
.. _`unittest.TestCase`:
.. _`unittest`:

unittest.TestCase Support
=========================

``pytest`` supports running Python ``unittest``-based tests out of the box.
It's meant for leveraging existing ``unittest``-based test suites
to use pytest as a test runner and also allow to incrementally adapt
the test suite to take full advantage of pytest's features.

To run an existing ``unittest``-style test suite using ``pytest``, type::

    pytest tests


pytest will automatically collect ``unittest.TestCase`` subclasses and
their ``test`` methods in ``test_*.py`` or ``*_test.py`` files.

Almost all ``unittest`` features are supported:

* ``@unittest.skip`` style decorators;
* ``setUp/tearDown``;
* ``setUpClass/tearDownClass``;
* ``setUpModule/tearDownModule``;

.. _`load_tests protocol`: https://docs.python.org/3/library/unittest.html#load-tests-protocol
.. _`subtests`: https://docs.python.org/3/library/unittest.html#distinguishing-test-iterations-using-subtests

Up to this point pytest does not have support for the following features:

* `load_tests protocol`_;
* `subtests`_;

Benefits out of the box
-----------------------

By running your test suite with pytest you can make use of several features,
in most cases without having to modify existing code:

* Obtain :ref:`more informative tracebacks <tbreportdemo>`;
* :ref:`stdout and stderr <captures>` capturing;
* :ref:`Test selection options <select-tests>` using ``-k`` and ``-m`` flags;
* :ref:`maxfail`;
* :ref:`--pdb <pdb-option>` command-line option for debugging on test failures
  (see :ref:`note <pdb-unittest-note>` below);
* Distribute tests to multiple CPUs using the `pytest-xdist <https://pypi.org/project/pytest-xdist/>`_ plugin;
* Use :ref:`plain assert-statements <assert>` instead of ``self.assert*`` functions (`unittest2pytest
  <https://pypi.org/project/unittest2pytest/>`__ is immensely helpful in this);


pytest features in ``unittest.TestCase`` subclasses
---------------------------------------------------

The following pytest features work in ``unittest.TestCase`` subclasses:

* :ref:`Marks <mark>`: :ref:`skip <skip>`, :ref:`skipif <skipif>`, :ref:`xfail <xfail>`;
* :ref:`Auto-use fixtures <mixing-fixtures>`;

The following pytest features **do not** work, and probably
never will due to different design philosophies:

* :ref:`Fixtures <fixture>` (except for ``autouse`` fixtures, see :ref:`below <mixing-fixtures>`);
* :ref:`Parametrization <parametrize>`;
* :ref:`Custom hooks <writing-plugins>`;


Third party plugins may or may not work well, depending on the plugin and the test suite.

.. _mixing-fixtures:

Mixing pytest fixtures into ``unittest.TestCase`` subclasses using marks
------------------------------------------------------------------------

Running your unittest with ``pytest`` allows you to use its
:ref:`fixture mechanism <fixture>` with ``unittest.TestCase`` style
tests.  Assuming you have at least skimmed the pytest fixture features,
let's jump-start into an example that integrates a pytest ``db_class``
fixture, setting up a class-cached database object, and then reference
it from a unittest-style test::

    # content of conftest.py

    # we define a fixture function below and it will be "used" by
    # referencing its name from tests

    import pytest

    @pytest.fixture(scope="class")
    def db_class(request):
        class DummyDB(object):
            pass
        # set a class attribute on the invoking test context
        request.cls.db = DummyDB()

This defines a fixture function ``db_class`` which - if used - is
called once for each test class and which sets the class-level
``db`` attribute to a ``DummyDB`` instance.  The fixture function
achieves this by receiving a special ``request`` object which gives
access to :ref:`the requesting test context <request-context>` such
as the ``cls`` attribute, denoting the class from which the fixture
is used.  This architecture de-couples fixture writing from actual test
code and allows re-use of the fixture by a minimal reference, the fixture
name.  So let's write an actual ``unittest.TestCase`` class using our
fixture definition::

    # content of test_unittest_db.py

    import unittest
    import pytest

    @pytest.mark.usefixtures("db_class")
    class MyTest(unittest.TestCase):
        def test_method1(self):
            assert hasattr(self, "db")
            assert 0, self.db   # fail for demo purposes

        def test_method2(self):
            assert 0, self.db   # fail for demo purposes

The ``@pytest.mark.usefixtures("db_class")`` class-decorator makes sure that
the pytest fixture function ``db_class`` is called once per class.
Due to the deliberately failing assert statements, we can take a look at
the ``self.db`` values in the traceback:

.. code-block:: pytest

    $ pytest test_unittest_db.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 2 items

    test_unittest_db.py FF                                               [100%]

    ================================= FAILURES =================================
    ___________________________ MyTest.test_method1 ____________________________

    self = <test_unittest_db.MyTest testMethod=test_method1>

        def test_method1(self):
            assert hasattr(self, "db")
    >       assert 0, self.db   # fail for demo purposes
    E       AssertionError: <conftest.db_class.<locals>.DummyDB object at 0xdeadbeef>
    E       assert 0

    test_unittest_db.py:9: AssertionError
    ___________________________ MyTest.test_method2 ____________________________

    self = <test_unittest_db.MyTest testMethod=test_method2>

        def test_method2(self):
    >       assert 0, self.db   # fail for demo purposes
    E       AssertionError: <conftest.db_class.<locals>.DummyDB object at 0xdeadbeef>
    E       assert 0

    test_unittest_db.py:12: AssertionError
    ========================= 2 failed in 0.12 seconds =========================

This default pytest traceback shows that the two test methods
share the same ``self.db`` instance which was our intention
when writing the class-scoped fixture function above.


Using autouse fixtures and accessing other fixtures
---------------------------------------------------

Although it's usually better to explicitly declare use of fixtures you need
for a given test, you may sometimes want to have fixtures that are
automatically used in a given context.  After all, the traditional
style of unittest-setup mandates the use of this implicit fixture writing
and chances are, you are used to it or like it.

You can flag fixture functions with ``@pytest.fixture(autouse=True)``
and define the fixture function in the context where you want it used.
Let's look at an ``initdir`` fixture which makes all test methods of a
``TestCase`` class execute in a temporary directory with a
pre-initialized ``samplefile.ini``.  Our ``initdir`` fixture itself uses
the pytest builtin :ref:`tmpdir <tmpdir>` fixture to delegate the
creation of a per-test temporary directory::

    # content of test_unittest_cleandir.py
    import pytest
    import unittest

    class MyTest(unittest.TestCase):

        @pytest.fixture(autouse=True)
        def initdir(self, tmpdir):
            tmpdir.chdir() # change to pytest-provided temporary directory
            tmpdir.join("samplefile.ini").write("# testdata")

        def test_method(self):
            with open("samplefile.ini") as f:
                s = f.read()
            assert "testdata" in s

Due to the ``autouse`` flag the ``initdir`` fixture function will be
used for all methods of the class where it is defined.  This is a
shortcut for using a ``@pytest.mark.usefixtures("initdir")`` marker
on the class like in the previous example.

Running this test module ...:

.. code-block:: pytest

    $ pytest -q test_unittest_cleandir.py
    .                                                                    [100%]
    1 passed in 0.12 seconds

... gives us one passed test because the ``initdir`` fixture function
was executed ahead of the ``test_method``.

.. note::

   ``unittest.TestCase`` methods cannot directly receive fixture
   arguments as implementing that is likely to inflict
   on the ability to run general unittest.TestCase test suites.

   The above ``usefixtures`` and ``autouse`` examples should help to mix in
   pytest fixtures into unittest suites.

   You can also gradually move away from subclassing from ``unittest.TestCase`` to *plain asserts*
   and then start to benefit from the full pytest feature set step by step.

.. _pdb-unittest-note:

.. note::

    Running tests from ``unittest.TestCase`` subclasses with ``--pdb`` will
    disable tearDown and cleanup methods for the case that an Exception
    occurs. This allows proper post mortem debugging for all applications
    which have significant logic in their tearDown machinery. However,
    supporting this feature has the following side effect: If people
    overwrite ``unittest.TestCase`` ``__call__`` or ``run``, they need to
    to overwrite ``debug`` in the same way  (this is also true for standard
    unittest).

.. note::

    Due to architectural differences between the two frameworks, setup and
    teardown for ``unittest``-based tests is performed during the ``call`` phase
    of testing instead of in ``pytest``'s standard ``setup`` and ``teardown``
    stages. This can be important to understand in some situations, particularly
    when reasoning about errors. For example, if a ``unittest``-based suite
    exhibits errors during setup, ``pytest`` will report no errors during its
    ``setup`` phase and will instead raise the error during ``call``.
