path: root/doc/en/assert.rst

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 <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 <tbreportdemo>` 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 <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 <https://docs.pytest.org/en/stable/writing_plugins.html#assertion-rewriting>`_
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 <http://pybites.blogspot.com/2011/07/behind-scenes-of-pytests-new-assertion.html>`_.

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``.