136 lines
4.3 KiB
Plaintext
136 lines
4.3 KiB
Plaintext
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Writing and reporting of assertions in tests
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============================================
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assert with the ``assert`` statement
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---------------------------------------------------------
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``py.test`` allows to use the standard python ``assert`` for verifying
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expectations and values in Python tests. For example, you can write the
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following in your tests::
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# content of test_assert1.py
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def f():
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return 3
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def test_function():
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assert f() == 4
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to state that your object has a certain ``attribute``. In case this
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assertion fails you will see the value of ``x``::
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$ py.test test_assert1.py
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=========================== test session starts ============================
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platform linux2 -- Python 2.6.5 -- pytest-2.0.0.dev22
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test path 1: test_assert1.py
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test_assert1.py F
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================================= FAILURES =================================
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______________________________ test_function _______________________________
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def test_function():
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> assert f() == 4
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E assert 3 == 4
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E + where 3 = f()
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test_assert1.py:5: AssertionError
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========================= 1 failed in 0.05 seconds =========================
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Reporting details about the failing assertion is achieved by re-evaluating
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the assert expression and recording intermediate values.
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Note: If evaluating the assert expression has side effects you may get a
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warning that the intermediate values could not be determined safely. A
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common example for this issue is reading from a file and comparing in one
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line::
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assert f.read() != '...'
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This might fail but when re-interpretation comes along it might pass.
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You can rewrite this (and any other expression with side effects) easily, though:
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content = f.read()
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assert content != '...'
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assertions about expected exceptions
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------------------------------------------
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In order to write assertions about raised exceptions, you can use
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``py.test.raises`` as a context manager like this::
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with py.test.raises(ZeroDivisionError):
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1 / 0
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and if you need to have access to the actual exception info you may use::
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with py.test.raises(RuntimeError) as excinfo:
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def f():
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f()
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f()
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# do checks related to excinfo.type, excinfo.value, excinfo.traceback
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If you want to write test code that works on Python2.4 as well,
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you may also use two other ways to test for an expected exception::
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py.test.raises(ExpectedException, func, *args, **kwargs)
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py.test.raises(ExpectedException, "func(*args, **kwargs)")
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both of which execute the specified function with args and kwargs and
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asserts that the given ``ExpectedException`` is raised. The reporter will
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provide you with helpful output in case of failures such as *no
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exception* or *wrong exception*.
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Making use of context-sensitive comparisons
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-------------------------------------------------
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.. versionadded:: 2.0
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py.test has rich support for providing context-sensitive informations
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when it encounters comparisons. For example::
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# content of test_assert2.py
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def test_set_comparison():
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set1 = set("1308")
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set2 = set("8035")
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assert set1 == set2
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if you run this module::
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$ py.test test_assert2.py
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=========================== test session starts ============================
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platform linux2 -- Python 2.6.5 -- pytest-2.0.0.dev22
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test path 1: test_assert2.py
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test_assert2.py F
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================================= FAILURES =================================
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___________________________ test_set_comparison ____________________________
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def test_set_comparison():
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set1 = set("1308")
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set2 = set("8035")
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> assert set1 == set2
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E assert set(['0', '1', '3', '8']) == set(['0', '3', '5', '8'])
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E Extra items in the left set:
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E '1'
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E Extra items in the right set:
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E '5'
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test_assert2.py:5: AssertionError
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========================= 1 failed in 0.02 seconds =========================
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Special comparisons are done for a number of cases:
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* comparing long strings: a context diff is shown
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* comparing long sequences: first failing indices
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* comparing dicts: different entries
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..
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Defining your own comparison
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----------------------------------------------
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