path: root/doc/en/example/parametrize.rst

.. _paramexamples:

Parametrizing tests
=================================================

.. currentmodule:: _pytest.python

``pytest`` allows to easily parametrize test functions.
For basic docs, see :ref:`parametrize-basics`.

In the following we provide some examples using
the builtin mechanisms.

Generating parameters combinations, depending on command line
----------------------------------------------------------------------------

.. regendoc:wipe

Let's say we want to execute a test with different computation
parameters and the parameter range shall be determined by a command
line argument.  Let's first write a simple (do-nothing) computation test:

.. code-block:: python

    # content of test_compute.py


    def test_compute(param1):
        assert param1 < 4

Now we add a test configuration like this:

.. code-block:: python

    # content of conftest.py


    def pytest_addoption(parser):
        parser.addoption("--all", action="store_true", help="run all combinations")


    def pytest_generate_tests(metafunc):
        if "param1" in metafunc.fixturenames:
            if metafunc.config.getoption("all"):
                end = 5
            else:
                end = 2
            metafunc.parametrize("param1", range(end))

This means that we only run 2 tests if we do not pass ``--all``:

.. code-block:: pytest

    $ pytest -q test_compute.py
    ..                                                                   [100%]
    2 passed in 0.01s

We run only two computations, so we see two dots.
let's run the full monty:

.. code-block:: pytest

    $ pytest -q --all
    ....F                                                                [100%]
    ================================= FAILURES =================================
    _____________________________ test_compute[4] ______________________________

    param1 = 4

        def test_compute(param1):
    >       assert param1 < 4
    E       assert 4 < 4

    test_compute.py:4: AssertionError
    1 failed, 4 passed in 0.02s

As expected when running the full range of ``param1`` values
we'll get an error on the last one.


Different options for test IDs
------------------------------------

pytest will build a string that is the test ID for each set of values in a
parametrized test. These IDs can be used with ``-k`` to select specific cases
to run, and they will also identify the specific case when one is failing.
Running pytest with ``--collect-only`` will show the generated IDs.

Numbers, strings, booleans and None will have their usual string representation
used in the test ID. For other objects, pytest will make a string based on
the argument name:

.. code-block:: python

    # content of test_time.py

    import pytest

    from datetime import datetime, timedelta

    testdata = [
        (datetime(2001, 12, 12), datetime(2001, 12, 11), timedelta(1)),
        (datetime(2001, 12, 11), datetime(2001, 12, 12), timedelta(-1)),
    ]


    @pytest.mark.parametrize("a,b,expected", testdata)
    def test_timedistance_v0(a, b, expected):
        diff = a - b
        assert diff == expected


    @pytest.mark.parametrize("a,b,expected", testdata, ids=["forward", "backward"])
    def test_timedistance_v1(a, b, expected):
        diff = a - b
        assert diff == expected


    def idfn(val):
        if isinstance(val, (datetime,)):
            # note this wouldn't show any hours/minutes/seconds
            return val.strftime("%Y%m%d")


    @pytest.mark.parametrize("a,b,expected", testdata, ids=idfn)
    def test_timedistance_v2(a, b, expected):
        diff = a - b
        assert diff == expected


    @pytest.mark.parametrize(
        "a,b,expected",
        [
            pytest.param(
                datetime(2001, 12, 12), datetime(2001, 12, 11), timedelta(1), id="forward"
            ),
            pytest.param(
                datetime(2001, 12, 11), datetime(2001, 12, 12), timedelta(-1), id="backward"
            ),
        ],
    )
    def test_timedistance_v3(a, b, expected):
        diff = a - b
        assert diff == expected

In ``test_timedistance_v0``, we let pytest generate the test IDs.

In ``test_timedistance_v1``, we specified ``ids`` as a list of strings which were
used as the test IDs. These are succinct, but can be a pain to maintain.

In ``test_timedistance_v2``, we specified ``ids`` as a function that can generate a
string representation to make part of the test ID. So our ``datetime`` values use the
label generated by ``idfn``, but because we didn't generate a label for ``timedelta``
objects, they are still using the default pytest representation:

.. code-block:: pytest

    $ pytest test_time.py --collect-only
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 8 items
    <Module test_time.py>
      <Function test_timedistance_v0[a0-b0-expected0]>
      <Function test_timedistance_v0[a1-b1-expected1]>
      <Function test_timedistance_v1[forward]>
      <Function test_timedistance_v1[backward]>
      <Function test_timedistance_v2[20011212-20011211-expected0]>
      <Function test_timedistance_v2[20011211-20011212-expected1]>
      <Function test_timedistance_v3[forward]>
      <Function test_timedistance_v3[backward]>

    ========================== no tests ran in 0.12s ===========================

In ``test_timedistance_v3``, we used ``pytest.param`` to specify the test IDs
together with the actual data, instead of listing them separately.

A quick port of "testscenarios"
------------------------------------

.. _`test scenarios`: https://pypi.org/project/testscenarios/

Here is a quick port to run tests configured with `test scenarios`_,
an add-on from Robert Collins for the standard unittest framework. We
only have to work a bit to construct the correct arguments for pytest's
:py:func:`Metafunc.parametrize`:

.. code-block:: python

    # content of test_scenarios.py


    def pytest_generate_tests(metafunc):
        idlist = []
        argvalues = []
        for scenario in metafunc.cls.scenarios:
            idlist.append(scenario[0])
            items = scenario[1].items()
            argnames = [x[0] for x in items]
            argvalues.append([x[1] for x in items])
        metafunc.parametrize(argnames, argvalues, ids=idlist, scope="class")


    scenario1 = ("basic", {"attribute": "value"})
    scenario2 = ("advanced", {"attribute": "value2"})


    class TestSampleWithScenarios:
        scenarios = [scenario1, scenario2]

        def test_demo1(self, attribute):
            assert isinstance(attribute, str)

        def test_demo2(self, attribute):
            assert isinstance(attribute, str)

this is a fully self-contained example which you can run with:

.. code-block:: pytest

    $ pytest test_scenarios.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 4 items

    test_scenarios.py ....                                               [100%]

    ============================ 4 passed in 0.12s =============================

If you just collect tests you'll also nicely see 'advanced' and 'basic' as variants for the test function:

.. code-block:: pytest

    $ pytest --collect-only test_scenarios.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 4 items
    <Module test_scenarios.py>
      <Class TestSampleWithScenarios>
          <Function test_demo1[basic]>
          <Function test_demo2[basic]>
          <Function test_demo1[advanced]>
          <Function test_demo2[advanced]>

    ========================== no tests ran in 0.12s ===========================

Note that we told ``metafunc.parametrize()`` that your scenario values
should be considered class-scoped.  With pytest-2.3 this leads to a
resource-based ordering.

Deferring the setup of parametrized resources
---------------------------------------------------

.. regendoc:wipe

The parametrization of test functions happens at collection
time.  It is a good idea to setup expensive resources like DB
connections or subprocess only when the actual test is run.
Here is a simple example how you can achieve that. This test
requires a ``db`` object fixture:

.. code-block:: python

    # content of test_backends.py

    import pytest


    def test_db_initialized(db):
        # a dummy test
        if db.__class__.__name__ == "DB2":
            pytest.fail("deliberately failing for demo purposes")

We can now add a test configuration that generates two invocations of
the ``test_db_initialized`` function and also implements a factory that
creates a database object for the actual test invocations:

.. code-block:: python

    # content of conftest.py
    import pytest


    def pytest_generate_tests(metafunc):
        if "db" in metafunc.fixturenames:
            metafunc.parametrize("db", ["d1", "d2"], indirect=True)


    class DB1:
        "one database object"


    class DB2:
        "alternative database object"


    @pytest.fixture
    def db(request):
        if request.param == "d1":
            return DB1()
        elif request.param == "d2":
            return DB2()
        else:
            raise ValueError("invalid internal test config")

Let's first see how it looks like at collection time:

.. code-block:: pytest

    $ pytest test_backends.py --collect-only
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 2 items
    <Module test_backends.py>
      <Function test_db_initialized[d1]>
      <Function test_db_initialized[d2]>

    ========================== no tests ran in 0.12s ===========================

And then when we run the test:

.. code-block:: pytest

    $ pytest -q test_backends.py
    .F                                                                   [100%]
    ================================= FAILURES =================================
    _________________________ test_db_initialized[d2] __________________________

    db = <conftest.DB2 object at 0xdeadbeef>

        def test_db_initialized(db):
            # a dummy test
            if db.__class__.__name__ == "DB2":
    >           pytest.fail("deliberately failing for demo purposes")
    E           Failed: deliberately failing for demo purposes

    test_backends.py:8: Failed
    1 failed, 1 passed in 0.02s

The first invocation with ``db == "DB1"`` passed while the second with ``db == "DB2"`` failed.  Our ``db`` fixture function has instantiated each of the DB values during the setup phase while the ``pytest_generate_tests`` generated two according calls to the ``test_db_initialized`` during the collection phase.

.. regendoc:wipe

Apply indirect on particular arguments
---------------------------------------------------

Very often parametrization uses more than one argument name. There is opportunity to apply ``indirect``
parameter on particular arguments. It can be done by passing list or tuple of
arguments' names to ``indirect``. In the example below there is a function ``test_indirect`` which uses
two fixtures: ``x`` and ``y``. Here we give to indirect the list, which contains the name of the
fixture ``x``. The indirect parameter will be applied to this argument only, and the value ``a``
will be passed to respective fixture function:

.. code-block:: python

    # content of test_indirect_list.py

    import pytest


    @pytest.fixture(scope="function")
    def x(request):
        return request.param * 3


    @pytest.fixture(scope="function")
    def y(request):
        return request.param * 2


    @pytest.mark.parametrize("x, y", [("a", "b")], indirect=["x"])
    def test_indirect(x, y):
        assert x == "aaa"
        assert y == "b"

The result of this test will be successful:

.. code-block:: pytest

    $ pytest test_indirect_list.py --collect-only
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 1 item
    <Module test_indirect_list.py>
      <Function test_indirect[a-b]>

    ========================== no tests ran in 0.12s ===========================

.. regendoc:wipe

Parametrizing test methods through per-class configuration
--------------------------------------------------------------

.. _`unittest parametrizer`: https://github.com/testing-cabal/unittest-ext/blob/master/params.py


Here is an example ``pytest_generate_tests`` function implementing a
parametrization scheme similar to Michael Foord's `unittest
parametrizer`_ but in a lot less code:

.. code-block:: python

    # content of ./test_parametrize.py
    import pytest


    def pytest_generate_tests(metafunc):
        # called once per each test function
        funcarglist = metafunc.cls.params[metafunc.function.__name__]
        argnames = sorted(funcarglist[0])
        metafunc.parametrize(
            argnames, [[funcargs[name] for name in argnames] for funcargs in funcarglist]
        )


    class TestClass:
        # a map specifying multiple argument sets for a test method
        params = {
            "test_equals": [dict(a=1, b=2), dict(a=3, b=3)],
            "test_zerodivision": [dict(a=1, b=0)],
        }

        def test_equals(self, a, b):
            assert a == b

        def test_zerodivision(self, a, b):
            with pytest.raises(ZeroDivisionError):
                a / b

Our test generator looks up a class-level definition which specifies which
argument sets to use for each test function.  Let's run it:

.. code-block:: pytest

    $ pytest -q
    F..                                                                  [100%]
    ================================= FAILURES =================================
    ________________________ TestClass.test_equals[1-2] ________________________

    self = <test_parametrize.TestClass object at 0xdeadbeef>, a = 1, b = 2

        def test_equals(self, a, b):
    >       assert a == b
    E       assert 1 == 2

    test_parametrize.py:21: AssertionError
    1 failed, 2 passed in 0.03s

Indirect parametrization with multiple fixtures
--------------------------------------------------------------

Here is a stripped down real-life example of using parametrized
testing for testing serialization of objects between different python
interpreters.  We define a ``test_basic_objects`` function which
is to be run with different sets of arguments for its three arguments:

* ``python1``: first python interpreter, run to pickle-dump an object to a file
* ``python2``: second interpreter, run to pickle-load an object from a file
* ``obj``: object to be dumped/loaded

.. literalinclude:: multipython.py

Running it results in some skips if we don't have all the python interpreters installed and otherwise runs all combinations (3 interpreters times 3 interpreters times 3 objects to serialize/deserialize):

.. code-block:: pytest

   . $ pytest -rs -q multipython.py
   ssssssssssss...ssssssssssss                                          [100%]
   ========================= short test summary info ==========================
   SKIPPED [12] $REGENDOC_TMPDIR/CWD/multipython.py:30: 'python3.5' not found
   SKIPPED [12] $REGENDOC_TMPDIR/CWD/multipython.py:30: 'python3.7' not found
   3 passed, 24 skipped in 0.24s

Indirect parametrization of optional implementations/imports
--------------------------------------------------------------------

If you want to compare the outcomes of several implementations of a given
API, you can write test functions that receive the already imported implementations
and get skipped in case the implementation is not importable/available.  Let's
say we have a "base" implementation and the other (possibly optimized ones)
need to provide similar results:

.. code-block:: python

    # content of conftest.py

    import pytest


    @pytest.fixture(scope="session")
    def basemod(request):
        return pytest.importorskip("base")


    @pytest.fixture(scope="session", params=["opt1", "opt2"])
    def optmod(request):
        return pytest.importorskip(request.param)

And then a base implementation of a simple function:

.. code-block:: python

    # content of base.py
    def func1():
        return 1

And an optimized version:

.. code-block:: python

    # content of opt1.py
    def func1():
        return 1.0001

And finally a little test module:

.. code-block:: python

    # content of test_module.py


    def test_func1(basemod, optmod):
        assert round(basemod.func1(), 3) == round(optmod.func1(), 3)


If you run this with reporting for skips enabled:

.. code-block:: pytest

    $ pytest -rs test_module.py
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collected 2 items

    test_module.py .s                                                    [100%]

    ========================= short test summary info ==========================
    SKIPPED [1] $REGENDOC_TMPDIR/conftest.py:13: could not import 'opt2': No module named 'opt2'
    ======================= 1 passed, 1 skipped in 0.12s =======================

You'll see that we don't have an ``opt2`` module and thus the second test run
of our ``test_func1`` was skipped.  A few notes:

- the fixture functions in the ``conftest.py`` file are "session-scoped" because we
  don't need to import more than once

- if you have multiple test functions and a skipped import, you will see
  the ``[1]`` count increasing in the report

- you can put :ref:`@pytest.mark.parametrize <@pytest.mark.parametrize>` style
  parametrization on the test functions to parametrize input/output
  values as well.


Set marks or test ID for individual parametrized test
--------------------------------------------------------------------

Use ``pytest.param`` to apply marks or set test ID to individual parametrized test.
For example:

.. code-block:: python

    # content of test_pytest_param_example.py
    import pytest


    @pytest.mark.parametrize(
        "test_input,expected",
        [
            ("3+5", 8),
            pytest.param("1+7", 8, marks=pytest.mark.basic),
            pytest.param("2+4", 6, marks=pytest.mark.basic, id="basic_2+4"),
            pytest.param(
                "6*9", 42, marks=[pytest.mark.basic, pytest.mark.xfail], id="basic_6*9"
            ),
        ],
    )
    def test_eval(test_input, expected):
        assert eval(test_input) == expected

In this example, we have 4 parametrized tests. Except for the first test,
we mark the rest three parametrized tests with the custom marker ``basic``,
and for the fourth test we also use the built-in mark ``xfail`` to indicate this
test is expected to fail. For explicitness, we set test ids for some tests.

Then run ``pytest`` with verbose mode and with only the ``basic`` marker:

.. code-block:: pytest

    $ pytest -v -m basic
    =========================== test session starts ============================
    platform linux -- Python 3.x.y, pytest-5.x.y, py-1.x.y, pluggy-0.x.y -- $PYTHON_PREFIX/bin/python
    cachedir: $PYTHON_PREFIX/.pytest_cache
    rootdir: $REGENDOC_TMPDIR
    collecting ... collected 18 items / 15 deselected / 3 selected

    test_pytest_param_example.py::test_eval[1+7-8] PASSED                [ 33%]
    test_pytest_param_example.py::test_eval[basic_2+4] PASSED            [ 66%]
    test_pytest_param_example.py::test_eval[basic_6*9] XFAIL             [100%]

    =============== 2 passed, 15 deselected, 1 xfailed in 0.12s ================

As the result:

- Four tests were collected
- One test was deselected because it doesn't have the ``basic`` mark.
- Three tests with the ``basic`` mark was selected.
- The test ``test_eval[1+7-8]`` passed, but the name is autogenerated and confusing.
- The test ``test_eval[basic_2+4]`` passed.
- The test ``test_eval[basic_6*9]`` was expected to fail and did fail.

.. _`parametrizing_conditional_raising`:

Parametrizing conditional raising
--------------------------------------------------------------------

Use :func:`pytest.raises` with the
:ref:`pytest.mark.parametrize ref` decorator to write parametrized tests
in which some tests raise exceptions and others do not.

It is helpful to define a no-op context manager ``does_not_raise`` to serve
as a complement to ``raises``. For example:

.. code-block:: python

    from contextlib import contextmanager
    import pytest


    @contextmanager
    def does_not_raise():
        yield


    @pytest.mark.parametrize(
        "example_input,expectation",
        [
            (3, does_not_raise()),
            (2, does_not_raise()),
            (1, does_not_raise()),
            (0, pytest.raises(ZeroDivisionError)),
        ],
    )
    def test_division(example_input, expectation):
        """Test how much I know division."""
        with expectation:
            assert (6 / example_input) is not None

In the example above, the first three test cases should run unexceptionally,
while the fourth should raise ``ZeroDivisionError``.

If you're only supporting Python 3.7+, you can simply use ``nullcontext``
to define ``does_not_raise``:

.. code-block:: python

    from contextlib import nullcontext as does_not_raise

Or, if you're supporting Python 3.3+ you can use:

.. code-block:: python

    from contextlib import ExitStack as does_not_raise

Or, if desired, you can ``pip install contextlib2`` and use:

.. code-block:: python

    from contextlib2 import ExitStack as does_not_raise