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".
:app:`Pyramid` Uses Thread Local VariablesWhy and How
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` retrieves the thread local :term:`application registry` as a matter of course to find an :term:`authentication policy`. It uses the :func:`pyramid.threadlocal.get_current_registry` function to retrieve the application registry, from which it looks up the authentication policy; it then uses the authentication policy to retrieve the authenticated user id. This is how :app:`Pyramid` allows arbitrary authentication 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 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 data derived from the request around 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 :mod:`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.