.. _paramexamples:

parametrizing tests
=================================================

py.test allows to easily implement your own custom
parametrization scheme for tests.  Here we provide
some examples for inspiration and re-use.

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

.. regendoc:wipe

Let's say we want to execute a test with different parameters
and the parameter range shall be determined by a command
line argument.  Let's first write a simple computation test::

    # content of test_compute.py

    def test_compute(param1):
        assert param1 < 4

Now we add a test configuration like this::

    # 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.funcargnames:
            if metafunc.config.option.all:
                end = 5
            else:
                end = 2
            for i in range(end):
                metafunc.addcall(funcargs={'param1': i})

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

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

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

    $ py.test -q --all
    collecting ... collected 5 items
    ....F
    ================================= FAILURES =================================
    _____________________________ test_compute[4] ______________________________
    
    param1 = 4
    
        def test_compute(param1):
    >       assert param1 < 4
    E       assert 4 < 4
    
    test_compute.py:3: AssertionError
    1 failed, 4 passed in 0.03 seconds

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

Defering the setup of parametrizing resources
---------------------------------------------------

.. regendoc:wipe

The parametrization of test functions happens at collection
time.  It is often a good idea to setup possibly expensive
resources only when the actual test is run.  Here is a simple
example how you can achieve that::

    # 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")

Now we add a test configuration that takes care to generate
two invocations of the ``test_db_initialized`` function and
furthermore a factory that creates a database object when
each test is actually run::

    # content of conftest.py

    def pytest_generate_tests(metafunc):
        if 'db' in metafunc.funcargnames:
            metafunc.addcall(param="d1")
            metafunc.addcall(param="d2")

    class DB1:
        "one database object"
    class DB2:
        "alternative database object"
    
    def pytest_funcarg__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::

    $ py.test test_backends.py --collectonly
    <Module 'test_backends.py'>
      <Function 'test_db_initialized[0]'>
      <Function 'test_db_initialized[1]'>

And then when we run the test::

    $ py.test -q test_backends.py
    collecting ... collected 2 items
    .F
    ================================= FAILURES =================================
    __________________________ test_db_initialized[1] __________________________
    
    db = <conftest.DB2 instance at 0x1a5b488>
    
        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:6: Failed
    1 failed, 1 passed in 0.02 seconds

Now you see that one invocation of the test passes and another fails,
as it to be expected.

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

.. _`unittest parameterizer`: http://code.google.com/p/unittest-ext/source/browse/trunk/params.py

Here is an example ``pytest_generate_function`` function implementing a
parametrization scheme similar to Michael Foords `unittest
parameterizer`_ in a lot less code::

    # content of ./test_parametrize.py
    import pytest

    def pytest_generate_tests(metafunc):
        # called once per each test function
        for funcargs in metafunc.cls.params[metafunc.function.__name__]:
            # schedule a new test function run with applied **funcargs
            metafunc.addcall(funcargs=funcargs)

    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), dict(a=3, b=2)],
        }

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

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

Running it means we are two tests for each test functions, using
the respective settings::

    $ py.test -q
    collecting ... collected 6 items
    .FF..F
    ================================= FAILURES =================================
    __________________________ test_db_initialized[1] __________________________
    
    db = <conftest.DB2 instance at 0xf81c20>
    
        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:6: Failed
    _________________________ TestClass.test_equals[0] _________________________
    
    self = <test_parametrize.TestClass instance at 0xf93050>, a = 1, b = 2
    
        def test_equals(self, a, b):
    >       assert a == b
    E       assert 1 == 2
    
    test_parametrize.py:17: AssertionError
    ______________________ TestClass.test_zerodivision[1] ______________________
    
    self = <test_parametrize.TestClass instance at 0xf93098>, a = 3, b = 2
    
        def test_zerodivision(self, a, b):
    >       pytest.raises(ZeroDivisionError, "a/b")
    E       Failed: DID NOT RAISE
    
    test_parametrize.py:20: Failed
    3 failed, 3 passed in 0.04 seconds

Parametrizing test methods through a decorator
--------------------------------------------------------------

Modifying the previous example we can also allow decorators
for parametrizing test methods::

    # content of test_parametrize2.py

    import pytest

    # test support code
    def params(funcarglist):
        def wrapper(function):
            function.funcarglist = funcarglist
            return function
        return wrapper

    def pytest_generate_tests(metafunc):
        for funcargs in getattr(metafunc.function, 'funcarglist', ()):
            metafunc.addcall(funcargs=funcargs)

    # actual test code
    class TestClass:
        @params([dict(a=1, b=2), dict(a=3, b=3), ])
        def test_equals(self, a, b):
            assert a == b

        @params([dict(a=1, b=0), dict(a=3, b=2)])
        def test_zerodivision(self, a, b):
            pytest.raises(ZeroDivisionError, "a/b")

Running it gives similar results as before::

    $ py.test -q test_parametrize2.py
    collecting ... collected 4 items
    F..F
    ================================= FAILURES =================================
    _________________________ TestClass.test_equals[0] _________________________
    
    self = <test_parametrize2.TestClass instance at 0x27e15a8>, a = 1, b = 2
    
        @params([dict(a=1, b=2), dict(a=3, b=3), ])
        def test_equals(self, a, b):
    >       assert a == b
    E       assert 1 == 2
    
    test_parametrize2.py:19: AssertionError
    ______________________ TestClass.test_zerodivision[1] ______________________
    
    self = <test_parametrize2.TestClass instance at 0x2953bd8>, a = 3, b = 2
    
        @params([dict(a=1, b=0), dict(a=3, b=2)])
        def test_zerodivision(self, a, b):
    >       pytest.raises(ZeroDivisionError, "a/b")
    E       Failed: DID NOT RAISE
    
    test_parametrize2.py:23: Failed
    2 failed, 2 passed in 0.03 seconds

checking serialization between Python interpreters
--------------------------------------------------------------

Here is a stripped down real-life example of using parametrized
testing for testing serialization betwee different 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
* ``python2``: second python interpreter
* ``obj``: object to be dumped from first interpreter and loaded into second interpreter

.. literalinclude:: multipython.py

Running it (with Python-2.4 through to Python2.7 installed)::

   . $ py.test -q multipython.py
   collecting ... collected 75 items
   ....s....s....s....ssssss....s....s....s....ssssss....s....s....s....ssssss
   48 passed, 27 skipped in 1.59 seconds