refine and streamline Floris example for an assert_reprcompare hook.
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holger krekel
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doc/assert.txt
176
doc/assert.txt
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@ -22,14 +22,14 @@ 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 linux2 -- Python 2.6.6 -- pytest-2.0.3
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============================= test session starts ==============================
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platform darwin -- Python 2.7.0 -- pytest-2.1.0.dev4
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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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=================================== 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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@ -37,7 +37,7 @@ you will see the return value of the function call::
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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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=========================== 1 failed in 0.05 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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@ -104,14 +104,14 @@ when it encounters comparisons. For example::
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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 2.6.6 -- pytest-2.0.3
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============================= test session starts ==============================
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platform darwin -- Python 2.7.0 -- pytest-2.1.0.dev4
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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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=================================== 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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@ -124,7 +124,7 @@ if you run this module::
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E '5'
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test_assert2.py:5: AssertionError
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========================= 1 failed in 0.03 seconds =========================
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=========================== 1 failed in 0.05 seconds ===========================
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Special comparisons are done for a number of cases:
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@ -134,6 +134,54 @@ Special comparisons are done for a number of cases:
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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 0.05 seconds
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.. _assert-details:
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.. _`assert introspection`:
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@ -198,111 +246,3 @@ All assert introspection can be turned off by passing ``--assertmode=off``.
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``--nomagic``.
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Defining your own comparison
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----------------------------------------------
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As already shown in examples py.test is able to provide detailed
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explanations when an assertial fails. E.g. when comparing a
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dictionary it will show you which elements differ::
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$ py.test example.py
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============================= test session starts ==========================
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platform linux2 -- Python 2.7.1 -- pytest-2.0.3
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collected 1 items
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example.py F
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=================================== FAILURES ===============================
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__________________________________ test_text _______________________________
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def test_text():
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> assert {'foo': 0, 'bar': 1} == {'foo': 0, 'bar': 0}
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E assert {'bar': 1, 'foo': 0} == {'bar': 0, 'foo': 0}
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E - {'bar': 1, 'foo': 0}
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E ? ^
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E + {'bar': 0, 'foo': 0}
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E ? ^
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example.py:2: AssertionError
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=========================== 1 failed in 0.03 seconds =======================
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py.test has builtin knowledge about displaying detailed information
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for a number of types. If the objects compared do not match those it
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will fall back to a less detailed genric comparison, e.g.::
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$ py.test example2.py
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============================= test session starts ==========================
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platform linux2 -- Python 2.7.1 -- pytest-2.0.3
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collected 1 items
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example2.py F
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=================================== FAILURES ===============================
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________________________________ test_foo __________________________________
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def test_foo():
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f = Foo(1)
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g = Foo(2)
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> assert f == g
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E assert <example2.Foo object at 0x190de90> == <example2.Foo object at 0x190df10>
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example2.py:24: AssertionError
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=========================== 1 failed in 0.03 seconds =======================
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The detailed builtin comparisons are currently only present for
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strings, sequences, sets and dictionaries which are compared for
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equality (``==``) and for strings with a ``not in`` comparison.
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However it is possible to add your own detailed explanations using the
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``pytest_assertrepr_compare`` hook.
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.. py:function:: pytest_assertrepr_compare(config, op, left, right)
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The *config* argument is a ``_pytest.config.Config``
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instance. *op* will be the comparison operator: ``==``, ``<``,
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``in``, etc. While *left* and *right* will contain the objects
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which are being compared.
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The return value must be either *None* in case this hook does not
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provide an explanation for the arguments passed in, or a list of
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strings. Each string in the list will be regarded as a line and
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displayed on a line by itself. The first line is slightly special
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and is meant to be a summary with the detailed explanation
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following on the other lines.
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As an example consider adding following hook in a conftest.py which
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provides an alternative explanation for the ``Foo`` type used above::
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def pytest_assertrepr_compare(op, left, right):
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if (not isinstance(left, example2.Foo) or
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not isinstance(right, example2.Foo) or
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op != '=='):
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return None
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return ['Comparing Foo instances:',
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' vals: %s != %s' % (left.val, right.val)]
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When re-running the above example after adding this in the conftest.py
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file the output now contains a more detailed description::
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$ python pytest.py example2.py
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============================= test session starts ==========================
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platform linux2 -- Python 2.7.1 -- pytest-2.1.0.dev4
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collected 1 items
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example2.py F
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=================================== FAILURES ===============================
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___________________________________ test_foo _______________________________
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def test_foo():
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f = Foo(1)
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g = Foo(2)
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> assert f == g
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E assert Comparing Foo instances:
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E vals: 1 != 2
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example2.py:24: AssertionError
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=========================== 1 failed in 0.03 seconds =======================
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