--- name: "python-flask-principal" section: "1" platform: "linux" package: "python-flask-principal" version: "0.5.0-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/python-flask-principal" --- # python-flask-principal(1) Flask-Principal Documentation "I am that I am" ## NAME flask-principal - Flask-Principal Documentation *"I am that I am"* ## INTRODUCTION Flask-Principal provides a very loose framework to tie in providers of two types of service, often located in different parts of a web application: - 1\. Authentication providers - 2\. User information providers For example, an authentication provider may be oauth, using Flask-OAuth and the user information may be stored in a relational database. Looseness of the framework is provided by using signals as the interface. The major components are the Identity, Needs, Permission, and the IdentityContext. - 1\. The Identity represents the user, and is stored/loaded from various locations (eg session) for each request. The Identity is the user's avatar to the system. It contains the access rights that the user has. - 2\. A Need is the smallest grain of access control, and represents a specific parameter for the situation. For example "has the admin role", "can edit blog posts". Needs are any tuple, or probably could be object you like, but a tuple fits perfectly. The predesigned Need types (for saving your typing) are either pairs of (method, value) where method is used to specify common things such as *"role"*, *"user"*, etc. And the value is the value. An example of such is *('role', 'admin')*. Which would be a Need for an admin role. Or Triples for use-cases such as "The permission to edit a particular instance of an object or row", which might be represented as the triple *('article', 'edit', 46)*, where 46 is the key/ID for that row/object. Essentially, how and what Needs are is very much down to the user, and is designed loosely so that any effect can be achieved by using custom instances as Needs. Whilst a Need is a permission to access a resource, an Identity should provide a set of Needs that it has access to. - 3\. A Permission is a set of requirements, any of which should be present for access to a resource. - 4\. An IdentityContext is the context of a certain identity against a certain Permission. It can be used as a context manager, or a decorator. \[graph\].SH LINKS - • source \ - • changelog \<\> ## PROTECTING ACCESS TO RESOURCES For users of Flask-Principal (not authentication providers), access restriction is easy to define as both a decorator and a context manager. A simple quickstart example is presented with commenting: ``` from flask import Flask, Response from flask_principal import Principal, Permission, RoleNeed app = Flask(__name__) # load the extension principals = Principal(app) # Create a permission with a single Need, in this case a RoleNeed. admin_permission = Permission(RoleNeed('admin')) # protect a view with a principal for that need @app.route('/admin') @admin_permission.require() def do_admin_index(): return Response('Only if you are an admin') # this time protect with a context manager @app.route('/articles') def do_articles(): with admin_permission.require(): return Response('Only if you are admin') ``` ## AUTHENTICATION PROVIDERS Authentication providers should use the *identity-changed* signal to indicate that a request has been authenticated. For example, the following code is a hypothetical example of how one might combine the popular Flask-Login \ extension with Flask-Principal: ``` from flask import Flask, current_app, request, session, \ render_template, redirect from flask_login import LoginManager, login_user, logout_user, \ login_required from wtforms import StringField, PasswordField from flask_wtf import FlaskForm from flask_principal import Principal, Identity, AnonymousIdentity, \ identity_changed app = Flask(__name__) app.config['SECRET_KEY'] = 'notsosecret' Principal(app) login_manager = LoginManager(app) class User: def __init__(self, id='1', email=None, password=None): self.id = id self.email = email self.password = 'abcd' self.is_authenticated = False self.is_active = True self.is_anonymous = True def get_id(self): return self.id class DB: def __init__(self): pass def find_user(self, id=None, email=None): example_user = User(id=id, email=email) return example_user @login_manager.user_loader def load_user(userid): # Return an instance of the User model datastore = DB() return datastore.find_user(id=userid) class LoginForm(FlaskForm): email = StringField('email') password = PasswordField('password') @app.route('/login', methods=['GET', 'POST']) def login(): # A hypothetical login form that uses Flask-WTF form = LoginForm() if request.method == 'POST': # Validate form input if form.validate_on_submit(): # Retrieve the user from the hypothetical datastore datastore = DB() user = datastore.find_user(email=form.email.data) # Compare passwords (use password hashing production) if form.password.data == user.password: # Keep the user info in the session using Flask-Login login_user(user) # Tell Flask-Principal the identity changed identity_changed.send(current_app._get_current_object(), identity=Identity(user.id)) print('logged in ') return redirect(request.args.get('next') or '/') return render_template('login.html', form=form) @app.route('/logout') @login_required def logout(): # Remove the user information from the session logout_user() # Remove session keys set by Flask-Principal for key in ('identity.name', 'identity.auth_type'): session.pop(key, None) # Tell Flask-Principal the user is anonymous identity_changed.send(current_app._get_current_object(), identity=AnonymousIdentity()) return redirect(request.args.get('next') or '/') @app.route('/') def home(): return 'home page' if __name__ == '__main__': app.run(debug=True) ``` The login page snippets looks like this: - **\** - **\** - **\
** {{ form.csrf\_token }} {{ form.email.label }} {{ form.email }}\ {{ form.csrf\_token }} {{ form.password.label }} {{ form.password }}\ \ \ \ \ ## USER INFORMATION PROVIDERS User information providers should connect to the *identity-loaded* signal to add any additional information to the Identity instance such as roles. The following is another hypothetical example using Flask-Login and could be combined with the previous example. It shows how one might use a role based permission scheme: ``` from flask_login import current_user from flask_principal import identity_loaded, RoleNeed, UserNeed @identity_loaded.connect_via(app) def on_identity_loaded(sender, identity): # Set the identity user object identity.user = current_user # Add the UserNeed to the identity if hasattr(current_user, 'id'): identity.provides.add(UserNeed(current_user.id)) # Assuming the User model has a list of roles, update the # identity with the roles that the user provides if hasattr(current_user, 'roles'): for role in current_user.roles: identity.provides.add(RoleNeed(role.name)) ``` ## GRANULAR RESOURCE PROTECTION Now lets say, for example, you only want the author of a blog post to be able to edit said article. This can be achieved by creating the necessary *Need* and *Permission* objects, and adding more logic into the *identity\_loaded* signal handler. For example: ``` from collections import namedtuple from functools import partial from flask_login import current_user from flask_principal import identity_loaded, Permission, RoleNeed, \ UserNeed BlogPostNeed = namedtuple('blog_post', ['method', 'value']) EditBlogPostNeed = partial(BlogPostNeed, 'edit') class EditBlogPostPermission(Permission): def __init__(self, post_id): need = EditBlogPostNeed(unicode(post_id)) super(EditBlogPostPermission, self).__init__(need) @identity_loaded.connect_via(app) def on_identity_loaded(sender, identity): # Set the identity user object identity.user = current_user # Add the UserNeed to the identity if hasattr(current_user, 'id'): identity.provides.add(UserNeed(current_user.id)) # Assuming the User model has a list of roles, update the # identity with the roles that the user provides if hasattr(current_user, 'roles'): for role in current_user.roles: identity.provides.add(RoleNeed(role.name)) # Assuming the User model has a list of posts the user # has authored, add the needs to the identity if hasattr(current_user, 'posts'): for post in current_user.posts: identity.provides.add(EditBlogPostNeed(unicode(post.id))) ``` The next step will be to protect the endpoint that allows a user to edit an article. This is done by creating a permission object on the fly using the ID of the resource, in this case the blog post: ``` @app.route('/posts/', methods=['PUT', 'PATCH']) def edit_post(post_id): permission = EditBlogPostPermission(post_id) if permission.can(): # Save the edits ... return render_template('edit_post.html') abort(403) # HTTP Forbidden ``` ## COMBINING PERMISSIONS While the core constructs provided are sufficient for the common simple use cases, for more complex cases it is possible to combine existing permissions through the use of bitwise operators to result in a new permission object that can do the complex validations. For instance, it is to create a permission where the identity has the both the *"blog\_poster"* or *"blog\_reviewer"* role but not the *"under\_probation"* role. Example: ``` blog_admin = Permission(RoleNeed('blog_admin')) blog_poster = Permission(RoleNeed('blog_poster')) blog_reviewer = Permission(RoleNeed('blog_reviewer')) under_probation = Permission(RoleNeed('under_probation')) prize_permission = ((blog_poster | blog_reviewer) & ~under_probation) @app.route('/blog/prizes') @prize_permission.require() def prize_redeem(): # find out what prizes are available to blog users that are not # under probation return render_template('prize_redeem.html') ``` Any number of these can be chained, but it is also possible to use the constructor classes directly to combine these things together: ``` from flask.ext.principal import AndPermission, OrPermission allperms = AndPermission(blog_poster, blog_reviewer, blog_admin) anyperms = OrPermission(blog_poster, blog_reviewer, blog_admin) ``` ## CUSTOM PERMISSIONS Sometimes your permissions may be determined by other circumstances specific to whatever you need to implement. For that you can create custom classes to address your needs like so: ``` from flask.ext.principal import BasePermission class CustomPermission(BasePermission): def allows(self, identity): # Implement other conditions that allow this to pass return False ``` These custom permissions can be combined together via the bitwise operators as explained above. ## STARTING THE EXTENSION ## MAIN TYPES ## PREDEFINED NEED TYPES ## SIGNALS - **identity\_changed** Signal sent when the identity for a request has been changed. - **identity\_loaded** Signal sent when the identity has been initialised for a request. ## FLASK-PRINCIPAL CHANGELOG Here you can see the full list of changes between each Flask-Principal release. ### Released 2026-05-31 - • Added Pallets projects scaffolding - • Updated docstrings. Changed **flask.ext.principal** imports to **flask\_principal**. - • Updated **Permission** needs and excludes to never conflict with each other. - • Updated docs: Flask-login sections - • Fix Mypy --strict warnings - • Added combining permissions feature \[#111\](\) - • Changed intersphinx mapping \[#109\](\) ### Version 0.4.0 Released June 14th 2013 - • Added Python 3 support - • Dropped support for Python 2.5 ### Version 0.3.5 Released April 3rd 2013 - • Fixed possible bug with **AnonymousIdentity** supplying "anon" as the username - • Changed Indentity **name** property to **id** to be more generic ### Version 0.3.4 Released February 1st 2013 - • Add **\_\_repr\_\_** method to Identity and Permission classes - • Optimized **\_is\_static\_resource** method ### Version 0.3.3 Released September 4th 2012 - • Add **init\_app** method to accomodate usage with a factory pattern. ### Version 0.3.2 Released August 25th 2012 - • Update to check for **static\_url\_path** in Flask 0.9 ### Version 0.3.1 Released August 16th 2012 - • Fixed bug with re-raising exceptions/tracebacks ### Version 0.3 Released June 20th 2012 - • Python 2.5/GAE support - • New extension structure - • Added ignore\_static option - • Updated docs ### Version 0.2 Initial development by Ali Asfshar. Original repository \ - • Index \<\> - • Module Index \<\> - • Search Page \<\> ## Author Matt Wright ## Copyright 2012, Ali Afshar