.. _fixture:
.. _fixtures:
.. _`fixture functions`:

pytest fixtures: explicit, modular, scalable
========================================================

.. currentmodule:: _pytest.python

.. versionadded:: 2.0/2.3/2.4

.. _`xUnit`: http://en.wikipedia.org/wiki/XUnit
.. _`purpose of test fixtures`: http://en.wikipedia.org/wiki/Test_fixture#Software
.. _`Dependency injection`: http://en.wikipedia.org/wiki/Dependency_injection#Definition

The `purpose of test fixtures`_ is to provide a fixed baseline
upon which tests can reliably and repeatedly execute.   pytest fixtures
offer dramatic improvements over the classic xUnit style of setup/teardown
functions:

* fixtures have explicit names and are activated by declaring their use
  from test functions, modules, classes or whole projects.

* fixtures are implemented in a modular manner, as each fixture name
  triggers a *fixture function* which can itself use other fixtures.

* fixture management scales from simple unit to complex
  functional testing, allowing to parametrize fixtures and tests according
  to configuration and component options, or to re-use fixtures
  across class, module or whole test session scopes.

In addition, pytest continues to support :ref:`xunitsetup`.  You can mix
both styles, moving incrementally from classic to new style, as you
prefer.  You can also start out from existing :ref:`unittest.TestCase
style <unittest.TestCase>` or :ref:`nose based <nosestyle>` projects.

.. note::

    pytest-2.4 introduced an additional experimental
    :ref:`yield fixture mechanism <yieldfixture>` for easier context manager
    integration and more linear writing of teardown code.

.. _`funcargs`:
.. _`funcarg mechanism`:
.. _`fixture function`:
.. _`@pytest.fixture`:
.. _`pytest.fixture`:

Fixtures as Function arguments
-----------------------------------------

Test functions can receive fixture objects by naming them as an input
argument. For each argument name, a fixture function with that name provides
the fixture object.  Fixture functions are registered by marking them with
:py:func:`@pytest.fixture <_pytest.python.fixture>`.  Let's look at a simple
self-contained test module containing a fixture and a test function
using it::

    # content of ./test_smtpsimple.py
    import pytest

    @pytest.fixture
    def smtp():
        import smtplib
        return smtplib.SMTP("smtp.gmail.com")

    def test_ehlo(smtp):
        response, msg = smtp.ehlo()
        assert response == 250
        assert 0 # for demo purposes

Here, the ``test_ehlo`` needs the ``smtp`` fixture value.  pytest
will discover and call the :py:func:`@pytest.fixture <_pytest.python.fixture>`
marked ``smtp`` fixture function.  Running the test looks like this::

    $ py.test test_smtpsimple.py
    ======= test session starts ========
    platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
    rootdir: $REGENDOC_TMPDIR, inifile: 
    collected 1 items
    
    test_smtpsimple.py F
    
    ======= FAILURES ========
    _______ test_ehlo ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_ehlo(smtp):
            response, msg = smtp.ehlo()
            assert response == 250
    >       assert 0 # for demo purposes
    E       assert 0
    
    test_smtpsimple.py:11: AssertionError
    ======= 1 failed in 0.12 seconds ========

In the failure traceback we see that the test function was called with a
``smtp`` argument, the ``smtplib.SMTP()`` instance created by the fixture
function.  The test function fails on our deliberate ``assert 0``.  Here is
the exact protocol used by ``pytest`` to call the test function this way:

1. pytest :ref:`finds <test discovery>` the ``test_ehlo`` because
   of the ``test_`` prefix.  The test function needs a function argument
   named ``smtp``.  A matching fixture function is discovered by
   looking for a fixture-marked function named ``smtp``.

2. ``smtp()`` is called to create an instance.

3. ``test_ehlo(<SMTP instance>)`` is called and fails in the last
   line of the test function.

Note that if you misspell a function argument or want
to use one that isn't available, you'll see an error
with a list of available function arguments.

.. Note::

    You can always issue::

        py.test --fixtures test_simplefactory.py

    to see available fixtures.

    In versions prior to 2.3 there was no ``@pytest.fixture`` marker
    and you had to use a magic ``pytest_funcarg__NAME`` prefix
    for the fixture factory.  This remains and will remain supported
    but is not anymore advertised as the primary means of declaring fixture
    functions.

"Funcargs" a prime example of dependency injection
---------------------------------------------------

When injecting fixtures to test functions, pytest-2.0 introduced the
term "funcargs" or "funcarg mechanism" which continues to be present
also in docs today.  It now refers to the specific case of injecting
fixture values as arguments to test functions.  With pytest-2.3 there are
more possibilities to use fixtures but "funcargs" remain as the main way
as they allow to directly state the dependencies of a test function.

As the following examples show in more detail, funcargs allow test
functions to easily receive and work against specific pre-initialized
application objects without having to care about import/setup/cleanup
details.  It's a prime example of `dependency injection`_ where fixture
functions take the role of the *injector* and test functions are the
*consumers* of fixture objects.

.. _smtpshared:

Sharing a fixture across tests in a module (or class/session)
-----------------------------------------------------------------

.. regendoc:wipe

Fixtures requiring network access depend on connectivity and are
usually time-expensive to create.  Extending the previous example, we
can add a ``scope='module'`` parameter to the
:py:func:`@pytest.fixture <_pytest.python.fixture>` invocation
to cause the decorated ``smtp`` fixture function to only be invoked once
per test module.  Multiple test functions in a test module will thus
each receive the same ``smtp`` fixture instance.  The next example puts
the fixture function into a separate ``conftest.py`` file so
that tests from multiple test modules in the directory can
access the fixture function::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="module")
    def smtp():
        return smtplib.SMTP("smtp.gmail.com")

The name of the fixture again is ``smtp`` and you can access its result by
listing the name ``smtp`` as an input parameter in any test or fixture
function (in or below the directory where ``conftest.py`` is located)::

    # content of test_module.py

    def test_ehlo(smtp):
        response, msg = smtp.ehlo()
        assert response == 250
        assert b"smtp.gmail.com" in msg
        assert 0  # for demo purposes   

    def test_noop(smtp):
        response, msg = smtp.noop()
        assert response == 250
        assert 0  # for demo purposes

We deliberately insert failing ``assert 0`` statements in order to
inspect what is going on and can now run the tests::

    $ py.test test_module.py
    ======= test session starts ========
    platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
    rootdir: $REGENDOC_TMPDIR, inifile: 
    collected 2 items
    
    test_module.py FF
    
    ======= FAILURES ========
    _______ test_ehlo ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_ehlo(smtp):
            response, msg = smtp.ehlo()
            assert response == 250
            assert b"smtp.gmail.com" in msg
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:6: AssertionError
    _______ test_noop ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_noop(smtp):
            response, msg = smtp.noop()
            assert response == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError
    ======= 2 failed in 0.12 seconds ========

You see the two ``assert 0`` failing and more importantly you can also see
that the same (module-scoped) ``smtp`` object was passed into the two
test functions because pytest shows the incoming argument values in the
traceback.  As a result, the two test functions using ``smtp`` run as
quick as a single one because they reuse the same instance.

If you decide that you rather want to have a session-scoped ``smtp``
instance, you can simply declare it:

.. code-block:: python

    @pytest.fixture(scope="session")
    def smtp(...):
        # the returned fixture value will be shared for
        # all tests needing it

.. _`finalization`:

Fixture finalization / executing teardown code
-------------------------------------------------------------

pytest supports execution of fixture specific finalization code
when the fixture goes out of scope.  By accepting a ``request`` object
into your fixture function you can call its ``request.addfinalizer`` one
or multiple times::

    # content of conftest.py

    import smtplib
    import pytest

    @pytest.fixture(scope="module")
    def smtp(request):
        smtp = smtplib.SMTP("smtp.gmail.com")
        def fin():
            print ("teardown smtp")
            smtp.close()
        request.addfinalizer(fin)
        return smtp  # provide the fixture value

The ``fin`` function will execute when the last test using
the fixture in the module has finished execution.

Let's execute it::

    $ py.test -s -q --tb=no
    FFteardown smtp
    
    2 failed in 0.12 seconds

We see that the ``smtp`` instance is finalized after the two
tests finished execution.  Note that if we decorated our fixture
function with ``scope='function'`` then fixture setup and cleanup would
occur around each single test.  In either case the test
module itself does not need to change or know about these details
of fixture setup.


.. _`request-context`:

Fixtures can introspect the requesting test context
-------------------------------------------------------------

Fixture function can accept the :py:class:`request <FixtureRequest>` object
to introspect the "requesting" test function, class or module context.
Further extending the previous ``smtp`` fixture example, let's
read an optional server URL from the test module which uses our fixture::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="module")
    def smtp(request):
        server = getattr(request.module, "smtpserver", "smtp.gmail.com")
        smtp = smtplib.SMTP(server)

        def fin():
            print ("finalizing %s (%s)" % (smtp, server))
            smtp.close()
        request.addfinalizer(fin)
        return smtp

We use the ``request.module`` attribute to optionally obtain an
``smtpserver`` attribute from the test module.  If we just execute
again, nothing much has changed::

    $ py.test -s -q --tb=no
    FFfinalizing <smtplib.SMTP object at 0xdeadbeef> (smtp.gmail.com)
    
    2 failed in 0.12 seconds

Let's quickly create another test module that actually sets the
server URL in its module namespace::

    # content of test_anothersmtp.py

    smtpserver = "mail.python.org"  # will be read by smtp fixture

    def test_showhelo(smtp):
        assert 0, smtp.helo()

Running it::

    $ py.test -qq --tb=short test_anothersmtp.py
    F
    ======= FAILURES ========
    _______ test_showhelo ________
    test_anothersmtp.py:5: in test_showhelo
        assert 0, smtp.helo()
    E   AssertionError: (250, b'mail.python.org')
    E   assert 0

voila! The ``smtp`` fixture function picked up our mail server name
from the module namespace.

.. _`fixture-parametrize`:

Parametrizing a fixture
-----------------------------------------------------------------

Fixture functions can be parametrized in which case they will be called
multiple times, each time executing the set of dependent tests, i. e. the
tests that depend on this fixture.  Test functions do usually not need
to be aware of their re-running.  Fixture parametrization helps to
write exhaustive functional tests for components which themselves can be
configured in multiple ways.

Extending the previous example, we can flag the fixture to create two
``smtp`` fixture instances which will cause all tests using the fixture
to run twice.  The fixture function gets access to each parameter
through the special :py:class:`request <FixtureRequest>` object::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="module",
                    params=["smtp.gmail.com", "mail.python.org"])
    def smtp(request):
        smtp = smtplib.SMTP(request.param)
        def fin():
            print ("finalizing %s" % smtp)
            smtp.close()
        request.addfinalizer(fin)
        return smtp

The main change is the declaration of ``params`` with
:py:func:`@pytest.fixture <_pytest.python.fixture>`, a list of values
for each of which the fixture function will execute and can access
a value via ``request.param``.  No test function code needs to change.
So let's just do another run::

    $ py.test -q test_module.py
    FFFF
    ======= FAILURES ========
    _______ test_ehlo[smtp.gmail.com] ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_ehlo(smtp):
            response, msg = smtp.ehlo()
            assert response == 250
            assert b"smtp.gmail.com" in msg
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:6: AssertionError
    _______ test_noop[smtp.gmail.com] ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_noop(smtp):
            response, msg = smtp.noop()
            assert response == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError
    _______ test_ehlo[mail.python.org] ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_ehlo(smtp):
            response, msg = smtp.ehlo()
            assert response == 250
    >       assert b"smtp.gmail.com" in msg
    E       assert b'smtp.gmail.com' in b'mail.python.org\nSIZE 51200000\nETRN\nSTARTTLS\nENHANCEDSTATUSCODES\n8BITMIME\nDSN\nSMTPUTF8'
    
    test_module.py:5: AssertionError
    -------------------------- Captured stdout setup ---------------------------
    finalizing <smtplib.SMTP object at 0xdeadbeef>
    _______ test_noop[mail.python.org] ________
    
    smtp = <smtplib.SMTP object at 0xdeadbeef>
    
        def test_noop(smtp):
            response, msg = smtp.noop()
            assert response == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError
    4 failed in 0.12 seconds

We see that our two test functions each ran twice, against the different
``smtp`` instances.  Note also, that with the ``mail.python.org``
connection the second test fails in ``test_ehlo`` because a
different server string is expected than what arrived.

pytest will build a string that is the test ID for each fixture value
in a parametrized fixture, e.g. ``test_ehlo[smtp.gmail.com]`` and
``test_ehlo[mail.python.org]`` in the above examples.  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.  It is possible to customise
the string used in a test ID for a certain fixture value by using the
``ids`` keyword argument::

   # content of test_ids.py
   import pytest

   @pytest.fixture(params=[0, 1], ids=["spam", "ham"])
   def a(request):
       return request.param

   def test_a(a):
       pass

   def idfn(fixture_value):
       if fixture_value == 0:
           return "eggs"
       else:
           return None

   @pytest.fixture(params=[0, 1], ids=idfn)
   def b(request):
       return request.param

   def test_b(b):
       pass

The above shows how ``ids`` can be either a list of strings to use or
a function which will be called with the fixture value and then
has to return a string to use.  In the latter case if the function
return ``None`` then pytest's auto-generated ID will be used.

Running the above tests results in the following test IDs being used::

   $ py.test --collect-only
   ======= test session starts ========
   platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
   rootdir: $REGENDOC_TMPDIR, inifile: 
   collected 10 items
   <Module 'test_anothersmtp.py'>
     <Function 'test_showhelo[smtp.gmail.com]'>
     <Function 'test_showhelo[mail.python.org]'>
   <Module 'test_ids.py'>
     <Function 'test_a[spam]'>
     <Function 'test_a[ham]'>
     <Function 'test_b[eggs]'>
     <Function 'test_b[1]'>
   <Module 'test_module.py'>
     <Function 'test_ehlo[smtp.gmail.com]'>
     <Function 'test_noop[smtp.gmail.com]'>
     <Function 'test_ehlo[mail.python.org]'>
     <Function 'test_noop[mail.python.org]'>
   
   ======= no tests ran in 0.12 seconds ========

.. _`interdependent fixtures`:

Modularity: using fixtures from a fixture function
----------------------------------------------------------

You can not only use fixtures in test functions but fixture functions
can use other fixtures themselves.  This contributes to a modular design
of your fixtures and allows re-use of framework-specific fixtures across
many projects.  As a simple example, we can extend the previous example
and instantiate an object ``app`` where we stick the already defined
``smtp`` resource into it::

    # content of test_appsetup.py

    import pytest

    class App:
        def __init__(self, smtp):
            self.smtp = smtp

    @pytest.fixture(scope="module")
    def app(smtp):
        return App(smtp)

    def test_smtp_exists(app):
        assert app.smtp

Here we declare an ``app`` fixture which receives the previously defined
``smtp`` fixture and instantiates an ``App`` object with it.  Let's run it::

    $ py.test -v test_appsetup.py
    ======= test session starts ========
    platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1 -- $PYTHON_PREFIX/bin/python3.4
    cachedir: .cache
    rootdir: $REGENDOC_TMPDIR, inifile: 
    collecting ... collected 2 items
    
    test_appsetup.py::test_smtp_exists[smtp.gmail.com] PASSED
    test_appsetup.py::test_smtp_exists[mail.python.org] PASSED
    
    ======= 2 passed in 0.12 seconds ========

Due to the parametrization of ``smtp`` the test will run twice with two
different ``App`` instances and respective smtp servers.  There is no
need for the ``app`` fixture to be aware of the ``smtp`` parametrization
as pytest will fully analyse the fixture dependency graph.

Note, that the ``app`` fixture has a scope of ``module`` and uses a
module-scoped ``smtp`` fixture.  The example would still work if ``smtp``
was cached on a ``session`` scope: it is fine for fixtures to use
"broader" scoped fixtures but not the other way round:
A session-scoped fixture could not use a module-scoped one in a
meaningful way.


.. _`automatic per-resource grouping`:

Automatic grouping of tests by fixture instances
----------------------------------------------------------

.. regendoc: wipe

pytest minimizes the number of active fixtures during test runs.
If you have a parametrized fixture, then all the tests using it will
first execute with one instance and then finalizers are called
before the next fixture instance is created.  Among other things,
this eases testing of applications which create and use global state.

The following example uses two parametrized funcargs, one of which is
scoped on a per-module basis, and all the functions perform ``print`` calls
to show the setup/teardown flow::

    # content of test_module.py
    import pytest

    @pytest.fixture(scope="module", params=["mod1", "mod2"])
    def modarg(request):
        param = request.param
        print ("create", param)
        def fin():
            print ("fin %s" % param)
        return param

    @pytest.fixture(scope="function", params=[1,2])
    def otherarg(request):
        return request.param

    def test_0(otherarg):
        print ("  test0", otherarg)
    def test_1(modarg):
        print ("  test1", modarg)
    def test_2(otherarg, modarg):
        print ("  test2", otherarg, modarg)

Let's run the tests in verbose mode and with looking at the print-output::

    $ py.test -v -s test_module.py
    ======= test session starts ========
    platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1 -- $PYTHON_PREFIX/bin/python3.4
    cachedir: .cache
    rootdir: $REGENDOC_TMPDIR, inifile: 
    collecting ... collected 8 items
    
    test_module.py::test_0[1]   test0 1
    PASSED
    test_module.py::test_0[2]   test0 2
    PASSED
    test_module.py::test_1[mod1] create mod1
      test1 mod1
    PASSED
    test_module.py::test_2[1-mod1]   test2 1 mod1
    PASSED
    test_module.py::test_2[2-mod1]   test2 2 mod1
    PASSED
    test_module.py::test_1[mod2] create mod2
      test1 mod2
    PASSED
    test_module.py::test_2[1-mod2]   test2 1 mod2
    PASSED
    test_module.py::test_2[2-mod2]   test2 2 mod2
    PASSED
    
    ======= 8 passed in 0.12 seconds ========

You can see that the parametrized module-scoped ``modarg`` resource caused
an ordering of test execution that lead to the fewest possible "active" resources. The finalizer for the ``mod1`` parametrized resource was executed
before the ``mod2`` resource was setup.


.. _`usefixtures`:

Using fixtures from classes, modules or projects
----------------------------------------------------------------------

.. regendoc:wipe

Sometimes test functions do not directly need access to a fixture object.
For example, tests may require to operate with an empty directory as the
current working directory but otherwise do not care for the concrete
directory.  Here is how you can can use the standard `tempfile
<http://docs.python.org/library/tempfile.html>`_ and pytest fixtures to
achieve it.  We separate the creation of the fixture into a conftest.py
file::

    # content of conftest.py

    import pytest
    import tempfile
    import os

    @pytest.fixture()
    def cleandir():
        newpath = tempfile.mkdtemp()
        os.chdir(newpath)

and declare its use in a test module via a ``usefixtures`` marker::

    # content of test_setenv.py
    import os
    import pytest

    @pytest.mark.usefixtures("cleandir")
    class TestDirectoryInit:
        def test_cwd_starts_empty(self):
            assert os.listdir(os.getcwd()) == []
            with open("myfile", "w") as f:
                f.write("hello")

        def test_cwd_again_starts_empty(self):
            assert os.listdir(os.getcwd()) == []

Due to the ``usefixtures`` marker, the ``cleandir`` fixture
will be required for the execution of each test method, just as if
you specified a "cleandir" function argument to each of them.  Let's run it
to verify our fixture is activated and the tests pass::

    $ py.test -q
    ..
    2 passed in 0.12 seconds

You can specify multiple fixtures like this:

.. code-block:: python

    @pytest.mark.usefixtures("cleandir", "anotherfixture")

and you may specify fixture usage at the test module level, using
a generic feature of the mark mechanism:

.. code-block:: python

    pytestmark = pytest.mark.usefixtures("cleandir")

Note that the assigned variable *must* be called ``pytestmark``, assigning e.g.
``foomark`` will not activate the fixtures.

Lastly you can put fixtures required by all tests in your project
into an ini-file:

.. code-block:: ini

    # content of pytest.ini
    [pytest]
    usefixtures = cleandir


.. _`autouse`:
.. _`autouse fixtures`:

Autouse fixtures (xUnit setup on steroids)
----------------------------------------------------------------------

.. regendoc:wipe

Occasionally, you may want to have fixtures get invoked automatically
without a `usefixtures`_ or `funcargs`_ reference.   As a practical
example, suppose we have a database fixture which has a
begin/rollback/commit architecture and we want to automatically surround
each test method by a transaction and a rollback.  Here is a dummy
self-contained implementation of this idea::

    # content of test_db_transact.py

    import pytest

    class DB:
        def __init__(self):
            self.intransaction = []
        def begin(self, name):
            self.intransaction.append(name)
        def rollback(self):
            self.intransaction.pop()

    @pytest.fixture(scope="module")
    def db():
        return DB()

    class TestClass:
        @pytest.fixture(autouse=True)
        def transact(self, request, db):
            db.begin(request.function.__name__)
            request.addfinalizer(db.rollback)

        def test_method1(self, db):
            assert db.intransaction == ["test_method1"]

        def test_method2(self, db):
            assert db.intransaction == ["test_method2"]

The class-level ``transact`` fixture is marked with *autouse=true*
which implies that all test methods in the class will use this fixture
without a need to state it in the test function signature or with a
class-level ``usefixtures`` decorator.

If we run it, we get two passing tests::

    $ py.test -q
    ..
    2 passed in 0.12 seconds

Here is how autouse fixtures work in other scopes:

- if an autouse fixture is defined in a test module, all its test
  functions automatically use it.

- if an autouse fixture is defined in a conftest.py file then all tests in
  all test modules below its directory will invoke the fixture.

- lastly, and **please use that with care**: if you define an autouse
  fixture in a plugin, it will be invoked for all tests in all projects
  where the plugin is installed.  This can be useful if a fixture only
  anyway works in the presence of certain settings e. g. in the ini-file.  Such
  a global fixture should always quickly determine if it should do
  any work and avoid otherwise expensive imports or computation.

Note that the above ``transact`` fixture may very well be a fixture that
you want to make available in your project without having it generally
active.  The canonical way to do that is to put the transact definition
into a conftest.py file **without** using ``autouse``::

    # content of conftest.py
    @pytest.fixture()
    def transact(self, request, db):
        db.begin()
        request.addfinalizer(db.rollback)

and then e.g. have a TestClass using it by declaring the need::

    @pytest.mark.usefixtures("transact")
    class TestClass:
        def test_method1(self):
            ...

All test methods in this TestClass will use the transaction fixture while
other test classes or functions in the module will not use it unless
they also add a ``transact`` reference.

Shifting (visibility of) fixture functions
----------------------------------------------------

If during implementing your tests you realize that you
want to use a fixture function from multiple test files you can move it
to a :ref:`conftest.py <conftest.py>` file or even separately installable
:ref:`plugins <plugins>` without changing test code.  The discovery of
fixtures functions starts at test classes, then test modules, then
``conftest.py`` files and finally builtin and third party plugins.

Overriding fixtures on various levels
-------------------------------------

In relatively large test suite, you most likely need to ``override`` a ``global`` or ``root`` fixture with a ``locally``
defined one, keeping the test code readable and maintainable.

Override a fixture on a folder (conftest) level
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given the tests file structure is:

::

    tests/
        __init__.py

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

            @pytest.fixture
            def username():
                return 'username'

        test_something.py
            # content of tests/test_something.py
            def test_username(username):
                assert username == 'username'

        subfolder/
            __init__.py

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

                @pytest.fixture
                def username(username):
                    return 'overridden-' + username

            test_something.py
                # content of tests/subfolder/test_something.py
                def test_username(username):
                    assert username == 'overridden-username'

As you can see, a fixture with the same name can be overridden for certain test folder level.
Note that the ``base`` or ``super`` fixture can be accessed from the ``overriding``
fixture easily - used in the example above.

Override a fixture on a test module level
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given the tests file structure is:

::

    tests/
        __init__.py

        conftest.py
            # content of tests/conftest.py
            @pytest.fixture
            def username():
                return 'username'

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

            @pytest.fixture
            def username(username):
                return 'overridden-' + username

            def test_username(username):
                assert username == 'overridden-username'

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

            @pytest.fixture
            def username(username):
                return 'overridden-else-' + username

            def test_username(username):
                assert username == 'overridden-else-username'

In the example above, a fixture with the same name can be overridden for certain test module.


Override a fixture with direct test parametrization
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given the tests file structure is:

::

    tests/
        __init__.py

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

            @pytest.fixture
            def username():
                return 'username'

            @pytest.fixture
            def other_username(username):
                return 'other-' + username

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

            @pytest.mark.parametrize('username', ['directly-overridden-username'])
            def test_username(username):
                assert username == 'directly-overridden-username'

            @pytest.mark.parametrize('username', ['directly-overridden-username-other'])
            def test_username_other(other_username):
                assert username == 'other-directly-overridden-username-other'

In the example above, a fixture value is overridden by the test parameter value. Note that the value of the fixture
can be overridden this way even if the test doesn't use it directly (doesn't mention it in the function prototype).


Override a parametrized fixture with non-parametrized one and vice versa
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given the tests file structure is:

::

    tests/
        __init__.py

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

            @pytest.fixture(params=['one', 'two', 'three'])
            def parametrized_username(request):
                return request.param

            @pytest.fixture
            def non_parametrized_username(request):
                return 'username'

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

            @pytest.fixture
            def parametrized_username():
                return 'overridden-username'

            @pytest.fixture(params=['one', 'two', 'three'])
            def non_parametrized_username(request):
                return request.param

            def test_username(parametrized_username):
                assert parametrized_username == 'overridden-username'

            def test_parametrized_username(non_parametrized_username):
                assert non_parametrized_username in ['one', 'two', 'three']

        test_something_else.py
            # content of tests/test_something_else.py
            def test_username(parametrized_username):
                assert parametrized_username in ['one', 'two', 'three']

            def test_username(non_parametrized_username):
                assert non_parametrized_username == 'username'

In the example above, a parametrized fixture is overridden with a non-parametrized version, and
a non-parametrized fixture is overridden with a parametrized version for certain test module.
The same applies for the test folder level obviously.