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: .. code-block:: python # 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: .. code-block:: pytest $ pytest test_assert1.py =========================== test session starts ============================ platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y cachedir: $PYTHON_PREFIX/.pytest_cache rootdir: $REGENDOC_TMPDIR collected 1 item test_assert1.py F [100%] ================================= FAILURES ================================= ______________________________ test_function _______________________________ def test_function(): > assert f() == 4 E assert 3 == 4 E + where 3 = f() test_assert1.py:6: AssertionError ========================= short test summary info ========================== FAILED test_assert1.py::test_function - assert 3 == 4 ============================ 1 failed in 0.12s ============================= ``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: .. code-block:: python 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 :func:`pytest.raises` as a context manager like this: .. code-block:: python 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: .. code-block:: python def test_recursion_depth(): with pytest.raises(RuntimeError) as excinfo: def f(): f() f() assert "maximum recursion" in str(excinfo.value) ``excinfo`` is an ``ExceptionInfo`` instance, which is a wrapper around the actual exception raised. The main attributes of interest are ``.type``, ``.value`` and ``.traceback``. You can pass a ``match`` keyword parameter to the context-manager to test that a regular expression matches on the string representation of an exception (similar to the ``TestCase.assertRaisesRegexp`` method from ``unittest``): .. code-block:: python import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises(ValueError, match=r".* 123 .*"): myfunc() The regexp parameter of the ``match`` method is matched with the ``re.search`` function, so in the above example ``match='123'`` would have worked as well. There's an alternate form of the :func:`pytest.raises` function where you pass a function that will be executed with the given ``*args`` and ``**kwargs`` and assert that the given exception is raised: .. code-block:: python pytest.raises(ExpectedException, func, *args, **kwargs) 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: .. code-block:: python @pytest.mark.xfail(raises=IndexError) def test_f(): f() Using :func:`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. .. _`assertwarns`: Assertions about expected warnings ----------------------------------------- You can check that code raises a particular warning using :ref:`pytest.warns `. .. _newreport: Making use of context-sensitive comparisons ------------------------------------------------- ``pytest`` has rich support for providing context-sensitive information when it encounters comparisons. For example: .. code-block:: python # content of test_assert2.py def test_set_comparison(): set1 = set("1308") set2 = set("8035") assert set1 == set2 if you run this module: .. code-block:: pytest $ pytest test_assert2.py =========================== test session starts ============================ platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y cachedir: $PYTHON_PREFIX/.pytest_cache rootdir: $REGENDOC_TMPDIR collected 1 item test_assert2.py F [100%] ================================= FAILURES ================================= ___________________________ test_set_comparison ____________________________ def test_set_comparison(): set1 = set("1308") set2 = set("8035") > assert set1 == set2 E AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'} E Extra items in the left set: E '1' E Extra items in the right set: E '5' E Use -v to get the full diff test_assert2.py:6: AssertionError ========================= short test summary info ========================== FAILED test_assert2.py::test_set_comparison - AssertionError: assert {'0'... ============================ 1 failed in 0.12s ============================= 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 explanation for failed assertions --------------------------------------------------- It is possible to add your own detailed explanations by implementing the ``pytest_assertrepr_compare`` hook. .. autofunction:: _pytest.hookspec.pytest_assertrepr_compare :noindex: As an example consider adding the following hook in a :ref:`conftest.py ` file which provides an alternative explanation for ``Foo`` objects: .. code-block:: python # 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: {} != {}".format(left.val, right.val), ] now, given this test module: .. code-block:: python # content of test_foocompare.py class Foo: def __init__(self, val): self.val = val def __eq__(self, other): return self.val == other.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: .. code-block:: pytest $ pytest -q test_foocompare.py F [100%] ================================= 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:12: AssertionError ========================= short test summary info ========================== FAILED test_foocompare.py::test_compare - assert Comparing Foo instances: 1 failed in 0.12s .. _assert-details: .. _`assert introspection`: Assertion introspection details ------------------------------- Reporting details about a failing assertion is achieved by rewriting assert statements before they are run. 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**. You can manually enable assertion rewriting for an imported module by calling `register_assert_rewrite `_ before you import it (a good place to do that is in your root ``conftest.py``). For further information, Benjamin Peterson wrote up `Behind the scenes of pytest's new assertion rewriting `_. Assertion rewriting caches files on disk ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``pytest`` will write back the rewritten modules to disk for caching. You can disable this behavior (for example to avoid leaving stale ``.pyc`` files around in projects that move files around a lot) by adding this to the top of your ``conftest.py`` file: .. code-block:: python import sys sys.dont_write_bytecode = True Note that you still get the benefits of assertion introspection, the only change is that the ``.pyc`` files won't be cached on disk. Additionally, rewriting will silently skip caching if it cannot write new ``.pyc`` files, i.e. in a read-only filesystem or a zipfile. Disabling assert rewriting ~~~~~~~~~~~~~~~~~~~~~~~~~~ ``pytest`` rewrites test modules on import by using an import hook to write new ``pyc`` files. Most of the time this works transparently. However, if you are working with the import machinery yourself, the import hook may interfere. If this is the case you have two options: * Disable rewriting for a specific module by adding the string ``PYTEST_DONT_REWRITE`` to its docstring. * Disable rewriting for all modules by using ``--assert=plain``.