Pyramid provides an optional declarative authorization system that can prevent a view from being invoked based on an authorization policy. Before a view is invoked, the authorization system can use the credentials in the request along with the context resource to determine if access will be allowed. Here’s how it works at a high level:
Security in Pyramid, unlike many systems, cleanly and explicitly separates authentication and authorization. Authentication is merely the mechanism by which credentials provided in the request are resolved to one or more principal identifiers. These identifiers represent the users and groups in effect during the request. Authorization then determines access based on the principal identifiers, the view callable being invoked, and the context resource.
Authorization is enabled by modifying your application to include an authentication policy and authorization policy. Pyramid comes with a variety of implementations of these policies. To provide maximal flexibility, Pyramid also allows you to create custom authentication policies and authorization policies.
To protect a view callable from invocation based on a user’s security settings when a particular type of resource becomes the context, you must pass a permission to view configuration. Permissions are usually just strings, and they have no required composition: you can name permissions whatever you like.
For example, the following view declaration protects the view named add_entry.html when the context resource is of type Blog with the add permission using the pyramid.config.Configurator.add_view() API:
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# config is an instance of pyramid.config.Configurator config.add_view('mypackage.views.blog_entry_add_view', name='add_entry.html', context='mypackage.resources.Blog', permission='add')
The equivalent view registration including the add permission name may be performed via the @view_config decorator:
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from pyramid.view import view_config from resources import Blog @view_config(context=Blog, name='add_entry.html', permission='add') def blog_entry_add_view(request): """ Add blog entry code goes here """ pass
As a result of any of these various view configuration statements, if an authorization policy is in place when the view callable is found during normal application operations, the requesting user will need to possess the add permission against the context resource in order to be able to invoke the blog_entry_add_view view. If he does not, the Forbidden view will be invoked.
If a permission is not supplied to a view configuration, the registered view will always be executable by entirely anonymous users: any authorization policy in effect is ignored.
In support of making it easier to configure applications which are “secure by default”, Pyramid allows you to configure a default permission. If supplied, the default permission is used as the permission string to all view registrations which don’t otherwise name a permission argument.
These APIs are in support of configuring a default permission for an application:
When a default permission is registered:
When you register a default permission, all views (even exception view views) are protected by a permission. For all views which are truly meant to be anonymously accessible, you will need to associate the view’s configuration with the __no_permission_required__ permission.
When the default Pyramid authorization policy determines whether a user possesses a particular permission with respect to a resource, it examines the ACL associated with the resource. An ACL is associated with a resource by adding an __acl__ attribute to the resource object. This attribute can be defined on the resource instance if you need instance-level security, or it can be defined on the resource class if you just need type-level security.
For example, an ACL might be attached to the resource for a blog via its class:
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from pyramid.security import Everyone from pyramid.security import Allow class Blog(object): __acl__ = [ (Allow, Everyone, 'view'), (Allow, 'group:editors', 'add'), (Allow, 'group:editors', 'edit'), ]
Or, if your resources are persistent, an ACL might be specified via the __acl__ attribute of an instance of a resource:
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from pyramid.security import Everyone from pyramid.security import Allow class Blog(object): pass blog = Blog() blog.__acl__ = [ (Allow, Everyone, 'view'), (Allow, 'group:editors', 'add'), (Allow, 'group:editors', 'edit'), ]
Whether an ACL is attached to a resource’s class or an instance of the resource itself, the effect is the same. It is useful to decorate individual resource instances with an ACL (as opposed to just decorating their class) in applications such as “CMS” systems where fine-grained access is required on an object-by-object basis.
Here’s an example ACL:
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from pyramid.security import Everyone from pyramid.security import Allow __acl__ = [ (Allow, Everyone, 'view'), (Allow, 'group:editors', 'add'), (Allow, 'group:editors', 'edit'), ]
The example ACL indicates that the pyramid.security.Everyone principal – a special system-defined principal indicating, literally, everyone – is allowed to view the blog, the group:editors principal is allowed to add to and edit the blog.
Each element of an ACL is an ACE or access control entry. For example, in the above code block, there are three ACEs: (Allow, Everyone, 'view'), (Allow, 'group:editors', 'add'), and (Allow, 'group:editors', 'edit').
The first element of any ACE is either pyramid.security.Allow, or pyramid.security.Deny, representing the action to take when the ACE matches. The second element is a principal. The third argument is a permission or sequence of permission names.
A principal is usually a user id, however it also may be a group id if your authentication system provides group information and the effective authentication policy policy is written to respect group information. For example, the pyramid.authentication.RepozeWho1AuthenicationPolicy respects group information if you configure it with a callback.
Each ACE in an ACL is processed by an authorization policy in the order dictated by the ACL. So if you have an ACL like this:
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from pyramid.security import Everyone from pyramid.security import Allow from pyramid.security import Deny __acl__ = [ (Allow, Everyone, 'view'), (Deny, Everyone, 'view'), ]
The default authorization policy will allow everyone the view permission, even though later in the ACL you have an ACE that denies everyone the view permission. On the other hand, if you have an ACL like this:
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from pyramid.security import Everyone from pyramid.security import Allow from pyramid.security import Deny __acl__ = [ (Deny, Everyone, 'view'), (Allow, Everyone, 'view'), ]
The authorization policy will deny everyone the view permission, even though later in the ACL is an ACE that allows everyone.
The third argument in an ACE can also be a sequence of permission names instead of a single permission name. So instead of creating multiple ACEs representing a number of different permission grants to a single group:editors group, we can collapse this into a single ACE, as below.
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from pyramid.security import Everyone from pyramid.security import Allow __acl__ = [ (Allow, Everyone, 'view'), (Allow, 'group:editors', ('add', 'edit')), ]
Literally, everyone, no matter what. This object is actually a string “under the hood” (system.Everyone). Every user “is” the principal named Everyone during every request, even if a security policy is not in use.
Any user with credentials as determined by the current security policy. You might think of it as any user that is “logged in”. This object is actually a string “under the hood” (system.Authenticated).
Special permission names exist in the pyramid.security module. These can be imported for use in ACLs.
An object representing, literally, all permissions. Useful in an ACL like so: (Allow, 'fred', ALL_PERMISSIONS). The ALL_PERMISSIONS object is actually a stand-in object that has a __contains__ method that always returns True, which, for all known authorization policies, has the effect of indicating that a given principal “has” any permission asked for by the system.
A convenience ACE is defined representing a deny to everyone of all permissions in pyramid.security.DENY_ALL. This ACE is often used as the last ACE of an ACL to explicitly cause inheriting authorization policies to “stop looking up the traversal tree” (effectively breaking any inheritance). For example, an ACL which allows only fred the view permission for a particular resource despite what inherited ACLs may say when the default authorization policy is in effect might look like so:
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from pyramid.security import Allow from pyramid.security import DENY_ALL __acl__ = [ (Allow, 'fred', 'view'), DENY_ALL ]
“Under the hood”, the pyramid.security.DENY_ALL ACE equals the following:
from pyramid.security import ALL_PERMISSIONS __acl__ = [ (Deny, Everyone, ALL_PERMISSIONS) ]
While the default authorization policy is in place, if a resource object does not have an ACL when it is the context, its parent is consulted for an ACL. If that object does not have an ACL, its parent is consulted for an ACL, ad infinitum, until we’ve reached the root and there are no more parents left.
In order to allow the security machinery to perform ACL inheritance, resource objects must provide location-awareness. Providing location-awareness means two things: the root object in the resource tree must have a _name__ attribute and a __parent__ attribute.
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class Blog(object): __name__ = '' __parent__ = None
An object with a __parent__ attribute and a __name__ attribute is said to be location-aware. Location-aware objects define an __parent__ attribute which points at their parent object. The root object’s __parent__ is None.
When Pyramid denies a view invocation due to an authorization denial, the special forbidden view is invoked. “Out of the box”, this forbidden view is very plain. See Changing the Forbidden View within Using Hooks for instructions on how to create a custom forbidden view and arrange for it to be called when view authorization is denied.
Pyramid ships with a number of useful out-of-the-box security policies (see pyramid.authentication). However, creating your own authentication policy is often necessary when you want to control the “horizontal and vertical” of how your users authenticate. Doing so is a matter of creating an instance of something that implements the following interface:
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class AuthenticationPolicy(object): """ An object representing a Pyramid authentication policy. """ def authenticated_userid(self, request): """ Return the authenticated userid or ``None`` if no authenticated userid can be found. """ def effective_principals(self, request): """ Return a sequence representing the effective principals including the userid and any groups belonged to by the current user, including 'system' groups such as ``pyramid.security.Everyone`` and ``pyramid.security.Authenticated``. """ def remember(self, request, principal, **kw): """ Return a set of headers suitable for 'remembering' the principal named ``principal`` when set in a response. An individual authentication policy and its consumers can decide on the composition and meaning of **kw. """ def forget(self, request): """ Return a set of headers suitable for 'forgetting' the current user on subsequent requests. """
After you do so, you can pass an instance of such a class into the Configurator class at configuration time as authentication_policy to use it.