.. index::
   single: thread locals
   single: get_current_request
   single: get_current_registry

.. _threadlocals_chapter:

Thread Locals
=============

A :term:`thread local` variable is a variable that appears to be a "global"
variable to an application which uses it.  However, unlike a true global
variable, one thread or process serving the application may receive a different
value than another thread or process when that variable is "thread local".

When a request is processed, :app:`Pyramid` makes two :term:`thread local`
variables available to the application: a "registry" and a "request".

Why and How :app:`Pyramid` Uses Thread Local Variables
------------------------------------------------------

How are thread locals beneficial to :app:`Pyramid` and application developers
who use :app:`Pyramid`?  Well, usually they're decidedly **not**.  Using a
global or a thread local variable in any application usually makes it a lot
harder to understand for a casual reader.  Use of a thread local or a global is
usually just a way to avoid passing some value around between functions, which
is itself usually a very bad idea, at least if code readability counts as an
important concern.

For historical reasons, however, thread local variables are indeed consulted by
various :app:`Pyramid` API functions.  For example, the implementation of the
:mod:`pyramid.security` function named
:func:`~pyramid.security.authenticated_userid` (deprecated as of 1.5) retrieves
the thread local :term:`application registry` as a matter of course to find an
:term:`security policy`.  It uses the
:func:`pyramid.threadlocal.get_current_registry` function to retrieve the
application registry, from which it looks up the security policy; it then
uses the security policy to retrieve the authenticated user id.  This is
how :app:`Pyramid` allows arbitrary security policies to be "plugged in".

When they need to do so, :app:`Pyramid` internals use two API functions to
retrieve the :term:`request` and :term:`application registry`:
:func:`~pyramid.threadlocal.get_current_request` and
:func:`~pyramid.threadlocal.get_current_registry`.  The former returns the
"current" request; the latter returns the "current" registry.  Both
``get_current_*`` functions retrieve an object from a thread-local data
structure.  These API functions are documented in :ref:`threadlocal_module`.

These values are thread locals rather than true globals because one Python
process may be handling multiple simultaneous requests or even multiple
:app:`Pyramid` applications.  If they were true globals, :app:`Pyramid` could
not handle multiple simultaneous requests or allow more than one :app:`Pyramid`
application instance to exist in a single Python process.

Because one :app:`Pyramid` application is permitted to call *another*
:app:`Pyramid` application from its own :term:`view` code (perhaps as a
:term:`WSGI` app with help from the :func:`pyramid.wsgi.wsgiapp2` decorator),
these variables are managed in a *stack* during normal system operations.  The
stack instance itself is a :class:`threading.local`.

During normal operations, the thread locals stack is managed by a
:term:`Router` object.  At the beginning of a request, the Router pushes the
application's registry and the request on to the stack.  At the end of a
request, the stack is popped.  The topmost request and registry on the stack
are considered "current".  Therefore, when the system is operating normally,
the very definition of "current" is defined entirely by the behavior of a
pyramid :term:`Router`.

However, during unit testing, no Router code is ever invoked, and the
definition of "current" is defined by the boundary between calls to the
:meth:`pyramid.config.Configurator.begin` and
:meth:`pyramid.config.Configurator.end` methods (or between calls to the
:func:`pyramid.testing.setUp` and :func:`pyramid.testing.tearDown` functions).
These functions push and pop the threadlocal stack when the system is under
test.  See :ref:`test_setup_and_teardown` for the definitions of these
functions.

Scripts which use :app:`Pyramid` machinery but never actually start a WSGI
server or receive requests via HTTP, such as scripts which use the
:mod:`pyramid.scripting` API, will never cause any Router code to be executed.
However, the :mod:`pyramid.scripting` APIs also push some values on to the
thread locals stack as a matter of course. Such scripts should expect the
:func:`~pyramid.threadlocal.get_current_request` function to always return
``None``, and should expect the
:func:`~pyramid.threadlocal.get_current_registry` function to return exactly
the same :term:`application registry` for every request.

Why You Shouldn't Abuse Thread Locals
-------------------------------------

You probably should almost never use the
:func:`~pyramid.threadlocal.get_current_request` or
:func:`~pyramid.threadlocal.get_current_registry` functions, except perhaps in
tests.  In particular, it's almost always a mistake to use
``get_current_request`` or ``get_current_registry`` in application code because
its usage makes it possible to write code that can be neither easily tested nor
scripted.  Inappropriate usage is defined as follows:

- ``get_current_request`` should never be called within the body of a
  :term:`view callable`, or within code called by a view callable. View
  callables already have access to the request (it's passed in to each as
  ``request``).

- ``get_current_request`` should never be called in :term:`resource` code. If a
  resource needs access to the request, it should be passed the request by a
  :term:`view callable`.

- ``get_current_request`` function should never be called because it's "easier"
  or "more elegant" to think about calling it than to pass a request through a
  series of function calls when creating some API design.  Your application
  should instead, almost certainly, pass around data derived from the request
  rather than relying on being able to call this function to obtain the request
  in places that actually have no business knowing about it.  Parameters are
  *meant* to be passed around as function arguments; this is why they exist.
  Don't try to "save typing" or create "nicer APIs" by using this function in
  the place where a request is required; this will only lead to sadness later.

- Neither ``get_current_request`` nor ``get_current_registry`` should ever be
  called within application-specific forks of third-party library code.  The
  library you've forked almost certainly has nothing to do with :app:`Pyramid`,
  and making it dependent on :app:`Pyramid` (rather than making your
  :app:`pyramid` application depend upon it) means you're forming a dependency
  in the wrong direction.

Use of the :func:`~pyramid.threadlocal.get_current_request` function in
application code *is* still useful in very limited circumstances. As a rule of
thumb, usage of ``get_current_request`` is useful **within code which is meant
to eventually be removed**.  For instance, you may find yourself wanting to
deprecate some API that expects to be passed a request object in favor of one
that does not expect to be passed a request object.  But you need to keep
implementations of the old API working for some period of time while you
deprecate the older API.  So you write a "facade" implementation of the new API
which calls into the code which implements the older API.  Since the new API
does not require the request, your facade implementation doesn't have local
access to the request when it needs to pass it into the older API
implementation.  After some period of time, the older implementation code is
disused and the hack that uses ``get_current_request`` is removed.  This would
be an appropriate place to use the ``get_current_request``.

Use of the :func:`~pyramid.threadlocal.get_current_registry` function should be
limited to testing scenarios.  The registry made current by use of the
:meth:`pyramid.config.Configurator.begin` method during a test (or via
:func:`pyramid.testing.setUp`) when you do not pass one in is available to you
via this API.
