knave 0.2

Configuration

To start using knave, you need to define roles and permissions:

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

class Permissions:

    #: Can manage user accounts
    USER_MANAGE = Permission('user_manage')

    #: Can author articles
    ARTICLE_CREATE = Permission('article_create')

    #: Can publish articles
    ARTICLE_PUBLISH = Permission('article_publish')

class Roles:

    ADMINS = Role('admins')
    EDITORS = Role('editors')

Then you can map permissions to the roles that should be authorized for them:

role_permssions = {
    Permissions.USER_MANAGE: {Roles.ADMINS},
    Permissions.ARTICLE_CREATE: {Roles.ADMINS, Roles.EDITORS},
    Permissions.ARTICLE_PUBLISH: {Roles.ADMINS, Roles.EDITORS},
}

Finally, you need to tell knave which users belong to which roles. The demonstration purposes we’ve used a static mapping of user names to roles:

role_provider = StaticRoleProvider({
    'spike': {Roles.ADMINS},
    'harry': {Roles.EDITORS}
})

You can subclass knave.roles.RoleProvider to look up role membership from a dynamic source such as a database.

With everything defined, you can link all this together in an ACL:

acl = ACL([role_provider], role_permssions)

You should use knave.middleware.KnaveMiddleware to link the ACL into your WSGI application:

from knave import KnaveMiddleware

app = KnaveMiddleware(app, acl)

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

def wsgi_app(environ, start_response):
    ...

    Permissions('user_manage').check(environ)

The middleware also takes care of catching any knave.predicates.Unauthorized exceptions and returning an HTTP 401 response instead.

Integrating with an authentication system

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

You can change this behaviour by supplying a different identity_adapter when configuring 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 permission(environ) to test a permission:

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

Or you can call permission.check(environ) to test the permission and raise an unauthorized exception if it isn’t met:

Permissions.user_manage.check(environ)

knave.middleware.KnaveMiddleware will trap this exception and return the appropriate WSGI response.

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"

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

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

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,
                      acl,
                      unauthorized_response=redirect_on_unauthorized)