Request Processing

Once a Pyramid application is up and running, it is ready to accept requests and return responses.

What happens from the time a WSGI request enters a Pyramid application through to the point that Pyramid hands off a response back to WSGI for upstream processing?

1. A user initiates a request from his browser to the hostname and port number of the WSGI server used by the Pyramid application.
2. The WSGI server used by the Pyramid application passes the WSGI environment to the __call__ method of the Pyramid router object.
3. A request object is created based on the WSGI environment.
4. The application registry and the request object created in the last step are pushed on to the thread local stack that Pyramid uses to allow the functions named get_current_request() and get_current_registry() to work.
5. A NewRequest event is sent to any subscribers.
6. If any route has been defined within application configuration, the Pyramid router calls a URL dispatch “route mapper.” The job of the mapper is to examine the request to determine whether any user-defined route matches the current WSGI environment. The router passes the request as an argument to the mapper.
7. If any route matches, the request is mutated; a matchdict and matched_route attributes are added to the request object; the former contains a dictionary representing the matched dynamic elements of the request’s PATH_INFO value, the latter contains the IRoute object representing the route which matched. The root object associated with the route found is also generated: if the route configuration which matched has an associated a factory argument, this factory is used to generate the root object, otherwise a default root factory is used.
8. If a route match was not found, and a root_factory argument was passed to the Configurator constructor, that callable is used to generate the root object. If the root_factory argument passed to the Configurator constructor was None, a default root factory is used to generate a root object.
9. The Pyramid router calls a “traverser” function with the root object and the request. The traverser function attempts to traverse the root object (using any existing __getitem__ on the root object and subobjects) to find a context. If the root object has no __getitem__ method, the root itself is assumed to be the context. The exact traversal algorithm is described in Traversal. The traverser function returns a dictionary, which contains a context and a view name as well as other ancillary information.
10. The request is decorated with various names returned from the traverser (such as context, view_name, and so forth), so they can be accessed via e.g. request.context within view code.
11. A ContextFound event is sent to any subscribers.
12. Pyramid looks up a view callable using the context, the request, and the view name. If a view callable doesn’t exist for this combination of objects (based on the type of the context, the type of the request, and the value of the view name, and any predicate attributes applied to the view configuration), Pyramid raises a HTTPNotFound exception, which is meant to be caught by a surrounding exception view.
13. If a view callable was found, Pyramid attempts to call the view function.
14. If an authorization policy is in use, and the view was protected by a permission, Pyramid passes the context, the request, and the view_name to a function which determines whether the view being asked for can be executed by the requesting user, based on credential information in the request and security information attached to the context. If it returns True, Pyramid calls the view callable to obtain a response. If it returns False, it raises a HTTPForbidden exception, which is meant to be called by a surrounding exception view.
15. If any exception was raised within a root factory, by traversal, by a view callable or by Pyramid itself (such as when it raises HTTPNotFound or HTTPForbidden), the router catches the exception, and attaches it to the request as the exception attribute. It then attempts to find a exception view for the exception that was caught. If it finds an exception view callable, that callable is called, and is presumed to generate a response. If an exception view that matches the exception cannot be found, the exception is reraised.
16. The following steps occur only when a response could be successfully generated by a normal view callable or an exception view callable. Pyramid will attempt to execute any response callback functions attached via add_response_callback(). A NewResponse event is then sent to any subscribers. The response object’s __call__ method is then used to generate a WSGI response. The response is sent back to the upstream WSGI server.
17. Pyramid will attempt to execute any finished callback functions attached via add_finished_callback().
18. The thread local stack is popped.

This is a very high-level overview that leaves out various details. For more detail about subsystems invoked by the Pyramid router such as traversal, URL dispatch, views, and event processing, see URL Dispatch, Views, and Using Events.