313 lines
10 KiB
ReStructuredText
313 lines
10 KiB
ReStructuredText
|
|
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:
|
|
|
|
.. code-block:: pytest
|
|
|
|
$ pytest test_assert1.py
|
|
=========================== test session starts ============================
|
|
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
|
|
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('$REGENDOC_TMPDIR/.hypothesis/examples')
|
|
rootdir: $REGENDOC_TMPDIR, inifile:
|
|
plugins: hypothesis-3.x.y
|
|
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:5: AssertionError
|
|
========================= 1 failed in 0.12 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 ``ExceptionInfo`` instance, which is a wrapper around
|
|
the actual exception raised. The main attributes of interest are
|
|
``.type``, ``.value`` and ``.traceback``.
|
|
|
|
.. versionchanged:: 3.0
|
|
|
|
In the context manager form you may use the keyword argument
|
|
``message`` to specify a custom failure message::
|
|
|
|
>>> with raises(ZeroDivisionError, message="Expecting ZeroDivisionError"):
|
|
... pass
|
|
... Failed: Expecting ZeroDivisionError
|
|
|
|
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)
|
|
|
|
which will execute the specified function with args and kwargs and
|
|
assert 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.
|
|
|
|
Also, the context manager form accepts a ``match`` keyword parameter to test
|
|
that a regular expression matches on the string representation of an exception
|
|
(like the ``TestCase.assertRaisesRegexp`` method from ``unittest``)::
|
|
|
|
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.
|
|
|
|
|
|
.. _`assertwarns`:
|
|
|
|
Assertions about expected warnings
|
|
-----------------------------------------
|
|
|
|
.. versionadded:: 2.8
|
|
|
|
You can check that code raises a particular warning using
|
|
:ref:`pytest.warns <warns>`.
|
|
|
|
|
|
.. _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:
|
|
|
|
.. code-block:: pytest
|
|
|
|
$ pytest test_assert2.py
|
|
=========================== test session starts ============================
|
|
platform linux -- Python 3.x.y, pytest-4.x.y, py-1.x.y, pluggy-0.x.y
|
|
hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('$REGENDOC_TMPDIR/.hypothesis/examples')
|
|
rootdir: $REGENDOC_TMPDIR, inifile:
|
|
plugins: hypothesis-3.x.y
|
|
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:5: AssertionError
|
|
========================= 1 failed in 0.12 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 <tbreportdemo>` 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
|
|
: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::
|
|
|
|
# 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(object):
|
|
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:11: AssertionError
|
|
1 failed in 0.12 seconds
|
|
|
|
.. _assert-details:
|
|
.. _`assert introspection`:
|
|
|
|
Advanced assertion introspection
|
|
----------------------------------
|
|
|
|
.. versionadded:: 2.1
|
|
|
|
|
|
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/latest/writing_plugins.html#assertion-rewriting>`_
|
|
before you import it (a good place to do that is in ``conftest.py``).
|
|
|
|
.. note::
|
|
|
|
``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 messing with import 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``.
|
|
|
|
Additionally, rewriting will fail silently if it cannot write new ``.pyc`` files,
|
|
i.e. in a read-only filesystem or a zipfile.
|
|
|
|
|
|
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>`_.
|
|
|
|
.. versionadded:: 2.1
|
|
Add assert rewriting as an alternate introspection technique.
|
|
|
|
.. versionchanged:: 2.1
|
|
Introduce the ``--assert`` option. Deprecate ``--no-assert`` and
|
|
``--nomagic``.
|
|
|
|
.. versionchanged:: 3.0
|
|
Removes the ``--no-assert`` and ``--nomagic`` options.
|
|
Removes the ``--assert=reinterp`` option.
|