=============
Compatibility
=============

Hypothesis does its level best to be compatible with everything you could
possibly need it to be compatible with. Generally you should just try it and
expect it to work. If it doesn't, you can be surprised and check this document
for the details.

-------------------
Hypothesis versions
-------------------

Backwards compatibility is better than backporting fixes, so we use
:ref:`semantic versioning <release-policy>` and only support the most recent
version of Hypothesis.

Documented APIs will not break except between major version bumps.
All APIs mentioned in this documentation are public unless explicitly
noted as provisional, in which case they may be changed in minor releases.
Undocumented attributes, modules, and behaviour may include breaking
changes in patch releases.


.. _deprecation-policy:

------------
Deprecations
------------

Deprecated features will emit warnings for at least six
months, and then be removed in the following major release.

Note however that not all warnings are subject to this grace period;
sometimes we strengthen validation by adding a warning and these may
become errors immediately at a major release.

We use custom exception and warning types, so you can see
exactly where an error came from, or turn only our warnings into errors.

.. autoclass:: hypothesis.errors.HypothesisDeprecationWarning


---------------
Python versions
---------------

Hypothesis is supported and tested on CPython 3.9+, i.e.
`all versions of CPython with upstream support <https://devguide.python.org/versions/>`_,
along with PyPy for the same versions.
32-bit builds of CPython also work, though we only test them on Windows.

In general Hypothesis does not officially support anything except the latest
patch release of any version of Python it supports. Earlier releases should work
and bugs in them will get fixed if reported, but they're not tested in CI and
no guarantees are made.

-----------------
Operating systems
-----------------

In theory Hypothesis should work anywhere that Python does. In practice it is
only known to work and regularly tested on OS X, Windows and Linux, and you may
experience issues running it elsewhere.

If you're using something else and it doesn't work, do get in touch and I'll try
to help, but unless you can come up with a way for me to run a CI server on that
operating system it probably won't stay fixed due to the inevitable march of time.

.. _framework-compatibility:

------------------
Testing frameworks
------------------

In general Hypothesis goes to quite a lot of effort to generate things that
look like normal Python test functions that behave as closely to the originals
as possible, so it should work sensibly out of the box with every test framework.

If your testing relies on doing something other than calling a function and seeing
if it raises an exception then it probably *won't* work out of the box. In particular
things like tests which return generators and expect you to do something with them
(e.g. nose's yield based tests) will not work. Use a decorator or similar to wrap the
test to take this form, or ask the framework maintainer to support our
:ref:`hooks for inserting such a wrapper later <custom-function-execution>`.

In terms of what's actually *known* to work:

* Hypothesis integrates as smoothly with pytest and unittest as we can make it,
  and this is verified as part of the CI.
* :pypi:`pytest` fixtures work in the usual way for tests that have been decorated
  with |@given| - just avoid passing a strategy for
  each argument that will be supplied by a fixture.  However, function-scoped fixtures
  will run only once for the whole function, not per example. To proactively warn you about
  this case, we raise |HealthCheck.function_scoped_fixture|, unless suppressed with
  |settings.suppress_health_check|.
* The :func:`python:unittest.mock.patch` decorator works with
  |@given|, but we recommend using it as a context
  manager within the decorated test to ensure that the mock is per-test-case
  and avoid poor interactions with Pytest fixtures.
* Nose works fine with Hypothesis, and this is tested as part of the CI. ``yield`` based
  tests simply won't work.
* Integration with Django's testing requires use of the :ref:`hypothesis-django` extra.
  The issue is that in Django's tests' normal mode of execution it will reset the
  database once per test rather than once per example, which is not what you want.
* :pypi:`coverage` works out of the box with Hypothesis; our own test suite has
  100% branch coverage.

-----------------
Optional packages
-----------------

The supported versions of optional packages, for strategies in ``hypothesis.extra``,
are listed in the documentation for that extra.  Our general goal is to support
all versions that are supported upstream.


.. _thread-safety-policy:

--------------------
Thread-Safety Policy
--------------------

As discussed in :issue:`2719`, Hypothesis is not truly thread-safe and that's unlikely to change in the future.  This policy therefore describes what you *can* expect if you use Hypothesis with multiple threads.

**Running tests in multiple processes**, e.g. with ``pytest -n auto``, is fully supported and we test this regularly in CI - thanks to process isolation, we only need to ensure that :class:`~hypothesis.database.DirectoryBasedExampleDatabase` can't tread on its own toes too badly.  If you find a bug here we will fix it ASAP.

**Running separate tests in multiple threads** is not something we design or test for, and is not formally supported.  That said, anecdotally it does mostly work and we would like it to keep working - we accept reasonable patches and low-priority bug reports.  The main risks here are global state, shared caches, and cached strategies.

**Running the same test in multiple threads**, or using multiple threads within the same test, makes it pretty easy to trigger internal errors.  We usually accept patches for such issues unless readability or single-thread performance suffer.

Hypothesis assumes that tests are single-threaded, or do a sufficiently-good job of pretending to be single-threaded.  Tests that use helper threads internally should be OK, but the user must be careful to ensure that test outcomes are still deterministic. In particular it counts as nondeterministic if helper-thread timing changes the sequence of dynamic draws using e.g. the |st.data| strategy.

Interacting with any Hypothesis APIs from helper threads might do weird/bad things, so avoid that too - we rely on thread-local variables in a few places, and haven't explicitly tested/audited how they respond to cross-thread API calls.  While |st.data| and equivalents are the most obvious danger, other APIs might also be subtly affected.

----------
Type hints
----------

We ship type hints with Hypothesis itself. Though we always try to minimize breakage, we may make breaking changes to these between minor releases and do not commit to maintaining a fully stable interface for type hints.

We may also find more precise ways to describe the type of various interfaces, or change their type and runtime behaviour together in a way which is otherwise backwards-compatible.

There are known issues with inferring the type of examples generated by |st.deferred|, |st.recursive|, |st.one_of|, |st.dictionaries|, and |st.fixed_dictionaries|. We're following proposed updates to Python's typing standards, but unfortunately the long-standing interfaces of these strategies cannot (yet) be statically typechecked.
