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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
# content of
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 session starts ========
platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
rootdir: $REGENDOC_TMPDIR, inifile:
collected 1 items F
======= FAILURES ========
_______ test_function ________
def test_function():
> assert f() == 4
E assert 3 == 4
E + where 3 = f() 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():
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``.
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::
def test_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.
.. _`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
def test_set_comparison():
set1 = set("1308")
set2 = set("8035")
assert set1 == set2
if you run this module::
$ py.test
======= test session starts ========
platform linux -- Python 3.4.0, pytest-2.9.1, py-1.4.31, pluggy-0.3.1
rootdir: $REGENDOC_TMPDIR, inifile:
collected 1 items 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'
E Use -v to get the full diff 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
As an example consider adding the following hook in a which
provides an alternative explanation for ``Foo`` objects::
# content of
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
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::
$ py.test -q
======= FAILURES ========
_______ test_compare ________
def test_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E assert Comparing Foo instances:
E vals: 1 != 2 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 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``.
By default, ``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
.. 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 != '...'
If this assertion fails then the re-evaluation will probably succeed!
This is because ```` 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 =
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