Flask Principal
***************

*"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]


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:

   <html>
      <body>
         <form method="POST" action="/login">
            {{ form.csrf_token }} {{ form.email.label }} {{ form.email
            }}<br> {{ form.csrf_token }} {{ form.password.label }} {{
            form.password }}<br> <input type="submit" value="Go">

         </form>

      </body>

   </html>


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/<post_id>', 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.


API
***


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.


Changelog
*********

* Flask-Principal Changelog

  * Released 2026-05-31

  * Version 0.4.0

  * Version 0.3.5

  * Version 0.3.4

  * Version 0.3.3

  * Version 0.3.2

  * Version 0.3.1

  * Version 0.3

  * Version 0.2


Indices and tables
******************

* Index

* Module Index

* Search Page
