test_ok2/doc/en/fixture.txt

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

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

.. currentmodule:: _pytest.python

.. versionadded:: 2.0/2.3

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

The `general purpose of test fixtures`_ is to provide a fixed baseline
upon which tests can reliably and repeatedly execute.   pytest-2.3 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 easily 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` which it
originally introduced in 2005.  You can mix both styles, moving
incrementally from classic to new style, if you prefer.  You can also
start out from existing :ref:`unittest.TestCase style <unittest.TestCase>` 
or :ref:`nose based <nosestyle>` projects.

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

Fixtures as Function arguments (funcargs)
-----------------------------------------

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("merlinux.eu")

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

Here, the ``test_function`` 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 linux2 -- Python 2.7.3 -- pytest-2.3.1
    collected 1 items
    
    test_smtpsimple.py F
    
    ================================= FAILURES =================================
    ________________________________ test_ehlo _________________________________
    
    smtp = <smtplib.SMTP instance at 0x1ab68c0>
    
        def test_ehlo(smtp):
            response, msg = smtp.ehlo()
            assert response == 250
            assert "merlinux" in msg
    >       assert 0 # for demo purposes
    E       assert 0
    
    test_smtpsimple.py:12: 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
an exact protocol of how py.test comes 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 pytest-2.3 docs.  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" probably will remain
as the main way of dealing with fixtures.

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:

Working with a module-shared fixture
-----------------------------------------------------------------

.. 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 also
extracts the fixture function into a separate ``conftest.py`` file so
that all tests in 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("merlinux.eu")

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 setup
function (in or below the directory where ``conftest.py`` is located)::

    # content of test_module.py

    def test_ehlo(smtp):
        response = smtp.ehlo()
        assert response[0] == 250 
        assert "merlinux" in response[1]
        assert 0  # for demo purposes

    def test_noop(smtp):
        response = smtp.noop()
        assert response[0] == 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 linux2 -- Python 2.7.3 -- pytest-2.3.1
    collected 2 items
    
    test_module.py FF
    
    ================================= FAILURES =================================
    ________________________________ test_ehlo _________________________________
    
    smtp = <smtplib.SMTP instance at 0x2c35488>
    
        def test_ehlo(smtp):
            response = smtp.ehlo()
            assert response[0] == 250
            assert "merlinux" in response[1]
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:6: AssertionError
    ________________________________ test_noop _________________________________
    
    smtp = <smtplib.SMTP instance at 0x2c35488>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError
    ========================= 2 failed in 0.13 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::

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

Fixtures can interact with the requesting test context
-------------------------------------------------------------

Fixture functions can themselves use other fixtures by naming
them as an input argument just like test functions do, see 
:ref:`interdependent fixtures`.  Moreover, pytest 
provides a builtin :py:class:`request <FixtureRequest>` object,
which fixture functions can use to introspect the function, class or module
for which they are invoked or to register finalizing (cleanup)
functions which are called when the last test finished execution.  

Further extending the previous ``smtp`` fixture example, let's  
read an optional server URL from the module namespace and register 
a finalizer that closes the smtp connection after the last
test in a module finished execution::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="module")
    def smtp(request):
        server = getattr(request.module, "smtpserver", "merlinux.eu")
        smtp = smtplib.SMTP(server)
        def fin():
            print ("finalizing %s" % smtp)
            smtp.close()
        request.addfinalizer(fin)
        return smtp

The registered ``fin`` function will be called when the last test
using it has executed::

    $ py.test -s -q --tb=no
    FF
    finalizing <smtplib.SMTP instance at 0x2afd830>

We see that the ``smtp`` instance is finalized after the two
tests using it tests executed.  If we had specified ``scope='function'`` 
then fixture setup and cleanup would occur around each single test. 
Note that either case the test module itself does not need to change!

Let's quickly create another test module that actually sets the
server URL and has a test to verify the fixture picks it up::
    
    # 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, 'mail.python.org')

.. _`request`: :py:class:`_pytest.python.FixtureRequest`

.. _`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 `request`_ object::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="module", 
                    params=["merlinux.eu", "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[merlinux.eu] __________________________
    
    smtp = <smtplib.SMTP instance at 0x1bd3710>
    
        def test_ehlo(smtp):
            response = smtp.ehlo()
            assert response[0] == 250
            assert "merlinux" in response[1]
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:6: AssertionError
    __________________________ test_noop[merlinux.eu] __________________________
    
    smtp = <smtplib.SMTP instance at 0x1bd3710>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError
    ________________________ test_ehlo[mail.python.org] ________________________
    
    smtp = <smtplib.SMTP instance at 0x1c58128>
    
        def test_ehlo(smtp):
            response = smtp.ehlo()
            assert response[0] == 250
    >       assert "merlinux" in response[1]
    E       assert 'merlinux' in 'mail.python.org\nSIZE 10240000\nETRN\nSTARTTLS\nENHANCEDSTATUSCODES\n8BITMIME\nDSN'
    
    test_module.py:5: AssertionError
    ________________________ test_noop[mail.python.org] ________________________
    
    smtp = <smtplib.SMTP instance at 0x1c58128>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:11: AssertionError

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.


.. _`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 linux2 -- Python 2.7.3 -- pytest-2.3.1 -- /home/hpk/p/pytest/.tox/regen/bin/python
    collecting ... collected 2 items
    
    test_appsetup.py:12: test_smtp_exists[merlinux.eu] PASSED
    test_appsetup.py:12: test_smtp_exists[mail.python.org] PASSED
    
    ========================= 2 passed in 0.20 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", param
        request.addfinalizer(fin)
        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 linux2 -- Python 2.7.3 -- pytest-2.3.1 -- /home/hpk/p/pytest/.tox/regen/bin/python
    collecting ... collected 8 items
    
    test_module.py:16: test_0[1] PASSED
    test_module.py:16: test_0[2] PASSED
    test_module.py:18: test_1[mod1] PASSED
    test_module.py:20: test_2[1-mod1] PASSED
    test_module.py:20: test_2[2-mod1] PASSED
    test_module.py:18: test_1[mod2] PASSED
    test_module.py:20: test_2[1-mod2] PASSED
    test_module.py:20: test_2[2-mod2] PASSED
    
    ========================= 8 passed in 0.01 seconds =========================
      test0 1
      test0 2
    create mod1
      test1 mod1
      test2 1 mod1
      test2 2 mod1
    fin mod1
    create mod2
      test1 mod2
      test2 1 mod2
      test2 2 mod2
    fin mod2

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
    ..

You can specify multiple fixtures like this::

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

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

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

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

    # content of pytest.ini

    [pytest]
    usefixtures = cleandir


.. _`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
    ..

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 belows 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 expensive imports or computation otherwise.

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.