309 lines
10 KiB
ReStructuredText
309 lines
10 KiB
ReStructuredText
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The writing and reporting of assertions in tests
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==================================================
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.. _`assertfeedback`:
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.. _`assert with the assert statement`:
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.. _`assert`:
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Asserting with the ``assert`` statement
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---------------------------------------------------------
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``pytest`` allows you to use the standard python ``assert`` for verifying
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expectations and values in Python tests. For example, you can write the
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following::
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# content of test_assert1.py
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def f():
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return 3
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def test_function():
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assert f() == 4
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to assert that your function returns a certain value. If this assertion fails
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you will see the return value of the function call:
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.. code-block:: pytest
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$ pytest test_assert1.py
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=========================== test session starts ============================
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platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
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rootdir: $REGENDOC_TMPDIR, inifile:
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collected 1 item
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test_assert1.py F [100%]
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================================= FAILURES =================================
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______________________________ test_function _______________________________
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def test_function():
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> assert f() == 4
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E assert 3 == 4
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E + where 3 = f()
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test_assert1.py:5: AssertionError
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========================= 1 failed in 0.12 seconds =========================
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``pytest`` has support for showing the values of the most common subexpressions
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including calls, attributes, comparisons, and binary and unary
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operators. (See :ref:`tbreportdemo`). This allows you to use the
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idiomatic python constructs without boilerplate code while not losing
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introspection information.
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However, if you specify a message with the assertion like this::
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assert a % 2 == 0, "value was odd, should be even"
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then no assertion introspection takes places at all and the message
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will be simply shown in the traceback.
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See :ref:`assert-details` for more information on assertion introspection.
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.. _`assertraises`:
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Assertions about expected exceptions
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------------------------------------------
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In order to write assertions about raised exceptions, you can use
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``pytest.raises`` as a context manager like this::
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import pytest
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def test_zero_division():
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with pytest.raises(ZeroDivisionError):
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1 / 0
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and if you need to have access to the actual exception info you may use::
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def test_recursion_depth():
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with pytest.raises(RuntimeError) as excinfo:
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def f():
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f()
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f()
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assert 'maximum recursion' in str(excinfo.value)
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``excinfo`` is a ``ExceptionInfo`` instance, which is a wrapper around
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the actual exception raised. The main attributes of interest are
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``.type``, ``.value`` and ``.traceback``.
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.. versionchanged:: 3.0
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In the context manager form you may use the keyword argument
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``message`` to specify a custom failure message::
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>>> with raises(ZeroDivisionError, message="Expecting ZeroDivisionError"):
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... pass
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... Failed: Expecting ZeroDivisionError
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If you want to write test code that works on Python 2.4 as well,
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you may also use two other ways to test for an expected exception::
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pytest.raises(ExpectedException, func, *args, **kwargs)
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which will execute the specified function with args and kwargs and
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assert that the given ``ExpectedException`` is raised. The reporter will
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provide you with helpful output in case of failures such as *no
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exception* or *wrong exception*.
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Note that it is also possible to specify a "raises" argument to
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``pytest.mark.xfail``, which checks that the test is failing in a more
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specific way than just having any exception raised::
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@pytest.mark.xfail(raises=IndexError)
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def test_f():
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f()
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Using ``pytest.raises`` is likely to be better for cases where you are testing
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exceptions your own code is deliberately raising, whereas using
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``@pytest.mark.xfail`` with a check function is probably better for something
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like documenting unfixed bugs (where the test describes what "should" happen)
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or bugs in dependencies.
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Also, the context manager form accepts a ``match`` keyword parameter to test
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that a regular expression matches on the string representation of an exception
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(like the ``TestCase.assertRaisesRegexp`` method from ``unittest``)::
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import pytest
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def myfunc():
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raise ValueError("Exception 123 raised")
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def test_match():
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with pytest.raises(ValueError, match=r'.* 123 .*'):
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myfunc()
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The regexp parameter of the ``match`` method is matched with the ``re.search``
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function. So in the above example ``match='123'`` would have worked as
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well.
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.. _`assertwarns`:
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Assertions about expected warnings
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-----------------------------------------
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.. versionadded:: 2.8
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You can check that code raises a particular warning using
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:ref:`pytest.warns <warns>`.
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.. _newreport:
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Making use of context-sensitive comparisons
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-------------------------------------------------
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.. versionadded:: 2.0
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``pytest`` has rich support for providing context-sensitive information
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when it encounters comparisons. For example::
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# content of test_assert2.py
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def test_set_comparison():
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set1 = set("1308")
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set2 = set("8035")
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assert set1 == set2
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if you run this module:
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.. code-block:: pytest
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$ pytest test_assert2.py
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=========================== test session starts ============================
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platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
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rootdir: $REGENDOC_TMPDIR, inifile:
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collected 1 item
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test_assert2.py F [100%]
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================================= FAILURES =================================
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___________________________ test_set_comparison ____________________________
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def test_set_comparison():
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set1 = set("1308")
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set2 = set("8035")
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> assert set1 == set2
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E AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'}
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E Extra items in the left set:
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E '1'
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E Extra items in the right set:
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E '5'
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E Use -v to get the full diff
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test_assert2.py:5: AssertionError
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========================= 1 failed in 0.12 seconds =========================
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Special comparisons are done for a number of cases:
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* comparing long strings: a context diff is shown
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* comparing long sequences: first failing indices
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* comparing dicts: different entries
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See the :ref:`reporting demo <tbreportdemo>` for many more examples.
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Defining your own assertion comparison
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----------------------------------------------
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It is possible to add your own detailed explanations by implementing
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the ``pytest_assertrepr_compare`` hook.
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.. autofunction:: _pytest.hookspec.pytest_assertrepr_compare
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:noindex:
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As an example consider adding the following hook in a :ref:`conftest.py <conftest.py>`
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file which provides an alternative explanation for ``Foo`` objects::
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# content of conftest.py
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from test_foocompare import Foo
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def pytest_assertrepr_compare(op, left, right):
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if isinstance(left, Foo) and isinstance(right, Foo) and op == "==":
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return ['Comparing Foo instances:',
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' vals: %s != %s' % (left.val, right.val)]
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now, given this test module::
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# content of test_foocompare.py
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class Foo(object):
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def __init__(self, val):
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self.val = val
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def __eq__(self, other):
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return self.val == other.val
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def test_compare():
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f1 = Foo(1)
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f2 = Foo(2)
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assert f1 == f2
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you can run the test module and get the custom output defined in
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the conftest file:
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.. code-block:: pytest
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$ pytest -q test_foocompare.py
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F [100%]
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================================= FAILURES =================================
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_______________________________ test_compare _______________________________
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def test_compare():
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f1 = Foo(1)
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f2 = Foo(2)
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> assert f1 == f2
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E assert Comparing Foo instances:
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E vals: 1 != 2
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test_foocompare.py:11: AssertionError
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1 failed in 0.12 seconds
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.. _assert-details:
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.. _`assert introspection`:
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Advanced assertion introspection
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----------------------------------
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.. versionadded:: 2.1
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Reporting details about a failing assertion is achieved by rewriting assert
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statements before they are run. Rewritten assert statements put introspection
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information into the assertion failure message. ``pytest`` only rewrites test
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modules directly discovered by its test collection process, so **asserts in
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supporting modules which are not themselves test modules will not be rewritten**.
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You can manually enable assertion rewriting for an imported module by calling
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`register_assert_rewrite <https://docs.pytest.org/en/latest/writing_plugins.html#assertion-rewriting>`_
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before you import it (a good place to do that is in ``conftest.py``).
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.. note::
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``pytest`` rewrites test modules on import by using an import
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hook to write new ``pyc`` files. Most of the time this works transparently.
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However, if you are messing with import yourself, the import hook may
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interfere.
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If this is the case you have two options:
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* Disable rewriting for a specific module by adding the string
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``PYTEST_DONT_REWRITE`` to its docstring.
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* Disable rewriting for all modules by using ``--assert=plain``.
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Additionally, rewriting will fail silently if it cannot write new ``.pyc`` files,
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i.e. in a read-only filesystem or a zipfile.
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For further information, Benjamin Peterson wrote up `Behind the scenes of pytest's new assertion rewriting <http://pybites.blogspot.com/2011/07/behind-scenes-of-pytests-new-assertion.html>`_.
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.. versionadded:: 2.1
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Add assert rewriting as an alternate introspection technique.
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.. versionchanged:: 2.1
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Introduce the ``--assert`` option. Deprecate ``--no-assert`` and
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``--nomagic``.
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.. versionchanged:: 3.0
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Removes the ``--no-assert`` and ``--nomagic`` options.
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Removes the ``--assert=reinterp`` option.
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