251 lines
8.8 KiB
Plaintext
251 lines
8.8 KiB
Plaintext
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The writing and reporting of assertions in tests
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==================================================
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.. _`assert with the assert statement`:
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Asserting with the ``assert`` statement
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---------------------------------------------------------
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``py.test`` allows you 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::
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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 assert that your function returns a certain value. If this assertion fails
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you will see the return value of the function call::
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$ py.test test_assert1.py
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=========================== test session starts ============================
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platform darwin -- Python 2.7.1 -- pytest-2.2.0
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collecting ... collected 1 items
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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 0.02 seconds =========================
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py.test has support for showing the values of the most common subexpressions
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including calls, attributes, comparisons, and binary and unary
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operators. (See :ref:`tbreportdemo`). This allows you to use the
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idiomatic python constructs without boilerplate code while not losing
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introspection information.
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However, if you specify a message with the assertion like this::
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assert a % 2 == 0, "value was odd, should be even"
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then no assertion introspection takes places at all and the message
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will be simply shown in the traceback.
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See :ref:`assert-details` for more information on assertion introspection.
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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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``pytest.raises`` as a context manager like this::
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import pytest
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with pytest.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 pytest.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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pytest.raises(ExpectedException, func, *args, **kwargs)
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pytest.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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.. _newreport:
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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 information
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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 darwin -- Python 2.7.1 -- pytest-2.2.0
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collecting ... collected 1 items
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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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See the :ref:`reporting demo <tbreportdemo>` for many more examples.
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Defining your own assertion comparison
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----------------------------------------------
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It is possible to add your own detailed explanations by implementing
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the ``pytest_assertrepr_compare`` hook.
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.. autofunction:: _pytest.hookspec.pytest_assertrepr_compare
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As an example consider adding the following hook in a conftest.py which
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provides an alternative explanation for ``Foo`` objects::
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# content of conftest.py
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from test_foocompare import Foo
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def pytest_assertrepr_compare(op, left, right):
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if isinstance(left, Foo) and isinstance(right, Foo) and op == "==":
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return ['Comparing Foo instances:',
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' vals: %s != %s' % (left.val, right.val)]
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now, given this test module::
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# content of test_foocompare.py
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class Foo:
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def __init__(self, val):
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self.val = val
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def test_compare():
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f1 = Foo(1)
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f2 = Foo(2)
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assert f1 == f2
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you can run the test module and get the custom output defined in
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the conftest file::
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$ py.test -q test_foocompare.py
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collecting ... collected 1 items
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F
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================================= FAILURES =================================
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_______________________________ test_compare _______________________________
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def test_compare():
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f1 = Foo(1)
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f2 = Foo(2)
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> assert f1 == f2
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E assert Comparing Foo instances:
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E vals: 1 != 2
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test_foocompare.py:8: AssertionError
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1 failed in 0.02 seconds
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.. _assert-details:
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.. _`assert introspection`:
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Advanced assertion introspection
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----------------------------------
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.. versionadded:: 2.1
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Reporting details about a failing assertion is achieved either by rewriting
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assert statements before they are run or re-evaluating the assert expression and
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recording the intermediate values. Which technique is used depends on the
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location of the assert, py.test's configuration, and Python version being used
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to run py.test. Note that for assert statements with a manually provided
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message, i.e. ``assert expr, message``, no assertion introspection takes place
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and the manually provided message will be rendered in tracebacks.
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By default, if the Python version is greater than or equal to 2.6, py.test
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rewrites assert statements in test modules. Rewritten assert statements put
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introspection information into the assertion failure message. py.test only
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rewrites test modules directly discovered by its test collection process, so
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asserts in supporting modules which are not themselves test modules will not be
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rewritten.
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.. note::
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py.test rewrites test modules on import. It does this by using an import hook
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to write a new pyc files. Most of the time this works transparently. However,
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if you are messing with import yourself, the import hook may interfere. If
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this is the case, simply use ``--assert=reinterp`` or
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``--assert=plain``. Additionally, rewriting will fail silently if it cannot
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write new pycs, i.e. in a read-only filesystem or a zipfile.
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If an assert statement has not been rewritten or the Python version is less than
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2.6, py.test falls back on assert reinterpretation. In assert reinterpretation,
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py.test walks the frame of the function containing the assert statement to
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discover sub-expression results of the failing assert statement. You can force
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py.test to always use assertion reinterpretation by passing the
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``--assert=reinterp`` option.
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Assert reinterpretation has a caveat not present with assert rewriting: If
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evaluating the assert expression has side effects you may get a warning that the
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intermediate values could not be determined safely. A common example of this
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issue is an assertion which reads from a file::
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assert f.read() != '...'
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If this assertion fails then the re-evaluation will probably succeed!
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This is because ``f.read()`` will return an empty string when it is
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called the second time during the re-evaluation. However, it is
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easy to rewrite the assertion and avoid any trouble::
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content = f.read()
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assert content != '...'
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All assert introspection can be turned off by passing ``--assert=plain``.
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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>`_.
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.. versionadded:: 2.1
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Add assert rewriting as an alternate introspection technique.
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.. versionchanged:: 2.1
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Introduce the ``--assert`` option. Deprecate ``--no-assert`` and
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``--nomagic``.
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