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Authorization for web applications: permissions, groups, roles, ACL

Project description

Knave: an ACL library for authorization in WSGI apps

Knave provides roles/groups and permissions based authorization for web applications. Knave assigns roles to users (both site wide roles like ‘administrator’ and context-sensitive roles like ‘creator’), then uses those roles to decide whether the user has permission to carry out an action.

Knave doesn’t provide any facility for authentication (ie checking the user’s identity, perhaps via a username and password), but can hook into most authentication systems.


from knave.acl import Role, Permission, ACL
from knave.roles import StaticRoleProvider

# Create some permissions...
CREATE_PAGE = Permission('create_page')
PUBLISH_PAGE = Permission('publish_page')

# ...and some roles
ADMIN = Role('admin')
EDITOR = Role('editor')
WRITER = Role('writer')

# Then map the permissions to the roles allowed to execute them
role_permssions = {

# Assign roles to users
role_provider = StaticRoleProvider({
    'spike': {ADMIN},
    'tom': {EDITOR}
    'jerry': {WRITER}

# Now, later on...
def create_page(request):
    # The `test` method raises knave.permssions.Unauthorized,
    # which is trapped by the middleware, causing a 401 response to be
    # emitted.
    # ... create page logic ...

def publish_page(request):
    # ... publish page logic ...

Authorization configuration in detail

Configure knave by defining roles and permissions used for authorization:

from knave.acl import Role, Permission, ACL
from knave.roles import StaticRoleProvider

# These are the roles available to users:
ADMIN = Role('admin')
EDITOR = Role('editor')
WRITER = Role('writer')
OWNER = Role('owner')

MANAGE_USERS = Permission('user_manage')
PUBLISH_PAGE = Permission('publish_page')
CREATE_PAGE = Permission('create_page')
EDIT_PAGE = Permission('edit_page')

Then map permissions to the roles you want to authorize:

role_permssions = {

Then tell knave which users have which roles. The simplest way to do this a static mapping of user names to roles:

role_provider = StaticRoleProvider({
    'spike': {ADMIN},
    'tom': {EDITOR, WRITER}
    'jerry': {WRITER}

Finally, link all this together in an ACL:

acl = ACL([role_provider], role_permssions)

For most applications you will store roles in a database Once you have created an ACL, use the ACL.role_provider decorator to add functions that look up roles:

class DBRoleProvider(RoleProvider):

    def member_subset(self, roles, identity, context=None):
        cursor = conn.cursor()
        cursor.execute("SELECT role FROM user_role WHERE user_id=?",
        return roles - set(row[0] for row in cursor.fetchall())

You can also create functions to determine roles:

def is_owner(identity, context):
    return context and == identity

WSGI Middleware

You should use knave.middleware.KnaveMiddleware to enable authorization in your WSGI application:

from knave import KnaveMiddleware

app = KnaveMiddleware(app, acl)

This middleware makes it possible to access the ACL from within a WSGI request, eg:

def wsgi_app(environ, start_response):

    if ACL.of(environ).test(Permissions.USER_MANAGE):

The middleware also catches any knave.predicates.Unauthorized exceptions and returns an HTTP 401 response instead.

Integrating with an authentication system

By default knave looks at the REMOTE_USER environ key to retrieve the identity of the current user.

Change this behaviour by supplying a different identity_adapter to your ACL.

If you are using repoze.who, there is a built in adapter for this:

import knave.identity
acl = ACL(..., identity_adapter=knave.identity.RepozeWhoIdentityAdapter())

If you have a custom authentication layer, you may need to write your own IdentityAdapter. Here’s an example for an authentication system where the user id is saved in the session (using beaker sessions):

from knave.identity import IdentityAdapter

class SessionIdentityAdapter(IdentityAdapter):
    Extract the user identity from the current session
    def __call__(self, environ):
        return environ['beaker.session'].get('current_user')


acl = ACL(..., identity_adapter=SessionIdentityAdapter())

Checking permissions

From your WSGI application you can call ACL.of(environ).test(...) to test a permission:

if not ACL.of(environ).test(Permissions.USER_MANAGE):
    start_response('401 Unauthorized', [('Content-Type', 'text/html')]
    return ['<h1>Sorry, you're not authorized to view this page</h1>']

Or you can call ACL.of(environ).require(...) to raise an unauthorized exception if the permission isn’t met:


knave.middleware.KnaveMiddleware will trap this exception and return an appropriate 401 HTTP response.

Contextual roles and fancy permissions checks

All checks support an optional context argument. You can use this to add roles dynamically.

For example, suppose you have a blogging application that creates BlogEntry objects, which have an author attribute.

You can define a owner role and have it set dynamically so that only the BlogEntry author has the role:

class Permissions:
    ARTICLE_EDIT = Permission('article_edit')

class Roles:
    OWNER = Role('owner')
    ADMIN = Role('admin')

role_permssions = {
    Permissions.ARTICLE_EDIT: {Roles.ADMIN, Roles.OWNER},
role_provider = StaticRoleProvider({
    'spike': {Roles.ADMIN}

class OwnerRoleProvider(RoleProvider):
    "A role provider to tell the ACL when the user has the owner role"

    contextual = True
    determines = {Roles.OWNER}

    def member_subset(self, roles, identity, context=None):

        if context is None or Roles.OWNER not in roles:
            return set()

        if getattr(context, 'author', None) == identity:
            return set(Roles.OWNER)

        return set()

acl = ACL([StaticRoleProvider, OwnerRoleProvider], role_permssions)

Your application code would then need to pass the article object to the permissions check:

blogentry = store.get(BlogEntry, id=request.get('id'))
ACL.of(environ).test(Permissions.ARTICLE_EDIT, context=blogentry)

Note also the contextual = True and determines = {...} lines in the OwnerRoleProvider class. These are optimization hints, telling the system not to bother querying the RoleProvider unless a context object is provided and one of the listed roles is present in the query. You can safely omit these lines, in which case your RoleProvider will be called for every lookup. Note RoleProviders can be called directly, in which case these hints are ignored. Your member_subset logic should still account for cases where context is None, or where it is queried for other roles.

If you want to check for a single role, the @role_decider decorator is a convenient shortcut. The OwnerRoleProvider might have been more concisely written as:

from knave.roles import role_decider

@role_decider(Roles.OWNER, contextual=True)
def is_owner(identity, context=None):
    return context and getattr(context, 'author', None) == identity

Permissions can also implement custom checking logic, for example:

class DaytimePermission(Permission):
    Only allow access during daytime working hours

    def __call__(self, acl, identity, context=None):
        from datetime import datetime
        return (9 <= < 5)

Custom unauthorized responses

By default KnaveMiddleware returns a minimal HTTP 401 Not Authorized response when encountering an Unauthorized exception.

You can change what action to take when an by supplying an unauthorized_response argument to KnaveMiddleware. This must be a WSGI app, and as such can return any suitable response (for example, redirecting to a login page):

def redirect_on_unauthorized(environ, start_response):

    start_response('302 Found',
                   [('Location', '/login'), ('Content-Type', 'text/html')])
    return ['<html><body><a href="/login">Login</a></body></html>']

app = KnaveMiddleware(app,


Upgrading to v0.3

You will need to make the following changes in order to upgrade from previous versions:

Predicate classes have changed their signature. In v0.2 you would have written:

class MyPredicate(Predicate):
    def __call__(self, environ, context=None):

def my_custom_predicate(environ, context=None):

In v0.3 you should to change this to:

class MyPredicate(Predicate):
    def __call__(self, acl, identity, context=None):

def my_custom_predicate(acl, identity, context=None):

RoleProviders also have a different signature. Change from this:

    def member_subset(self, roles, identity, environ, context):

To this:

    def member_subset(self, roles, identity, context):

If your RoleProvider or Predicate depends on information from the WSGI environ, this is no longer directly supported. Your application must now explicitly pass any context information required to evaluate roles or predicates in the context argument.

Testing permissions now always requires an ACL object. Where in 0.2 you would have written this:

if some_other_permission.is_met(environ):

You should now change this to:

from knave import ACL
acl = ACL.of(environ)

if acl.test(some_other_permission):

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