The writing and reporting of assertions in tests ================================================== .. _`assertfeedback`: .. _`assert with the assert statement`: .. _`assert`: Asserting with the ``assert`` statement --------------------------------------------------------- ``pytest`` allows you to use the standard python ``assert`` for verifying expectations and values in Python tests. For example, you can write the following:: # content of test_assert1.py def f(): return 3 def test_function(): assert f() == 4 to assert that your function returns a certain value. If this assertion fails you will see the return value of the function call:: $ py.test test_assert1.py =========================== test session starts ============================ platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3 collected 1 items test_assert1.py F ================================= FAILURES ================================= ______________________________ test_function _______________________________ def test_function(): > assert f() == 4 E assert 3 == 4 E + where 3 = f() test_assert1.py:5: AssertionError ========================= 1 failed in 0.01 seconds ========================= ``pytest`` has support for showing the values of the most common subexpressions including calls, attributes, comparisons, and binary and unary operators. (See :ref:`tbreportdemo`). This allows you to use the idiomatic python constructs without boilerplate code while not losing introspection information. However, if you specify a message with the assertion like this:: assert a % 2 == 0, "value was odd, should be even" then no assertion introspection takes places at all and the message will be simply shown in the traceback. See :ref:`assert-details` for more information on assertion introspection. .. _`assertraises`: Assertions about expected exceptions ------------------------------------------ In order to write assertions about raised exceptions, you can use ``pytest.raises`` as a context manager like this:: import pytest def test_zero_division(): with pytest.raises(ZeroDivisionError): 1 / 0 and if you need to have access to the actual exception info you may use:: def test_recursion_depth(): with pytest.raises(RuntimeError) as excinfo: def f(): f() f() assert 'maximum recursion' in str(excinfo.value) ``excinfo`` is a `py.code.ExceptionInfo`_ instance, which is a wrapper around the actual exception raised. The main attributes of interest are ``.type``, ``.value`` and ``.traceback``. .. _py.code.ExceptionInfo: http://pylib.readthedocs.org/en/latest/code.html#py-code-exceptioninfo If you want to write test code that works on Python 2.4 as well, you may also use two other ways to test for an expected exception:: pytest.raises(ExpectedException, func, *args, **kwargs) pytest.raises(ExpectedException, "func(*args, **kwargs)") both of which execute the specified function with args and kwargs and asserts that the given ``ExpectedException`` is raised. The reporter will provide you with helpful output in case of failures such as *no exception* or *wrong exception*. Note that it is also possible to specify a "raises" argument to ``pytest.mark.xfail``, which checks that the test is failing in a more specific way than just having any exception raised:: @pytest.mark.xfail(raises=IndexError) def test_f(): f() Using ``pytest.raises`` is likely to be better for cases where you are testing exceptions your own code is deliberately raising, whereas using ``@pytest.mark.xfail`` with a check function is probably better for something like documenting unfixed bugs (where the test describes what "should" happen) or bugs in dependencies. .. _newreport: Making use of context-sensitive comparisons ------------------------------------------------- .. versionadded:: 2.0 ``pytest`` has rich support for providing context-sensitive information when it encounters comparisons. For example:: # content of test_assert2.py def test_set_comparison(): set1 = set("1308") set2 = set("8035") assert set1 == set2 if you run this module:: $ py.test test_assert2.py =========================== test session starts ============================ platform linux -- Python 3.4.0 -- py-1.4.25 -- pytest-2.6.3 collected 1 items test_assert2.py F ================================= FAILURES ================================= ___________________________ test_set_comparison ____________________________ def test_set_comparison(): set1 = set("1308") set2 = set("8035") > assert set1 == set2 E assert set(['0', '1', '3', '8']) == set(['0', '3', '5', '8']) E Extra items in the left set: E '1' E Extra items in the right set: E '5' test_assert2.py:5: AssertionError ========================= 1 failed in 0.01 seconds ========================= Special comparisons are done for a number of cases: * comparing long strings: a context diff is shown * comparing long sequences: first failing indices * comparing dicts: different entries See the :ref:`reporting demo ` for many more examples. Defining your own assertion comparison ---------------------------------------------- It is possible to add your own detailed explanations by implementing the ``pytest_assertrepr_compare`` hook. .. autofunction:: _pytest.hookspec.pytest_assertrepr_compare As an example consider adding the following hook in a conftest.py which provides an alternative explanation for ``Foo`` objects:: # content of conftest.py from test_foocompare import Foo def pytest_assertrepr_compare(op, left, right): if isinstance(left, Foo) and isinstance(right, Foo) and op == "==": return ['Comparing Foo instances:', ' vals: %s != %s' % (left.val, right.val)] now, given this test module:: # content of test_foocompare.py class Foo: def __init__(self, val): self.val = val def test_compare(): f1 = Foo(1) f2 = Foo(2) assert f1 == f2 you can run the test module and get the custom output defined in the conftest file:: $ py.test -q test_foocompare.py F ================================= FAILURES ================================= _______________________________ test_compare _______________________________ def test_compare(): f1 = Foo(1) f2 = Foo(2) > assert f1 == f2 E assert Comparing Foo instances: E vals: 1 != 2 test_foocompare.py:8: AssertionError 1 failed in 0.01 seconds .. _assert-details: .. _`assert introspection`: Advanced assertion introspection ---------------------------------- .. versionadded:: 2.1 Reporting details about a failing assertion is achieved either by rewriting assert statements before they are run or re-evaluating the assert expression and recording the intermediate values. Which technique is used depends on the location of the assert, ``pytest`` configuration, and Python version being used to run ``pytest``. Note that for assert statements with a manually provided message, i.e. ``assert expr, message``, no assertion introspection takes place and the manually provided message will be rendered in tracebacks. By default, if the Python version is greater than or equal to 2.6, ``pytest`` rewrites assert statements in test modules. Rewritten assert statements put introspection information into the assertion failure message. ``pytest`` only rewrites test modules directly discovered by its test collection process, so asserts in supporting modules which are not themselves test modules will not be rewritten. .. note:: ``pytest`` rewrites test modules on import. It does this by using an import hook to write a new pyc files. Most of the time this works transparently. However, if you are messing with import yourself, the import hook may interfere. If this is the case, simply use ``--assert=reinterp`` or ``--assert=plain``. Additionally, rewriting will fail silently if it cannot write new pycs, i.e. in a read-only filesystem or a zipfile. If an assert statement has not been rewritten or the Python version is less than 2.6, ``pytest`` falls back on assert reinterpretation. In assert reinterpretation, ``pytest`` walks the frame of the function containing the assert statement to discover sub-expression results of the failing assert statement. You can force ``pytest`` to always use assertion reinterpretation by passing the ``--assert=reinterp`` option. Assert reinterpretation has a caveat not present with assert rewriting: If evaluating the assert expression has side effects you may get a warning that the intermediate values could not be determined safely. A common example of this issue is an assertion which reads from a file:: assert f.read() != '...' If this assertion fails then the re-evaluation will probably succeed! This is because ``f.read()`` will return an empty string when it is called the second time during the re-evaluation. However, it is easy to rewrite the assertion and avoid any trouble:: content = f.read() assert content != '...' All assert introspection can be turned off by passing ``--assert=plain``. For further information, Benjamin Peterson wrote up `Behind the scenes of pytest's new assertion rewriting `_. .. versionadded:: 2.1 Add assert rewriting as an alternate introspection technique. .. versionchanged:: 2.1 Introduce the ``--assert`` option. Deprecate ``--no-assert`` and ``--nomagic``.