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Testing made easier with trio.testing

.. module:: trio.testing

The :mod:`trio.testing` module provides various utilities to make it easier to test trio code. Unlike the other submodules in the :mod:`trio` namespace, :mod:`trio.testing` is not automatically imported when you do import trio; you must import trio.testing explicitly.

Test harness integration

.. decorator:: trio_test

Time and timeouts

:class:`trio.testing.MockClock` is a :class:`` with a few tricks up its sleeve to help you efficiently test code involving timeouts:

  • By default, it starts at time 0, and clock time only advances when you explicitly call :meth:`~MockClock.jump`. This provides an extremely controllable clock for testing.
  • You can set :attr:`~MockClock.rate` to 1.0 if you want it to start running in real time like a regular clock. You can stop and start the clock within a test. You can set :attr:`~MockClock.rate` to 10.0 to make clock time pass at 10x real speed (so e.g. await trio.sleep(10) returns after 1 second).
  • But even more interestingly, you can set :attr:`~MockClock.autojump_threshold` to zero or a small value, and then it will watch the execution of the run loop, and any time things have settled down and everyone's waiting for a timeout, it jumps the clock forward to that timeout. In many cases this allows natural-looking code involving timeouts to be automatically run at near full CPU utilization with no changes. (Thanks to fluxcapacitor for this awesome idea.)
  • And of course these can be mixed and matched at will.

Regardless of these shenanigans, from "inside" trio the passage of time still seems normal so long as you restrict yourself to trio's time functions (see :ref:`time-and-clocks`). Below is an example demonstrating two different ways of making time pass quickly. Notice how in both cases, the two tasks keep a consistent view of reality and events happen in the expected order, despite being wildly divorced from real time:

.. literalinclude:: reference-testing/


.. literalinclude:: reference-testing/across-realtime.out
   :language: none

.. autoclass:: MockClock

Inter-task ordering

.. autoclass:: Sequencer

.. autofunction:: wait_all_tasks_blocked


Connecting to an in-process socket server

.. autofunction:: open_stream_to_socket_listener

Virtual, controllable streams

One particularly challenging problem when testing network protocols is making sure that your implementation can handle data whose flow gets broken up in weird ways and arrives with weird timings: localhost connections tend to be much better behaved than real networks, so if you only test on localhost then you might get bitten later. To help you out, trio provides some fully in-memory implementations of the stream interfaces (see :ref:`abstract-stream-api`), that let you write all kinds of interestingly evil tests.

There are a few pieces here, so here's how they fit together:

:func:`memory_stream_pair` gives you a pair of connected, bidirectional streams. It's like :func:`socket.socketpair`, but without any involvement from that pesky operating system and its networking stack.

To build a bidirectional stream, :func:`memory_stream_pair` uses two unidirectional streams. It gets these by calling :func:`memory_stream_one_way_pair`.

:func:`memory_stream_one_way_pair`, in turn, is implemented using the low-ish level classes :class:`MemorySendStream` and :class:`MemoryReceiveStream`. These are implementations of (you guessed it) :class:`` and :class:`` that on their own, aren't attached to anything – "sending" and "receiving" just put data into and get data out of a private internal buffer that each object owns. They also have some interesting hooks you can set, that let you customize the behavior of their methods. This is where you can insert the evil, if you want it. :func:`memory_stream_one_way_pair` takes advantage of these hooks in a relatively boring way: it just sets it up so that when you call sendall, or when you close the send stream, then it automatically triggers a call to :func:`memory_stream_pump`, which is a convenience function that takes data out of a :class:`MemorySendStream`´s buffer and puts it into a :class:`MemoryReceiveStream`´s buffer. But that's just the default – you can replace this with whatever arbitrary behavior you want.

Trio also provides some specialized functions for testing completely unbuffered streams: :func:`lockstep_stream_one_way_pair` and :func:`lockstep_stream_pair`. These aren't customizable, but they do exhibit an extreme kind of behavior that's good at catching out edge cases in protocol implementations.

API details

.. autoclass:: MemorySendStream

.. autoclass:: MemoryReceiveStream

.. autofunction:: memory_stream_pump

.. autofunction:: memory_stream_one_way_pair

.. autofunction:: memory_stream_pair

.. autofunction:: lockstep_stream_one_way_pair

.. autofunction:: lockstep_stream_pair

Testing custom stream implementations

Trio also provides some functions to help you test your custom stream implementations:

.. autofunction:: check_one_way_stream

.. autofunction:: check_two_way_stream

.. autofunction:: check_half_closeable_stream

Virtual networking for testing

In the previous section you learned how to use virtual in-memory streams to test protocols that are written against trio's :class:`` abstraction. But what if you have more complicated networking code – the kind of code that makes connections to multiple hosts, or opens a listening socket, or sends UDP packets?

Trio doesn't itself provide a virtual in-memory network implementation for testing – but :mod:`trio.socket` module does provide the hooks you need to write your own! And if you're interested in helping implement a reusable virtual network for testing, then please get in touch.

Note that these APIs are actually in :mod:`trio.socket` and :mod:``, but we document them here because they're primarily intended for testing.

.. currentmodule:: trio.socket

.. autofunction:: trio.socket.set_custom_hostname_resolver

.. currentmodule::

.. autoclass::

.. currentmodule:: trio.socket

.. autofunction:: trio.socket.set_custom_socket_factory

.. currentmodule::

.. autoclass::

.. currentmodule:: trio.testing

Testing checkpoints

.. autofunction:: assert_checkpoints

.. autofunction:: assert_no_checkpoints