.. _reference-fixtures:
.. _fixture:
.. _fixtures:
.. _`@pytest.fixture`:
.. _`pytest.fixture`:


Fixtures reference
========================================================

.. seealso:: :ref:`about-fixtures`
.. seealso:: :ref:`how-to-fixtures`

.. _`Dependency injection`: https://en.wikipedia.org/wiki/Dependency_injection


Built-in fixtures
-----------------

:ref:`Fixtures <fixtures-api>` are defined using the :ref:`@pytest.fixture
<pytest.fixture-api>` decorator. Pytest has several useful built-in fixtures:

   :fixture:`capfd`
        Capture, as text, output to file descriptors ``1`` and ``2``.

   :fixture:`capfdbinary`
        Capture, as bytes, output to file descriptors ``1`` and ``2``.

   :fixture:`caplog`
        Control logging and access log entries.

   :fixture:`capsys`
        Capture, as text, output to ``sys.stdout`` and ``sys.stderr``.

   :fixture:`capsysbinary`
        Capture, as bytes, output to ``sys.stdout`` and ``sys.stderr``.

   :fixture:`cache`
        Store and retrieve values across pytest runs.

   :fixture:`doctest_namespace`
        Provide a dict injected into the doctests namespace.

   :fixture:`monkeypatch`
       Temporarily modify classes, functions, dictionaries,
       ``os.environ``, and other objects.

   :fixture:`pytestconfig`
        Access to configuration values, pluginmanager and plugin hooks.

   :fixture:`record_property`
       Add extra properties to the test.

   :fixture:`record_testsuite_property`
       Add extra properties to the test suite.

   :fixture:`recwarn`
        Record warnings emitted by test functions.

   :fixture:`request`
       Provide information on the executing test function.

   :fixture:`testdir`
        Provide a temporary test directory to aid in running, and
        testing, pytest plugins.

   :fixture:`tmp_path`
       Provide a :class:`pathlib.Path` object to a temporary directory
       which is unique to each test function.

   :fixture:`tmp_path_factory`
        Make session-scoped temporary directories and return
        :class:`pathlib.Path` objects.

   :fixture:`tmpdir`
        Provide a `py.path.local <https://py.readthedocs.io/en/latest/path.html>`_ object to a temporary
        directory which is unique to each test function;
        replaced by :fixture:`tmp_path`.

   :fixture:`tmpdir_factory`
        Make session-scoped temporary directories and return
        ``py.path.local`` objects;
        replaced by :fixture:`tmp_path_factory`.


.. _`conftest.py`:
.. _`conftest`:

Fixture availability
---------------------

Fixture availability is determined from the perspective of the test. A fixture
is only available for tests to request if they are in the scope that fixture is
defined in. If a fixture is defined inside a class, it can only be requested by
tests inside that class. But if a fixture is defined inside the global scope of
the module, then every test in that module, even if it's defined inside a class,
can request it.

Similarly, a test can also only be affected by an autouse fixture if that test
is in the same scope that autouse fixture is defined in (see
:ref:`autouse order`).

A fixture can also request any other fixture, no matter where it's defined, so
long as the test requesting them can see all fixtures involved.

For example, here's a test file with a fixture (``outer``) that requests a
fixture (``inner``) from a scope it wasn't defined in:

.. literalinclude:: /example/fixtures/test_fixtures_request_different_scope.py

From the tests' perspectives, they have no problem seeing each of the fixtures
they're dependent on:

.. image:: /example/fixtures/test_fixtures_request_different_scope.*
    :align: center

So when they run, ``outer`` will have no problem finding ``inner``, because
pytest searched from the tests' perspectives.

.. note::
    The scope a fixture is defined in has no bearing on the order it will be
    instantiated in: the order is mandated by the logic described
    :ref:`here <fixture order>`.

``conftest.py``: sharing fixtures across multiple files
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``conftest.py`` file serves as a means of providing fixtures for an entire
directory. Fixtures defined in a ``conftest.py`` can be used by any test
in that package without needing to import them (pytest will automatically
discover them).

You can have multiple nested directories/packages containing your tests, and
each directory can have its own ``conftest.py`` with its own fixtures, adding on
to the ones provided by the ``conftest.py`` files in parent directories.

For example, given a test file structure like this:

::

    tests/
        __init__.py

        conftest.py
            # content of tests/conftest.py
            import pytest

            @pytest.fixture
            def order():
                return []

            @pytest.fixture
            def top(order, innermost):
                order.append("top")

        test_top.py
            # content of tests/test_top.py
            import pytest

            @pytest.fixture
            def innermost(order):
                order.append("innermost top")

            def test_order(order, top):
                assert order == ["innermost top", "top"]

        subpackage/
            __init__.py

            conftest.py
                # content of tests/subpackage/conftest.py
                import pytest

                @pytest.fixture
                def mid(order):
                    order.append("mid subpackage")

            test_subpackage.py
                # content of tests/subpackage/test_subpackage.py
                import pytest

                @pytest.fixture
                def innermost(order, mid):
                    order.append("innermost subpackage")

                def test_order(order, top):
                    assert order == ["mid subpackage", "innermost subpackage", "top"]

The boundaries of the scopes can be visualized like this:

.. image:: /example/fixtures/fixture_availability.*
    :align: center

The directories become their own sort of scope where fixtures that are defined
in a ``conftest.py`` file in that directory become available for that whole
scope.

Tests are allowed to search upward (stepping outside a circle) for fixtures, but
can never go down (stepping inside a circle) to continue their search. So
``tests/subpackage/test_subpackage.py::test_order`` would be able to find the
``innermost`` fixture defined in ``tests/subpackage/test_subpackage.py``, but
the one defined in ``tests/test_top.py`` would be unavailable to it because it
would have to step down a level (step inside a circle) to find it.

The first fixture the test finds is the one that will be used, so
:ref:`fixtures can be overridden <override fixtures>` if you need to change or
extend what one does for a particular scope.

You can also use the ``conftest.py`` file to implement
:ref:`local per-directory plugins <conftest.py plugins>`.

Fixtures from third-party plugins
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Fixtures don't have to be defined in this structure to be available for tests,
though. They can also be provided by third-party plugins that are installed, and
this is how many pytest plugins operate. As long as those plugins are installed,
the fixtures they provide can be requested from anywhere in your test suite.

Because they're provided from outside the structure of your test suite,
third-party plugins don't really provide a scope like `conftest.py` files and
the directories in your test suite do. As a result, pytest will search for
fixtures stepping out through scopes as explained previously, only reaching
fixtures defined in plugins *last*.

For example, given the following file structure:

::

    tests/
        __init__.py

        conftest.py
            # content of tests/conftest.py
            import pytest

            @pytest.fixture
            def order():
                return []

        subpackage/
            __init__.py

            conftest.py
                # content of tests/subpackage/conftest.py
                import pytest

                @pytest.fixture(autouse=True)
                def mid(order, b_fix):
                    order.append("mid subpackage")

            test_subpackage.py
                # content of tests/subpackage/test_subpackage.py
                import pytest

                @pytest.fixture
                def inner(order, mid, a_fix):
                    order.append("inner subpackage")

                def test_order(order, inner):
                    assert order == ["b_fix", "mid subpackage", "a_fix", "inner subpackage"]

If ``plugin_a`` is installed and provides the fixture ``a_fix``, and
``plugin_b`` is installed and provides the fixture ``b_fix``, then this is what
the test's search for fixtures would look like:

.. image:: /example/fixtures/fixture_availability_plugins.svg
    :align: center

pytest will only search for ``a_fix`` and ``b_fix`` in the plugins after
searching for them first in the scopes inside ``tests/``.

.. note:

    pytest can tell you what fixtures are available for a given test if you call
    ``pytests`` along with the test's name (or the scope it's in), and provide
    the ``--fixtures`` flag, e.g. ``pytest --fixtures test_something.py``
    (fixtures with names that start with ``_`` will only be shown if you also
    provide the ``-v`` flag).


.. _`fixture order`:

Fixture instantiation order
---------------------------

When pytest wants to execute a test, once it knows what fixtures will be
executed, it has to figure out the order they'll be executed in. To do this, it
considers 3 factors:

1. scope
2. dependencies
3. autouse

Names of fixtures or tests, where they're defined, the order they're defined in,
and the order fixtures are requested in have no bearing on execution order
beyond coincidence. While pytest will try to make sure coincidences like these
stay consistent from run to run, it's not something that should be depended on.
If you want to control the order, it's safest to rely on these 3 things and make
sure dependencies are clearly established.

Higher-scoped fixtures are executed first
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Within a function request for fixtures, those of higher-scopes (such as
``session``) are executed before lower-scoped fixtures (such as ``function`` or
``class``).

Here's an example:

.. literalinclude:: /example/fixtures/test_fixtures_order_scope.py

The test will pass because the larger scoped fixtures are executing first.

The order breaks down to this:

.. image:: /example/fixtures/test_fixtures_order_scope.*
    :align: center

Fixtures of the same order execute based on dependencies
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When a fixture requests another fixture, the other fixture is executed first.
So if fixture ``a`` requests fixture ``b``, fixture ``b`` will execute first,
because ``a`` depends on ``b`` and can't operate without it. Even if ``a``
doesn't need the result of ``b``, it can still request ``b`` if it needs to make
sure it is executed after ``b``.

For example:

.. literalinclude:: /example/fixtures/test_fixtures_order_dependencies.py

If we map out what depends on what, we get something that looks like this:

.. image:: /example/fixtures/test_fixtures_order_dependencies.*
    :align: center

The rules provided by each fixture (as to what fixture(s) each one has to come
after) are comprehensive enough that it can be flattened to this:

.. image:: /example/fixtures/test_fixtures_order_dependencies_flat.*
    :align: center

Enough information has to be provided through these requests in order for pytest
to be able to figure out a clear, linear chain of dependencies, and as a result,
an order of operations for a given test. If there's any ambiguity, and the order
of operations can be interpreted more than one way, you should assume pytest
could go with any one of those interpretations at any point.

For example, if ``d`` didn't request ``c``, i.e.the graph would look like this:

.. image:: /example/fixtures/test_fixtures_order_dependencies_unclear.*
    :align: center

Because nothing requested ``c`` other than ``g``, and ``g`` also requests ``f``,
it's now unclear if ``c`` should go before/after ``f``, ``e``, or ``d``. The
only rules that were set for ``c`` is that it must execute after ``b`` and
before ``g``.

pytest doesn't know where ``c`` should go in the case, so it should be assumed
that it could go anywhere between ``g`` and ``b``.

This isn't necessarily bad, but it's something to keep in mind. If the order
they execute in could affect the behavior a test is targeting, or could
otherwise influence the result of a test, then the order should be defined
explicitly in a way that allows pytest to linearize/"flatten" that order.

.. _`autouse order`:

Autouse fixtures are executed first within their scope
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Autouse fixtures are assumed to apply to every test that could reference them,
so they are executed before other fixtures in that scope. Fixtures that are
requested by autouse fixtures effectively become autouse fixtures themselves for
the tests that the real autouse fixture applies to.

So if fixture ``a`` is autouse and fixture ``b`` is not, but fixture ``a``
requests fixture ``b``, then fixture ``b`` will effectively be an autouse
fixture as well, but only for the tests that ``a`` applies to.

In the last example, the graph became unclear if ``d`` didn't request ``c``. But
if ``c`` was autouse, then ``b`` and ``a`` would effectively also be autouse
because ``c`` depends on them. As a result, they would all be shifted above
non-autouse fixtures within that scope.

So if the test file looked like this:

.. literalinclude:: /example/fixtures/test_fixtures_order_autouse.py

the graph would look like this:

.. image:: /example/fixtures/test_fixtures_order_autouse.*
    :align: center

Because ``c`` can now be put above ``d`` in the graph, pytest can once again
linearize the graph to this:

.. image:: /example/fixtures/test_fixtures_order_autouse_flat.*
    :align: center

In this example, ``c`` makes ``b`` and ``a`` effectively autouse fixtures as
well.

Be careful with autouse, though, as an autouse fixture will automatically
execute for every test that can reach it, even if they don't request it. For
example, consider this file:

.. literalinclude:: /example/fixtures/test_fixtures_order_autouse_multiple_scopes.py

Even though nothing in ``TestClassWithoutC1Request`` is requesting ``c1``, it still
is executed for the tests inside it anyway:

.. image:: /example/fixtures/test_fixtures_order_autouse_multiple_scopes.*
    :align: center

But just because one autouse fixture requested a non-autouse fixture, that
doesn't mean the non-autouse fixture becomes an autouse fixture for all contexts
that it can apply to. It only effectively becomes an autouse fixture for the
contexts the real autouse fixture (the one that requested the non-autouse
fixture) can apply to.

For example, take a look at this test file:

.. literalinclude:: /example/fixtures/test_fixtures_order_autouse_temp_effects.py

It would break down to something like this:

.. image:: /example/fixtures/test_fixtures_order_autouse_temp_effects.*
    :align: center

For ``test_req`` and ``test_no_req`` inside ``TestClassWithAutouse``, ``c3``
effectively makes ``c2`` an autouse fixture, which is why ``c2`` and ``c3`` are
executed for both tests, despite not being requested, and why ``c2`` and ``c3``
are executed before ``c1`` for ``test_req``.

If this made ``c2`` an *actual* autouse fixture, then ``c2`` would also execute
for the tests inside ``TestClassWithoutAutouse``, since they can reference
``c2`` if they wanted to. But it doesn't, because from the perspective of the
``TestClassWithoutAutouse`` tests, ``c2`` isn't an autouse fixture, since they
can't see ``c3``.


.. note:

    pytest can tell you what order the fixtures will execute in for a given test
    if you call ``pytests`` along with the test's name (or the scope it's in),
    and provide the ``--setup-plan`` flag, e.g.
    ``pytest --setup-plan test_something.py`` (fixtures with names that start
    with ``_`` will only be shown if you also provide the ``-v`` flag).
