test_ok2/doc/en/fixture.txt

.. _xunitsetup:
.. _setup:
.. _fixture:
.. _`fixture functions`:
.. _`@pytest.fixture`:

pytest fixtures: modular, re-useable, flexible
========================================================

.. versionadded:: 2.0,2.3

.. _`funcargs`: funcargs.html
.. _`test parametrization`: funcargs.html#parametrizing-tests
.. _`unittest plugin`: plugin/unittest.html
.. _`xUnit`: http://en.wikipedia.org/wiki/XUnit
.. _`general purpose of test fixtures`: http://en.wikipedia.org/wiki/Test_fixture#Software
.. _`django`: https://www.djangoproject.com/
.. _`pytest-django`: https://pypi.python.org/pytest-django
.. _`Dependency injection`: http://en.wikipedia.org/wiki/Dependency_injection#Definition

pytest allows to provide and use test fixtures in a modular and flexible
manner, offering major improvements over the classic xUnit style of
setup/teardown functions.  The `general purpose of test fixtures`_ is to
provide a fixed baseline upon which tests can reliably and
repeatedly execute.  With pytest, fixtures are implemented by 
**fixture functions** which may return a fixture object, put extra
attributes on test classes or perform side effects.  The name of a 
fixture function is significant and is used for invoking or activating it.

**Test functions can receive fixture objects by naming them as an input
argument.** For each argument name, a matching fixture
function will provide a fixture object.  This mechanism has been
introduced with pytest-2.0 and is also called the **funcarg mechanism**.
It allows test functions to easily receive and work against specific
pre-initialized application objects without having to care about the
details of setup/cleanup procedures.  This mechanism is a prime example of
`dependency injection`_ where fixture functions take the role of the
*injector* and test functions are the *consumers* of fixture objects.
With pytest-2.3 this mechanism has been much improved to help with
sharing and parametrizing fixtures across test runs.

**Test classes, modules or whole projects can declare a need for
one or more fixtures**.  All required fixture functions will execute 
before a test from the specifying context executes.  They will
typically not provide a fixture object but rather perform side effects 
like reading or preparing default config settings and pre-initializing 
an application.  For example, the Django_ project requires database 
initialization to be able to import from and use its model objects.  
Plugins like `pytest-django`_ provide baseline fixtures which your 
project can then easily depend or extend on.

**Fixtures can be shared throughout a test session, module or class.**.  
By means of a "scope" declaration on a fixture function, it will
only be invoked once per the specified scope.  Sharing expensive application
object setups between tests typically helps to speed up test runs.
Typical examples are the setup of test databases or establishing
required subprocesses or network connections.

**Fixture functions have controlled visilibity** which depends on where they
are defined.  If they are defined on a test class, only its test methods 
may use it. A fixture defined in a module can only be used
from that test module.  A fixture defined in a conftest.py file
can only be used by the tests below the directory of that file.
Lastly plugins can define fixtures which are available across all
projects.

**Fixture functions can interact with the requesting testcontext**.  By
accepting a special ``request`` object, fixture functions can introspect
the function, class or module for which they are invoked and can
optionally register cleanup functions which are called when the last
test finished execution.  A good example is `pytest-timeout`_ which
allows to limit the execution time of a test, and will read the
according parameter from a test function or from project-wide setting.

**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 functional tests for components which themselves can be 
configured in multiple ways.


Basic funcarg fixture example
-----------------------------------------------------------

.. versionadded:: 2.3


Let's look at a simple self-contained test module containing a module
visible fixture function and a test function using the provided fixture::

    # content of ./test_simplefactory.py
    import pytest

    @pytest.fixture
    def myfuncarg():
        return 42

    def test_function(myfuncarg):
        assert myfuncarg == 17

Here, the ``test_function`` needs an object named ``myfuncarg`` and thus
py.test will discover and call the ``@pytest.fixture`` marked ``myfuncarg`` 
factory function.  Running the tests looks like this::

    $ py.test test_simplefactory.py
    =========================== test session starts ============================
    platform linux2 -- Python 2.7.3 -- pytest-2.3.0.dev11
    plugins: xdist, bugzilla, cache, oejskit, cli, timeout, pep8, cov
    collecting ... collected 1 items
    
    test_simplefactory.py F
    
    ================================= FAILURES =================================
    ______________________________ test_function _______________________________
    
    myfuncarg = 42
    
        def test_function(myfuncarg):
    >       assert myfuncarg == 17
    E       assert 42 == 17
    
    test_simplefactory.py:8: AssertionError
    ========================= 1 failed in 0.01 seconds =========================

This shows that the test function was called with a ``myfuncarg``
argument value of ``42`` and the assert fails as expected.  Here is 
how py.test comes to call the test function this way:

1. py.test :ref:`finds <test discovery>` the ``test_function`` because
   of the ``test_`` prefix.  The test function needs a function argument
   named ``myfuncarg``.  A matching factory function is discovered by
   looking for a fixture function named ``myfuncarg``.

2. ``myfuncarg()`` is called to create a value ``42``.

3. ``test_function(42)`` is now called and results in the above
   reported exception because of the assertion mismatch.

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 instead use a magic ``pytest_funcarg__NAME`` prefix
    for the fixture factory.  This remains and will remain supported 
    but is not advertised as the primary means of declaring fixture
    functions.


Creating and using a session-shared fixture
-----------------------------------------------------------------

.. regendoc:wipe

Here is a simple example of a fixture function creating a shared 
``smtplib.SMTP`` connection fixture which test functions from 
test modules below the directory of a ``conftest.py`` file may use::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="session")
    def smtp():
        return smtplib.SMTP("merlinux.eu")

The name of the fixture is ``smtp`` and you can access its result by
listing the name ``smtp`` as an input parameter in any test or setup
function::

    # 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 -q test_module.py
    collecting ... collected 2 items
    FF
    ================================= FAILURES =================================
    ________________________________ test_ehlo _________________________________
    
    smtp = <smtplib.SMTP instance at 0x31bce18>
    
        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:5: AssertionError
    ________________________________ test_noop _________________________________
    
    smtp = <smtplib.SMTP instance at 0x31bce18>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:10: AssertionError
    2 failed in 0.26 seconds

you see the two ``assert 0`` failing and can also see that
the same (session-scoped) object was passed into the two test functions
because pytest shows the incoming arguments in the traceback.

Adding a finalizer to a fixture
--------------------------------------------------------

Further extending the ``smtp`` example, we now want to properly
close a smtp server connection after the last test using it
has been run.  We can do this by calling the ``request.addfinalizer()``
helper::

    # content of conftest.py
    import pytest
    import smtplib

    @pytest.fixture(scope="session")
    def smtp(request):
        smtp = smtplib.SMTP("merlinux.eu")
        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
    collecting ... collected 4 items
    FFFF
    4 failed in 6.40 seconds
    finalizing <smtplib.SMTP instance at 0x125d3b0>

We see that the ``smtp`` instance is finalized after all
tests executed.  If we had specified ``scope='function'`` 
then fixture setup and cleanup would occur around each 
single test. 

Parametrizing a session-shared funcarg resource
-----------------------------------------------------------------

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="session", 
                    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``, 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
    collecting ... collected 4 items
    FFFF
    ================================= FAILURES =================================
    __________________________ test_ehlo[merlinux.eu] __________________________
    
    smtp = <smtplib.SMTP instance at 0x28dc5a8>
    
        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:5: AssertionError
    __________________________ test_noop[merlinux.eu] __________________________
    
    smtp = <smtplib.SMTP instance at 0x28dc5a8>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:10: AssertionError
    ________________________ test_ehlo[mail.python.org] ________________________
    
    smtp = <smtplib.SMTP instance at 0x28e3e18>
    
        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:4: AssertionError
    ________________________ test_noop[mail.python.org] ________________________
    
    smtp = <smtplib.SMTP instance at 0x28e3e18>
    
        def test_noop(smtp):
            response = smtp.noop()
            assert response[0] == 250
    >       assert 0  # for demo purposes
    E       assert 0
    
    test_module.py:10: AssertionError
    4 failed in 6.17 seconds

We now get four failures because we are running the two tests twice with
different ``smtp`` fixture instances.  Note that with the
``mail.python.org`` connection the second test fails in ``test_ehlo``
because it expects a specific server string.

We also see that the two ``smtp`` instances are finalized appropriately.

Looking at test collection without running tests
------------------------------------------------------

You can also look at the tests which pytest collects without running them::

    $ py.test --collectonly
    =========================== test session starts ============================
    platform linux2 -- Python 2.7.3 -- pytest-2.3.0.dev11
    plugins: xdist, bugzilla, cache, oejskit, cli, timeout, pep8, cov
    collecting ... collected 4 items
    <Module 'test_module.py'>
      <Function 'test_ehlo[merlinux.eu]'>
      <Function 'test_noop[merlinux.eu]'>
      <Function 'test_ehlo[mail.python.org]'>
      <Function 'test_noop[mail.python.org]'>
    
    =============================  in 0.01 seconds =============================

Our fixture parameters show up in the test id of the test functions.
Note that pytest orders your test run by resource usage, minimizing
the number of active resources at any given time.


.. _`interdependent fixtures`:

Interdepdendent fixtures
----------------------------------------------------------

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-2.3.0.dev11 -- /home/hpk/venv/1/bin/python
    cachedir: /home/hpk/tmp/doc-exec-423/.cache
    plugins: xdist, bugzilla, cache, oejskit, cli, timeout, pep8, cov
    collecting ... collected 2 items
    
    test_appsetup.py:12: test_exists[merlinux.eu] PASSED
    test_appsetup.py:12: test_exists[mail.python.org] PASSED
    
    ========================= 2 passed in 6.82 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 also,
that the ``app`` fixture has a scope of ``module`` but uses a
session-scoped ``smtp``: 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`` call s
to show the flow of calls::

    # 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-2.3.0.dev11 -- /home/hpk/venv/1/bin/python
    cachedir: /home/hpk/tmp/doc-exec-423/.cache
    plugins: xdist, bugzilla, cache, oejskit, cli, timeout, pep8, cov
    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 0.02 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.


Marking test classes, modules, projects with required fixtures
----------------------------------------------------------------------

.. regendoc:wipe

Sometimes test functions do not directly get access to a fixture object.
For example, each test in a test class may require to operate with an
empty directory as the current working directory.  Here is how you can
can use the standard :ref:`tempfile <lib:tempfile>` 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 ``needs`` marker::

    # content of test_setenv.py
    import os
    import pytest

    @pytest.mark.needsfixtures("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 ``needs`` class marker, the ``cleandir`` fixture
will be required for the execution of each of the test methods, just as if
you specified a "cleandir" function argument to each of them.  Let's run it
to verify our fixture is activated::

    $ py.test -q
    collecting ... collected 2 items
    .
    2 passed in 0.01 seconds

You may specify the need for multiple fixtures::

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

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

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

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

    # content of pytest.ini

    [pytest]
    needsfixtures = cleandir

Implicit fixtures at class/module/directory/global level
----------------------------------------------------------------------

.. regendoc:wipe

Occasionally, you may want to have fixtures get invoked automatically
without any ``needs`` reference.   Also, if you are used to the classical 
xUnit setup/teardown functions you may have gotten used to fixture
functions executing always.  As a practical example, 
suppose we have a database fixture which has a begin/rollback/commit 
architecture and we want to surround each test method by a transaction 
and a rollback.  Here is a dummy self-contained implementation::

    # content of test_db_transact.py
    
    import pytest

    @pytest.fixture(scope="module")
    class db:
        def __init__(self):
            self.intransaction = False
        def begin(self):
            self.intransaction = True
        def rollback(Self):
            self.intransaction = False

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

        def test_method1(self, db):
            assert db.intransaction

        def test_method2(self):
            pass

The class-level ``transact`` fixture is marked with *auto=true* which will
mark all test methods in the class as needing the fixture.  

Here is how this maps to module, project and cross-project scopes:

- if an automatic fixture was defined in a test module, all its test 
  functions would automatically invoke it.  

- if 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 automatic
  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 in the ini-file.  Such
  a global fixture should thus 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 something that
you want to make available in your project without having each test function
in your project automatically using it.  The canonical way to do that is to put 
the transact definition into a conftest.py file without using ``auto``::

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

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

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

While all test methods in this TestClass will thus use the transaction
fixture, other test classes will not unless they state the need.

.. currentmodule:: _pytest.python

.. _`@pytest.fixture`:

``@pytest.fixture``: marking a fixture function
--------------------------------------------------------------

The ``@pytest.fixture`` marker allows to

* mark a function as a factory for fixtures, useable by test and other
  fixture functions

* declare a scope which determines the level of caching, i.e. how often
  the factory will be called. Valid scopes are ``session``, ``module``, 
  ``class`` and ``function``.

* define a list of parameters in order to run dependent tests multiple 
  times with different fixtures

.. _`request`:

``request``: interacting with test invocation context
--------------------------------------------------------------

The ``request`` object may be received by fixture functions
and provides methods to:

* to inspect attributes of the requesting test context, such as 
  ``function``, ``cls``, ``module``, ``session`` and the pytest 
  ``config`` object.  A request object passed to a parametrized factory
  will also carry a ``request.param`` object (A parametrized factory and
  all of its dependent tests will be called with each of the factory-specified
  ``params``).

* to add finalizers/teardowns to be invoked when the last
  test of the requesting test context executes

.. autoclass:: _pytest.python.FixtureRequest()
    :members: