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 a 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
=====

* documentation

* 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.ext.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
   from flask.ext.login import LoginManager, login_user, logout_user, \
        login_required, current_user
   from flask.ext.wtf import Form, TextField, PasswordField, Required, Email
   from flask.ext.principal import Principal, Identity, AnonymousIdentity, \
        identity_changed

   app = Flask(__name__)

   Principal(app)

   login_manager = LoginManager(app)

   @login_manager.user_loader
   def load_user(userid):
       # Return an instance of the User model
       return datastore.find_user(id=userid)

   class LoginForm(Form):
       email = TextField()
       password = PasswordField()

   @app.route('/login', methods=['GET', 'POST'])
   def login():
       # A hypothetical login form that uses Flask-WTF
       form = LoginForm()

       # Validate form input
       if form.validate_on_submit():
           # Retrieve the user from the hypothetical datastore
           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))

               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 '/')


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.ext.login import current_user
   from flask.ext.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.ext.login import current_user
   from flask.ext.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


API
***


Starting the extension
======================

class flask_principal.Principal(app=None, use_sessions=True, skip_static=False)

   Principal extension

   Parameters:
      * **app** -- The flask application to extend

      * **use_sessions** -- Whether to use sessions to extract and
        store identification.

      * **skip_static** -- Whether to ignore static endpoints.

   identity_loader(f)

      Decorator to define a function as an identity loader.

      An identity loader function is called before request to find any
      provided identities. The first found identity is used to load
      from.

      For example:

         app = Flask(__name__)

         principals = Principal(app)

         @principals.identity_loader
         def load_identity_from_weird_usecase():
             return Identity('ali')

   identity_saver(f)

      Decorator to define a function as an identity saver.

      An identity loader saver is called when the identity is set to
      persist it for the next request.

      For example:

         app = Flask(__name__)

         principals = Principal(app)

         @principals.identity_saver
         def save_identity_to_weird_usecase(identity):
             my_special_cookie['identity'] = identity

   set_identity(identity)

      Set the current identity.

      Parameters:
         **identity** -- The identity to set


Main Types
==========

class flask_principal.Permission(*needs)

   Represents needs, any of which must be present to access a resource

   Parameters:
      **needs** -- The needs for this permission

   allows(identity)

      Whether the identity can access this permission.

      Parameters:
         **identity** -- The identity

   can()

      Whether the required context for this permission has access

      This creates an identity context and tests whether it can access
      this permission

   difference(other)

      Create a new permission consisting of requirements in this
      permission and not in the other.

   issubset(other)

      Whether this permission needs are a subset of another

      Parameters:
         **other** -- The other permission

   require(http_exception=None)

      Create a principal for this permission.

      The principal may be used as a context manager, or a decroator.

      If "http_exception" is passed then "abort()" will be called with
      the HTTP exception code. Otherwise a "PermissionDenied"
      exception will be raised if the identity does not meet the
      requirements.

      Parameters:
         **http_exception** -- the HTTP exception code (403, 401 etc)

   reverse()

      Returns reverse of current state (needs->excludes,
      excludes->needs)

   test(http_exception=None)

      Checks if permission available and raises relevant exception if
      not. This is useful if you just want to check permission without
      wrapping everything in a require() block.

      This is equivalent to:

         with permission.require():
             pass

   union(other)

      Create a new permission with the requirements of the union of
      this and other.

      Parameters:
         **other** -- The other permission

class flask_principal.Identity(id, auth_type=None)

   Represent the user's identity.

   Parameters:
      * **id** -- The user id

      * **auth_type** -- The authentication type used to confirm the
        user's identity.

   The identity is used to represent the user's identity in the
   system. This object is created on login, or on the start of the
   request as loaded from the user's session.

   Once loaded it is sent using the *identity-loaded* signal, and
   should be populated with additional required information.

   Needs that are provided by this identity should be added to the
   *provides* set after loading.

   can(permission)

      Whether the identity has access to the permission.

      Parameters:
         **permission** -- The permission to test provision for.

class flask_principal.AnonymousIdentity

   An anonymous identity

class flask_principal.IdentityContext(permission, http_exception=None)

   The context of an identity for a permission.

   Note: The principal is usually created by the
     flaskext.Permission.require method call for normal use-cases.

   The principal behaves as either a context manager or a decorator.
   The permission is checked for provision in the identity, and if
   available the flow is continued (context manager) or the function
   is executed (decorator).

   can()

      Whether the identity has access to the permission

   http_exception = None

      The permission of this principal

   identity

      The identity of this principal


Predefined Need Types
=====================

class flask_principal.Need(method, value)

   A required need

   This is just a named tuple, and practically any tuple will do.

   The "method" attribute can be used to look up element 0, and the
   "value" attribute can be used to look up element 1.

flask_principal.RoleNeed

flask_principal.UserNeed

class flask_principal.ItemNeed(method, value, type)

   A required item need

   An item need is just a named tuple, and practically any tuple will
   do. In addition to other Needs, there is a type, for example this
   could be specified as:

      ItemNeed('update', 27, 'posts')
      ('update', 27, 'posts') # or like this

   And that might describe the permission to update a particular blog
   post. In reality, the developer is free to choose whatever
   convention the permissions are.


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

  * 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
