Fetching contributors…
Cannot retrieve contributors at this time
332 lines (265 sloc) 13.3 KB
.. index::
   single: subrequest

Invoking a Subrequest

.. versionadded:: 1.4

:app:`Pyramid` allows you to invoke a subrequest at any point during the processing of a request. Invoking a subrequest allows you to obtain a :term:`response` object from a view callable within your :app:`Pyramid` application while you're executing a different view callable within the same application.

Here's an example application which uses a subrequest:

When /view_one is visted in a browser, the text printed in the browser pane will be This came from view_two. The view_one view used the :meth:`pyramid.request.Request.invoke_subrequest` API to obtain a response from another view (view_two) within the same application when it executed. It did so by constructing a new request that had a URL that it knew would match the view_two view registration, and passed that new request along to :meth:`pyramid.request.Request.invoke_subrequest`. The view_two view callable was invoked, and it returned a response. The view_one view callable then simply returned the response it obtained from the view_two view callable.

Note that it doesn't matter if the view callable invoked via a subrequest actually returns a literal Response object. Any view callable that uses a renderer or which returns an object that can be interpreted by a response adapter when found and invoked via :meth:`pyramid.request.Request.invoke_subrequest` will return a Response object:

Even though the view_two view callable returned a string, it was invoked in such a way that the string renderer associated with the view registration that was found turned it into a "real" response object for consumption by view_one.

Being able to unconditionally obtain a response object by invoking a view callable indirectly is the main advantage to using :meth:`pyramid.request.Request.invoke_subrequest` instead of simply importing a view callable and executing it directly. Note that there's not much advantage to invoking a view using a subrequest if you can invoke a view callable directly. Subrequests are slower and are less convenient if you actually do want just the literal information returned by a function that happens to be a view callable.

Note that, by default, if a view callable invoked by a subrequest raises an exception, the exception will be raised to the caller of :meth:`~pyramid.request.Request.invoke_subrequest` even if you have a :term:`exception view` configured:

When we run the above code and visit /view_one in a browser, the excview :term:`exception view` will not be executed. Instead, the call to :meth:`~pyramid.request.Request.invoke_subrequest` will cause a :exc:`ValueError` exception to be raised and a response will never be generated. We can change this behavior; how to do so is described below in our discussion of the use_tweens argument.

.. index::
   pair: subrequest; use_tweens

Subrequests with Tweens

The :meth:`pyramid.request.Request.invoke_subrequest` API accepts two arguments: a required positional argument request, and an optional keyword argument use_tweens which defaults to False.

The request object passed to the API must be an object that implements the Pyramid request interface (such as a :class:`pyramid.request.Request` instance). If use_tweens is True, the request will be sent to the :term:`tween` in the tween stack closest to the request ingress. If use_tweens is False, the request will be sent to the main router handler, and no tweens will be invoked.

In the example above, the call to :meth:`~pyramid.request.Request.invoke_subrequest` will always raise an exception. This is because it's using the default value for use_tweens, which is False. Alternatively, you can pass use_tweens=True to ensure that it will convert an exception to a Response if an :term:`exception view` is configured, instead of raising the exception. This is because exception views are called by the exception view :term:`tween` as described in :ref:`exception_views` when any view raises an exception.

We can cause the subrequest to be run through the tween stack by passing use_tweens=True to the call to :meth:`~pyramid.request.Request.invoke_subrequest`, like this:

In the above case, the call to request.invoke_subrequest(subreq) will not raise an exception. Instead, it will retrieve a "500" response from the attempted invocation of view_two, because the tween which invokes an exception view to generate a response is run, and therefore excview is executed.

This is one of the major differences between specifying the use_tweens=True and use_tweens=False arguments to :meth:`~pyramid.request.Request.invoke_subrequest`. use_tweens=True may also imply invoking a transaction commit or abort for the logic executed in the subrequest if you've got pyramid_tm in the tween list, injecting debug HTML if you've got pyramid_debugtoolbar in the tween list, and other tween-related side effects as defined by your particular tween list.

The :meth:`~pyramid.request.Request.invoke_subrequest` function also unconditionally does the following:

The invocation of a subrequest has more or less exactly the same effect as the invocation of a request received by the :app:`Pyramid` router from a web client when use_tweens=True. When use_tweens=False, the tweens are skipped but all the other steps take place.

It's a poor idea to use the original request object as an argument to :meth:`~pyramid.request.Request.invoke_subrequest`. You should construct a new request instead as demonstrated in the above example, using :meth:`pyramid.request.Request.blank`. Once you've constructed a request object, you'll need to massage it to match the view callable that you'd like to be executed during the subrequest. This can be done by adjusting the subrequest's URL, its headers, its request method, and other attributes. The documentation for :class:`pyramid.request.Request` exposes the methods you should call and attributes you should set on the request that you create, then massage it into something that will actually match the view you'd like to call via a subrequest.

We've demonstrated use of a subrequest from within a view callable, but you can use the :meth:`~pyramid.request.Request.invoke_subrequest` API from within a tween or an event handler as well. Even though you can do it, it's usually a poor idea to invoke :meth:`~pyramid.request.Request.invoke_subrequest` from within a tween, because tweens already, by definition, have access to a function that will cause a subrequest (they are passed a handle function). It's fine to invoke :meth:`~pyramid.request.Request.invoke_subrequest` from within an event handler, however.

.. index::
   pair: subrequest; exception view

Invoking an Exception View

.. versionadded:: 1.7

:app:`Pyramid` apps may define :term:`exception views <exception view>` which can handle any raised exceptions that escape from your code while processing a request. By default an unhandled exception will be caught by the EXCVIEW :term:`tween`, which will then lookup an exception view that can handle the exception type, generating an appropriate error response.

In :app:`Pyramid` 1.7 the :meth:`pyramid.request.Request.invoke_exception_view` was introduced, allowing a user to invoke an exception view while manually handling an exception. This can be useful in a few different circumstances:

  • Manually handling an exception losing the current call stack or flow.
  • Handling exceptions outside of the context of the EXCVIEW tween. The tween only covers certain parts of the request processing pipeline (See :ref:`router_chapter`). There are also some corner cases where an exception can be raised that will still bubble up to middleware, and possibly to the web server in which case a generic 500 Internal Server Error will be returned to the client.

Below is an example usage of :meth:`pyramid.request.Request.invoke_exception_view`:

Please note that in most cases you do not need to write code like this, and you may rely on the EXCVIEW tween to handle this for you.