142 lines
4.4 KiB
Plaintext
142 lines
4.4 KiB
Plaintext
|
|
Writing and reporting of assertions in tests
|
|
============================================
|
|
|
|
.. _`assert with the assert statement`:
|
|
|
|
assert with the ``assert`` statement
|
|
---------------------------------------------------------
|
|
|
|
``py.test`` allows to use the standard python ``assert`` for verifying
|
|
expectations and values in Python tests. For example, you can write the
|
|
following in your tests::
|
|
|
|
# content of test_assert1.py
|
|
def f():
|
|
return 3
|
|
|
|
def test_function():
|
|
assert f() == 4
|
|
|
|
to state that your object has a certain ``attribute``. In case this
|
|
assertion fails you will see the value of ``x``::
|
|
|
|
$ py.test test_assert1.py
|
|
=========================== test session starts ============================
|
|
platform linux2 -- Python 2.6.5 -- pytest-2.0.0
|
|
collecting ... 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 0.02 seconds =========================
|
|
|
|
Reporting details about the failing assertion is achieved by re-evaluating
|
|
the assert expression and recording intermediate values.
|
|
|
|
Note: 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 for this issue is reading from a file and comparing in one
|
|
line::
|
|
|
|
assert f.read() != '...'
|
|
|
|
This might fail but when re-interpretation comes along it might pass. You can
|
|
rewrite this (and any other expression with side effects) easily, though::
|
|
|
|
content = f.read()
|
|
assert content != '...'
|
|
|
|
assertions about expected exceptions
|
|
------------------------------------------
|
|
|
|
In order to write assertions about raised exceptions, you can use
|
|
``pytest.raises`` as a context manager like this::
|
|
|
|
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
|
|
|
|
If you want to write test code that works on Python2.4 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 informations
|
|
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 2.6.5 -- pytest-2.0.0
|
|
collecting ... 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 0.02 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 comparison
|
|
----------------------------------------------
|
|
|
|
|