from kitchen import __version_info__, __version__
if __version_info__ < ((0, 2, 1),) or __version_info__ >= ((0, 3, 0),):
    print 'kitchen is present but not at the right version.'
    print 'We need at least version 0.2.1 and less than 0.3.0'
    print 'Currently found: kitchen-%s' % __version__

Exceptions

Kitchen has a hierarchy of exceptions that should make it easy to catch many errors emitted by kitchen itself.

Base kitchen exceptions

Exception classes for kitchen and the root of the exception hierarchy for all kitchen modules.

exception kitchen.exceptions.KitchenError: Base exception class for any error thrown directly by kitchen.

Kitchen.text exceptions

Exception classes thrown by kitchen's text processing routines.

exception kitchen.text.exceptions.ControlCharError: Exception thrown when an ascii control character is encountered.

exception kitchen.text.exceptions.XmlEncodeError: Exception thrown by error conditions when encoding an xml string.

1.0.0 Porting Guide

The 0.1 through 1.0.0 releases focused on bringing in functions from yum and python-fedora. This porting guide tells how to port from those APIs to their kitchen replacements.

python-fedora

python-fedora	kitchen replacement
fedora.iterutils.isiterable()	kitchen.iterutils.isiterable() [1]
fedora.textutils.to_unicode()	kitchen.text.converters.to_unicode()
fedora.textutils.to_bytes()	kitchen.text.converters.to_bytes()

[1]: isiterable() has changed slightly in kitchen. The include_string attribute has switched its default value from True to False. So you need to change code like:

>>> # Old code >>> isiterable('abcdef') True >>> # New code >>> isiterable('abcdef', include_string=True) True

yum

yum	kitchen replacement
yum.i18n.dummy_wrapper()	kitchen.i18n.DummyTranslations.ugettext() [2]
yum.i18n.dummyP_wrapper()	kitchen.i18n.DummyTanslations.ungettext() [2]
yum.i18n.utf8_width()	kitchen.text.display.textual_width()
yum.i18n.utf8_width_chop()	kitchen.text.display.textual_width_chop() and kitchen.text.display.textual_width() [3] [5]
yum.i18n.utf8_valid()	kitchen.text.misc.byte_string_valid_encoding()
yum.i18n.utf8_text_wrap()	kitchen.text.display.wrap() [4]
yum.i18n.utf8_text_fill()	kitchen.text.display.fill() [4]
yum.i18n.to_unicode()	kitchen.text.converters.to_unicode() [6]
yum.i18n.to_unicode_maybe()	kitchen.text.converters.to_unicode() [6]
yum.i18n.to_utf8()	kitchen.text.converters.to_bytes() [6]
yum.i18n.to_str()	kitchen.text.converters.to_unicode() or kitchen.text.converters.to_bytes() [7]
yum.i18n.str_eq()	kitchen.text.misc.str_eq()
yum.misc.to_xml()	kitchen.text.converters.unicode_to_xml() or kitchen.text.converters.byte_string_to_xml() [8]
yum.i18n._()	See: Initializing Yum i18n
yum.i18n.P_()	See: Initializing Yum i18n
yum.i18n.exception2msg()	kitchen.text.converters.exception_to_unicode() or kitchen.text.converter.exception_to_bytes() [9]

[2]: These yum methods provided fallback support for gettext functions in case either gaftonmode was set or gettext failed to return an object. In kitchen, we can use the kitchen.i18n.DummyTranslations object to fulfill that role. Please see Initializing Yum i18n for more suggestions on how to do this.

[3]: The yum version of these functions returned a byte str. The kitchen version listed here returns a unicode string. If you need a byte str simply call kitchen.text.converters.to_bytes() on the result.

[4]: The yum version of these functions would return either a byte str or a unicode string depending on what the input value was. The kitchen version always returns unicode strings.

[5]: yum.i18n.utf8_width_chop() performed two functions. It returned the piece of the message that fit in a specified width and the width of that message. In kitchen, you need to call two functions, one for each action:

>>> # Old way >>> utf8_width_chop(msg, 5) (5, 'く ku') >>> # New way >>> from kitchen.text.display import textual_width, textual_width_chop >>> (textual_width(msg), textual_width_chop(msg, 5)) (5, u'く ku')

[6]: If the yum version of to_unicode() or to_utf8() is given an object that is not a string, it returns the object itself. kitchen.text.converters.to_unicode() and kitchen.text.converters.to_bytes() default to returning the simplerepr of the object instead. If you want the yum behaviour, set the nonstring parameter to passthru:

>>> from kitchen.text.converters import to_unicode >>> to_unicode(5) u'5' >>> to_unicode(5, nonstring='passthru') 5

[7]: yum.i18n.to_str() could return either a byte str. or a unicode string In kitchen you can get the same effect but you get to choose whether you want a byte str or a unicode string. Use to_bytes() for str and to_unicode() for unicode.

[8]: yum.misc.to_xml() was buggy as written. I think the intention was for you to be able to pass a byte str or unicode string in and get out a byte str that was valid to use in an xml file. The two kitchen functions byte_string_to_xml() and unicode_to_xml() do that for each string type.

[9]

When porting yum.i18n.exception2msg() to use kitchen, you should setup two wrapper functions to aid in your port. They'll look like this:

from kitchen.text.converters import EXCEPTION_CONVERTERS, \
    BYTE_EXCEPTION_CONVERTERS, exception_to_unicode, \
    exception_to_bytes
def exception2umsg(e):
    '''Return a unicode representation of an exception'''
    c = [lambda e: e.value]
    c.extend(EXCEPTION_CONVERTERS)
    return exception_to_unicode(e, converters=c)
def exception2bmsg(e):
    '''Return a utf8 encoded str representation of an exception'''
    c = [lambda e: e.value]
    c.extend(BYTE_EXCEPTION_CONVERTERS)
    return exception_to_bytes(e, converters=c)

The reason to define this wrapper is that many of the exceptions in yum put the message in the value attribute of the Exception instead of adding it to the args attribute. So the default EXCEPTION_CONVERTERS don't know where to find the message. The wrapper tells kitchen to check the value attribute for the message. The reason to define two wrappers may be less obvious. yum.i18n.exception2msg() can return a unicode string or a byte str depending on a combination of what attributes are present on the Exception and what locale the function is being run in. By contrast, kitchen.text.converters.exception_to_unicode() only returns unicode strings and kitchen.text.converters.exception_to_bytes() only returns byte str. This is much safer as it keeps code that can only handle unicode or only handle byte str correctly from getting the wrong type when an input changes but it means you need to examine the calling code when porting from yum.i18n.exception2msg() and use the appropriate wrapper.

Initializing Yum i18n

Previously, yum had several pieces of code to initialize i18n. From the toplevel of yum/i18n.py:

try:.
    '''
    Setup the yum translation domain and make _() and P_() translation wrappers
    available.
    using ugettext to make sure translated strings are in Unicode.
    '''
    import gettext
    t = gettext.translation('yum', fallback=True)
    _ = t.ugettext
    P_ = t.ungettext
except:
    '''
    Something went wrong so we make a dummy _() wrapper there is just
    returning the same text
    '''
    _ = dummy_wrapper
    P_ = dummyP_wrapper

With kitchen, this can be changed to this:

from kitchen.i18n import easy_gettext_setup, DummyTranslations
try:
    _, P_ = easy_gettext_setup('yum')
except:
    translations = DummyTranslations()
    _ = translations.ugettext
    P_ = translations.ungettext

NOTE:

In Overcoming frustration: Correctly using unicode in python2, it is mentioned that for some things (like exception messages), using the byte str oriented functions is more appropriate. If this is desired, the setup portion is only a second call to kitchen.i18n.easy_gettext_setup():

b_, bP_ = easy_gettext_setup('yum', use_unicode=False)

The second place where i18n is setup is in yum.YumBase._getConfig() in yum/__init_.py if gaftonmode is in effect:

if startupconf.gaftonmode:
    global _
    _ = yum.i18n.dummy_wrapper

This can be changed to:

if startupconf.gaftonmode:
    global _
    _ = DummyTranslations().ugettext()

Conventions for contributing to kitchen

Style

•: Strive to be PEP 8 compliant

•: Run :command:`pylint ` over the code and try to resolve most of its nitpicking

Python 2.4 compatibility

At the moment, we're supporting python-2.4 and above. Understand that there's a lot of python features that we cannot use because of this.

Sometimes modules in the python standard library can be added to kitchen so that they're available. When we do that we need to be careful of several things:

1.: Keep the module in sync with the version in the python-2.x trunk. Use maintainers/sync-copied-files.py for this.

2.: Sync the unittests as well as the module.

3.: Be aware that not all modules are written to remain compatible with Python-2.4 and might use python language features that were not present then (generator expressions, relative imports, decorators, with, try: with both except: and finally:, etc) These are not good candidates for importing into kitchen as they require more work to keep synced.

Unittests

•: At least smoketest your code (make sure a function will return expected values for one set of inputs).

•: Note that even 100% coverage is not a guarantee of working code! Good tests will realize that you need to also give multiple inputs that test the code paths of called functions that are outside of your code. Example:

def to_unicode(msg, encoding='utf8', errors='replace'): return unicode(msg, encoding, errors) # Smoketest only. This will give 100% coverage for your code (it # tests all of the code inside of to_unicode) but it leaves a lot of # room for errors as it doesn't test all combinations of arguments # that are then passed to the unicode() function. tools.ok_(to_unicode('abc') == u'abc') # Better -- tests now cover non-ascii characters and that error conditions # occur properly. There's a lot of other permutations that can be # added along these same lines. tools.ok_(to_unicode(u'café', 'utf8', 'replace')) tools.assert_raises(UnicodeError, to_unicode, [u'cafè ñunru'.encode('latin1')])

•: We're using nose for unittesting. Rather than depend on unittest2 functionality, use the functions that nose provides.

•: Remember to maintain python-2.4 compatibility even in unittests.

Docstrings and documentation

We use sphinx to build our documentation. We use the sphinx autodoc extension to pull docstrings out of the modules for API documentation. This means that docstrings for subpackages and modules should follow a certain pattern. The general structure is:

•

Introductory material about a module in the module's top level docstring.

•: Introductory material should begin with a level two title: an overbar and underbar of '-'.

•

docstrings for every function.

•: The first line is a short summary of what the function does

•: This is followed by a blank line

•

The next lines are a field list <http://sphinx.pocoo.org/markup/desc.html#info-field-lists>_ giving information about the function's signature. We use the keywords: arg, kwarg, raises, returns, and sometimes rtype. Use these to describe all arguments, key word arguments, exceptions raised, and return values using these.

•: Parameters that are kwarg should specify what their default behaviour is.

Kitchen versioning

Currently the kitchen library is in early stages of development. While we're in this state, the main kitchen library uses the following pattern for version information:

•

Versions look like this::: __version_info__ = ((0, 1, 2),) __version__ = '0.1.2'

•: The Major version number remains at 0 until we decide to make the first 1.0 release of kitchen. At that point, we're declaring that we have some confidence that we won't need to break backwards compatibility for a while.

•: The Minor version increments for any backwards incompatible API changes. When this is updated, we reset micro to zero.

•: The Micro version increments for any other changes (backwards compatible API changes, pure bugfixes, etc).

NOTE:

Versioning is only updated for releases that generate sdists and new uploads to the download directory. Usually we update the version information for the library just before release. By contrast, we update kitchen Versioning when an API change is made. When in doubt, look at the version information in the last release.

I18N

All strings that are used as feedback for users need to be translated. kitchen sets up several functions for this. _() is used for marking things that are shown to users via print, GUIs, or other "standard" methods. Strings for exceptions are marked with b_(). This function returns a byte str which is needed for use with exceptions:

from kitchen import _, b_
def print_message(msg, username):
    print _('%(user)s, your message of the day is:  %(message)s') % {
            'message': msg, 'user': username}
    raise Exception b_('Test message')

This serves several purposes:

•: It marks the strings to be extracted by an xgettext-like program.

•: _() is a function that will substitute available translations at runtime.

NOTE:

By using the %()s with dict style of string formatting, we make this string friendly to translators that may need to reorder the variables when they're translating the string.

paver <http://www.blueskyonmars.com/projects/paver/>_ and babel <http://babel.edgewall.org/>_ are used to extract the strings.

API updates

Kitchen strives to have a long deprecation cycle so that people have time to switch away from any APIs that we decide to discard. Discarded APIs should raise a DeprecationWarning and clearly state in the warning message and the docstring how to convert old code to use the new interface. An example of deprecating a function:

import warnings
from kitchen import _
from  kitchen.text.converters import to_bytes, to_unicode
from kitchen.text.new_module import new_function
def old_function(param):
    '''**Deprecated**
    This function is deprecated.  Use
    :func:`kitchen.text.new_module.new_function` instead. If you want
    unicode strngs as output, switch to::
        >>> from kitchen.text.new_module import new_function
        >>> output = new_function(param)
    If you want byte strings, use::
        >>> from kitchen.text.new_module import new_function
        >>> from kitchen.text.converters import to_bytes
        >>> output = to_bytes(new_function(param))
    '''
    warnings.warn(_('kitchen.text.old_function is deprecated.  Use'
        ' kitchen.text.new_module.new_function instead'),
        DeprecationWarning, stacklevel=2)
    as_unicode = isinstance(param, unicode)
    message = new_function(to_unicode(param))
    if not as_unicode:
        message = to_bytes(message)
    return message

If a particular API change is very intrusive, it may be better to create a new version of the subpackage and ship both the old version and the new version.

NEWS file

Update the NEWS file when you make a change that will be visible to the users. This is not a ChangeLog file so we don't need to list absolutely everything but it should give the user an idea of how this version differs from prior versions. API changes should be listed here explicitly. bugfixes can be more general:

-----
0.2.0
-----
* Relicense to LGPLv2+
* Add kitchen.text.format module with the following functions:
  textual_width, textual_width_chop.
* Rename the kitchen.text.utils module to kitchen.text.misc.  use of the
  old names is deprecated but still available.
* bugfixes applied to kitchen.pycompat24.defaultdict that fixes some
  tracebacks

Kitchen subpackages

Kitchen itself is a namespace. The kitchen sdist (tarball) provides certain useful subpackages.

SEE ALSO:

Kitchen addon packages: For information about subpackages not distributed in the kitchen sdist that install into the kitchen namespace.

Versioning

Each subpackage should have its own version information which is independent of the other kitchen subpackages and the main kitchen library version. This is used so that code that depends on kitchen APIs can check the version information. The standard way to do this is to put something like this in the subpackage's __init__.py:

from kitchen.versioning import version_tuple_to_string
__version_info__ = ((1, 0, 0),)
__version__ = version_tuple_to_string(__version_info__)

__version_info__ is documented in kitchen.versioning. The values of the first tuple should describe API changes to the module. There are at least three numbers present in the tuple: (Major, minor, micro). The major version number is for backwards incompatible changes (For instance, removing a function, or adding a new mandatory argument to a function). Whenever one of these occurs, you should increment the major number and reset minor and micro to zero. The second number is the minor version. Anytime new but backwards compatible changes are introduced this number should be incremented and the micro version number reset to zero. The micro version should be incremented when a change is made that does not change the API at all. This is a common case for bugfixes, for instance.

Version information beyond the first three parts of the first tuple may be useful for versioning but semantically have similar meaning to the micro version.

NOTE:

We update the __version_info__ tuple when the API is updated. This way there's less chance of forgetting to update the API version when a new release is made. However, we try to only increment the version numbers a single step for any release. So if kitchen-0.1.0 has kitchen.text.__version__ == '1.0.1', kitchen-0.1.1 should have kitchen.text.__version__ == '1.0.2' or '1.1.0' or '2.0.0'.

Criteria for subpackages in kitchen

Supackages within kitchen should meet these criteria:

•: Generally useful or needed for other pieces of kitchen.

•

No mandatory requirements outside of the python standard library.

•: Optional requirements from outside the python standard library are allowed. Things with mandatory requirements are better placed in kitchen addon packages

•

Somewhat API stable -- this is not a hard requirement. We can change the kitchen api. However, it is better not to as people may come to depend on it.

SEE ALSO:

API Updates

Kitchen addon packages

Addon packages are very similar to subpackages integrated into the kitchen sdist. This section just lists some of the differences to watch out for.

setup.py

Your setup.py should contain entries like this:

# It's suggested to use a dotted name like this so the package is easily
# findable on pypi:
setup(name='kitchen.config',
    # Include kitchen in the keywords, again, for searching on pypi
    keywords=['kitchen', 'configuration'],
    # This package lives in the directory kitchen/config
    packages=['kitchen.config'],
    # [...]
)

Package directory layout

Create a kitchen directory in the toplevel. Place the addon subpackage in there. For example:

./                     <== toplevel with README, setup.py, NEWS, etc
kitchen/
kitchen/__init__.py
kitchen/config/        <== subpackage directory
kitchen/config/__init__.py

Fake kitchen module

The :file::__init__.py in the kitchen directory is special. It won't be installed. It just needs to pull in the kitchen from the system so that you are able to test your module. You should be able to use this boilerplate:

# Fake module.  This is not installed,  It's just made to import the real
# kitchen modules for testing this module
import pkgutil
# Extend the __path__ with everything in the real kitchen module
__path__ = pkgutil.extend_path(__path__, __name__)

NOTE:

kitchen needs to be findable by python for this to work. Installed in the site-packages directory or adding it to the PYTHONPATH will work.

Your unittests should now be able to find both your submodule and the main kitchen module.

Versioning

It is recommended that addon packages version similarly to Versioning. The __version_info__ and __version__ strings can be changed independently of the version exposed by setup.py so that you have both an API version (__version_info__) and release version that's easier for people to parse. However, you aren't required to do this and you could follow a different methodology if you want (for instance, Kitchen versioning)

Glossary

"Everything but the kitchen sink": An English idiom meaning to include nearly everything that you can think of.

API version: Version that is meant for computer consumption. This version is parsable and comparable by computers. It contains information about a library's API so that computer software can decide whether it works with the software.

ASCII

A character encoding that maps numbers to characters essential to American English. It maps 128 characters using 7bits.

SEE ALSO:

http://en.wikipedia.org/wiki/ASCII

ASCII compatible

An encoding in which the particular byte that maps to a character in the ASCII character set is only used to map to that character. This excludes EBDIC based encodings and many multi-byte fixed and variable width encodings since they reuse the bytes that make up the ASCII encoding for other purposes. UTF-8 is notable as a variable width encoding that is ASCII compatible.

SEE ALSO:

http://en.wikipedia.org/wiki/Variable-width_encoding: For another explanation of various ways bytes are mapped to characters in a possibly incompatible manner.

code points: code point

code point: A number that maps to a particular abstract character. Code points make it so that we have a number pointing to a character without worrying about implementation details of how those numbers are stored for the computer to read. Encodings define how the code points map to particular sequences of bytes on disk and in memory.

control characters: control character

control character

The set of characters in unicode that are used, not to display glyphs on the screen, but to tell the display in program to do something.

SEE ALSO:

http://en.wikipedia.org/wiki/Control_character

grapheme

characters or pieces of characters that you might write on a page to make words, sentences, or other pieces of text.

SEE ALSO:

http://en.wikipedia.org/wiki/Grapheme

I18N: I18N is an abbreviation for internationalization. It's often used to signify the need to translate words, number and date formats, and other pieces of data in a computer program so that it will work well for people who speak another language than yourself.

message catalogs: message catalog

message catalog

Message catalogs contain translations for user-visible strings that are present in your code. Normally, you need to mark the strings to be translated by wrapping them in one of several gettext functions. The function serves two purposes:

1.: It allows automated tools to find which strings are supposed to be extracted for translation.

2.: The functions perform the translation when the program is running.

SEE ALSO:

babel's documentation

for one method of extracting message catalogs from source code.

Murphy's Law

"Anything that can go wrong, will go wrong."

SEE ALSO:

http://en.wikipedia.org/wiki/Murphy%27s_Law

release version: Version that is meant for human consumption. This version is easy for a human to look at to decide how a particular version relates to other versions of the software.

textual width: The amount of horizontal space a character takes up on a monospaced screen. The units are number of character cells or columns that it takes the place of.

UTF-8: A character encoding that maps all unicode code points to a sequence of bytes. It is compatible with ASCII. It uses a variable number of bytes to encode all of unicode. ASCII characters take one byte. Characters from other parts of unicode take two to four bytes. It is widespread as an encoding on the internet and in Linux.

Requirements

Soft Requirements

Other recommended libraries

Contents

Using kitchen to write good code

Overcoming frustration: Correctly using unicode in python2

Frustration #1: Inconsistent Errors

Frustration #2: Inconsistent APIs

Frustration #3: Inconsistent treatment of output

Frustrations #4 and #5 -- The other shoes

Frustration #4: Now it doesn't take byte strings?!

Frustration #5: Inconsistent APIs Part deux

A few solutions

Convert text at the border

When the data needs to be treated as bytes (or unicode) use a naming convention

When outputting data, convert back into bytes

When writing unittests, include non-ASCII values and both unicode and str type

Be vigilant about spotting poor APIs

Example: Putting this all together with kitchen

Designing Unicode Aware APIs

Contents

Take either bytes or unicode, output only unicode

Take either bytes or unicode, output the same type

Separate functions

Deciding whether to take str or unicode when no value is returned

Writing to external data

Updating data structures

APIs to Avoid

Returning unicode unless a conversion fails

Ignoring values with no chance of recovery

Raising a UnicodeException with no chance of recovery

Knowing your data

Do you need to operate on both bytes and unicode?

Can you restrict the encodings?

Single byte encodings

Multibyte encodings

Fixed width

Variable Width

ASCII compatible

Escaped

Other

Kitchen API

Kitchen.i18n Module

Functions

Translation Objects

Kitchen.text: unicode and utf8 and xml oh my!

Kitchen.text.converters

Byte Strings and Unicode in Python2

Strategy for Explicit Conversion

When to use an alternate strategy

Gotchas and how to avoid them

str(obj)

print

Unicode, str, and dict keys

Functions

Unicode and byte str conversion

Transformation to XML

Working with exception messages

Format Text for Display

Internal Data

Miscellaneous functions for manipulating text

UTF-8

Kitchen.collections

StrictDict

Kitchen.iterutils Module

Helpers for versioning software

PEP-386 compliant versioning

Exceptions

Base kitchen exceptions

Kitchen.text exceptions

1.0.0 Porting Guide

python-fedora

yum

Initializing Yum i18n

Conventions for contributing to kitchen

Style

Python 2.4 compatibility

Unittests

Docstrings and documentation

Kitchen versioning