CHAPTER

TWENTYSEVEN

SECURITY

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:

•A request is generated when a user visits the application.

•Based on the request, a context resource is located through resource location. A context is located differently depending on whether the application uses traversal or URL dispatch, but a context is ultimately found in either case. See the URL Dispatch chapter for more information.

•A view callable is located by view lookup using the context as well as other attributes of the request.

•If an authentication policy is in effect, it is passed the request; it returns some number of principal identifiers.

•If an authorization policy is in effect and the view configuration associated with the view callable that was found has a permission associated with it, the authorization policy is passed the context, some number of principal identifiers returned by the authentication policy, and the permission associated with the view; it will allow or deny access.

•If the authorization policy allows access, the view callable is invoked.

•If the authorization policy denies access, the view callable is not invoked; instead the forbidden view is invoked.

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.

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