261 lines
9.0 KiB
Plaintext
261 lines
9.0 KiB
Plaintext
|
|
The writing and reporting of assertions in tests
|
|
==================================================
|
|
|
|
.. _`assertfeedback`:
|
|
.. _`assert with the assert statement`:
|
|
.. _`assert`:
|
|
|
|
|
|
Asserting with the ``assert`` statement
|
|
---------------------------------------------------------
|
|
|
|
``py.test`` 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 linux2 -- Python 2.7.3 -- pytest-2.5.0
|
|
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 =========================
|
|
|
|
py.test 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
|
|
with pytest.raises(ZeroDivisionError):
|
|
1 / 0
|
|
|
|
and if you need to have access to the actual exception info you may use::
|
|
|
|
with pytest.raises(RuntimeError) as excinfo:
|
|
def f():
|
|
f()
|
|
f()
|
|
|
|
# do checks related to excinfo.type, excinfo.value, excinfo.traceback
|
|
|
|
``excinfo`` is a `py.code.ExceptionInfo`_ instance, which is a wrapper around
|
|
the actual exception raised.
|
|
|
|
.. _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*.
|
|
|
|
.. _newreport:
|
|
|
|
Making use of context-sensitive comparisons
|
|
-------------------------------------------------
|
|
|
|
.. versionadded:: 2.0
|
|
|
|
py.test 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 linux2 -- Python 2.7.3 -- pytest-2.5.0
|
|
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 <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 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, py.test's configuration, and Python version being used
|
|
to run py.test. 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, py.test
|
|
rewrites assert statements in test modules. Rewritten assert statements put
|
|
introspection information into the assertion failure message. py.test 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::
|
|
|
|
py.test 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, py.test falls back on assert reinterpretation. In assert reinterpretation,
|
|
py.test walks the frame of the function containing the assert statement to
|
|
discover sub-expression results of the failing assert statement. You can force
|
|
py.test 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 py.test'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``.
|