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.. index::
single: request processing
single: request
single: router
.. _router_chapter:
Request Processing
==================
Once a :app:`Pyramid` application is up and running, it is ready to
accept requests and return responses.
What happens from the time a :term:`WSGI` request enters a
:app:`Pyramid` application through to the point that
:app:`Pyramid` hands off a response back to WSGI for upstream
processing?
#. A user initiates a request from his browser to the hostname and
port number of the WSGI server used by the :app:`Pyramid`
application.
#. The WSGI server used by the :app:`Pyramid` application passes
the WSGI environment to the ``__call__`` method of the
:app:`Pyramid` :term:`router` object.
#. A :term:`request` object is created based on the WSGI environment.
#. The :term:`application registry` and the :term:`request` object
created in the last step are pushed on to the :term:`thread local`
stack that :app:`Pyramid` uses to allow the functions named
:func:`pyramid.threadlocal.get_current_request` and
:func:`pyramid.threadlocal.get_current_registry` to work.
#. A :class:`pyramid.events.NewRequest` :term:`event` is sent to any
subscribers.
#. If any :term:`route` has been defined within application
configuration, the :app:`Pyramid` :term:`router` calls a
:term:`URL dispatch` "route mapper." The job of the mapper is to
examine the request to determine whether any user-defined
:term:`route` matches the current WSGI environment. The
:term:`router` passes the request as an argument to the mapper.
#. 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 representign the matched dynamic
elements of the request's ``PATH_INFO`` value, the latter contains
the :class:`pyramid.interfaces.IRoute` object representing the
route which matched. The root object associated with the route
found is also generated: if the :term:`route configuration` which
matched has an associated a ``factory`` argument, this factory is
used to generate the root object, otherwise a default :term:`root
factory` is used.
#. If a route match was *not* found, and a ``root_factory`` argument
was passed to the :term:`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.
#. The :app:`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 :term:`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
:ref:`traversal_chapter`. The traverser function returns a
dictionary, which contains a :term:`context` and a :term:`view
name` as well as other ancillary information.
#. 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
:term:`view` code.
#. A :class:`pyramid.events.ContextFound` :term:`event` is
sent to any subscribers.
#. :app:`Pyramid` looks up a :term:`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 :term:`predicate` attributes applied to the view
configuration), :app:`Pyramid` raises a
:class:`pyramid.exceptions.NotFound` exception, which is meant
to be caught by a surrounding exception handler.
#. If a view callable was found, :app:`Pyramid` attempts to call
the view function.
#. If an :term:`authorization policy` is in use, and the view was
protected by a :term:`permission`, :app:`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``, :app:`Pyramid` calls the view callable to obtain a
response. If it returns ``False``, it raises a
:class:`pyramid.exceptions.Forbidden` exception, which is meant
to be called by a surrounding exception handler.
#. If any exception was raised within a :term:`root factory`, by
:term:`traversal`, by a :term:`view callable` or by
:app:`Pyramid` itself (such as when it raises
:class:`pyramid.exceptions.NotFound` or
:class:`pyramid.exceptions.Forbidden`), the router catches the
exception, and attaches it to the request as the ``exception``
attribute. It then attempts to find a :term:`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 :term:`exception view` that matches the exception
cannot be found, the exception is reraised.
#. The following steps occur only when a :term:`response` could be
successfully generated by a normal :term:`view callable` or an
:term:`exception view` callable. :app:`Pyramid` will attempt to execute
any :term:`response callback` functions attached via
:meth:`pyramid.request.Request.add_response_callback`. A
:class:`pyramid.events.NewResponse` :term:`event` is then sent to any
subscribers. The response object's ``app_iter``, ``status``, and
``headerlist`` attributes are then used to generate a WSGI response. The
response is sent back to the upstream WSGI server.
#. :app:`Pyramid` will attempt to execute any :term:`finished
callback` functions attached via
:meth:`pyramid.request.Request.add_finished_callback`.
#. The :term:`thread local` stack is popped.
.. image:: router.png
This is a very high-level overview that leaves out various details.
For more detail about subsystems invoked by the :app:`Pyramid` router
such as traversal, URL dispatch, views, and event processing, see
:ref:`resourcelocation_chapter`, :ref:`views_chapter`, and
:ref:`events_chapter`.
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