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[Ned Batchelder]

If you have a long series of operations you want to test, I'm not sure 
how you can do better than simply making one long test.  Use many 
assertions in the test.  If one fails, the test will stop.  If you were 
to make a cascade of tests as you describe, the later tests couldn't run 
if an earlier one failed anyway, and the test runner wouldn't know to 
skip the remaining tests.

Another option is to put the state-building code in helper functions 
that you can call from your tests.

--Ned.

On 3/29/15 12:38 PM, dpb dpb wrote:
> Thanks to both of you.
>
> I understand that it's normally the case to want complete isolation 
> between tests.
>
> But on occasion it's useful to run tests on a series of cumulative 
> changes to state. That means forfeiting test independence, or anyway 
> building it into the sequence in which tests are run. The alternatives 
> seem to be placing many asserts into each test function, or running 
> tests based on simulated data.
>
> I see that I can keep track cumulative changes in state pretty simply, 
> using a non-test class (rather than the test class) to define the 
> state-bearing object.
>
> - dpb
>
> On Sun, Mar 29, 2015 at 12:01 PM, Ned Batchelder 
> <ned at nedbatchelder.com <mailto:ned at nedbatchelder.com>> wrote:
>
>     On 3/29/15 4:57 AM, dpb dpb wrote:
>>     Something I do not see addressed explicitly in the Pytest docs is
>>     this behavior: Unlike normal class writing in Python 3, changing
>>     the value of a class attribute in one function does not leave it
>>     changed in test functions that are called subsequently:
>>
>     As Holger mentioned, a new TestExample instance is created for
>     each test.  This is how all the test runners work, because you
>     want isolation between your tests.  One of the principles of the
>     xUnit style of testing is that each test is independent of all
>     other tests.  You want to be able to run a single test and not
>     have it depend on the results of previous tests.
>
>     --Ned.
>
>>         # test_pytest_class_attributes.py
>>         """Test the setting of class attributes."""
>>         class TestExample():
>>             def setup(self):
>>         self.attribute = 1 
>>
>>
>>             def test_changing_attr(self):
>>         """Change attribute on object."""
>>         self.attribute = 2
>>         assert self.attribute == 2 
>>
>>
>>             def test_attr_is_changed(self):
>>         """Assume attribute is changed."""
>>         assert self.attribute == 2 
>>
>>
>>             def test_attr_is_unchanged(self):
>>         """Assume attribute is unchanged."""
>>         assert self.attribute == 1
>>
>>
>>     Output:
>>
>>         $ py.test test_pytest_class_attributes.py -v
>>         ============================= test session starts
>>         ==============================
>>         platform darwin -- Python 3.4.1 -- py-1.4.25 -- pytest-2.6.3
>>         -- /Users/dpb/py34/bin/python3.4
>>         collected 3 items
>>         test_pytest_class_attributes.py::TestExample::test_changing_attr
>>         PASSED
>>         test_pytest_class_attributes.py::TestExample::test_attr_is_changed
>>         FAILED
>>         test_pytest_class_attributes.py::TestExample::test_attr_is_unchanged
>>         PASSED
>>         =================================== FAILURES
>>         ===================================
>>         _______________________ TestExample.test_attr_is_changed
>>         _______________________
>>         self = <test_pytest_class_attributes.TestExample object at
>>         0x10dd98ef0>
>>             def test_attr_is_changed(self):
>>         """Test whether attribute is changed."""
>>         >     assert self.attribute == 2
>>         E       assert 1 == 2
>>         E    +  where 1 = <test_pytest_class_attributes.TestExample
>>         object at 0x10dd98ef0>.attribute 
>>
>>         test_pytest_class_attributes.py:14: AssertionError
>>         ====================== 1 failed, 2 passed in 0.02 seconds
>>         ======================
>>         $
>>
>>
>>     Is there special syntax or some special structure to make class
>>     attributes behave In Pytest as they do in an ordinary Python
>>     class? Example:
>>
>>         # display_class_attributes.py
>>         """Test the setting of class attributes.""" 
>>
>>
>>         class TestExample():
>>         def __init__(self):
>>           self.attribute = 1 
>>
>>
>>         def test_changing_attr(self):
>>           """Change attribute on object."""
>>           self.attribute = 2
>>           assert self.attribute == 2
>>           print('Finished test_changing_attr.\n') 
>>
>>
>>         def test_attr_is_changed(self):
>>           """Test whether attribute is changed."""
>>           assert self.attribute == 2
>>           print('Finished test_attr_is_changed.\n') 
>>
>>
>>         def test_attr_is_unchanged(self):
>>           """Test whether attribute is unchanged."""
>>           assert self.attribute == 1
>>           print('Finiahed test_attr_is_unchanged.\n') 
>>
>>
>>         t = TestExample()
>>         t.test_changing_attr()
>>         t.test_attr_is_changed()
>>         t.test_attr_is_unchanged()
>>
>>
>>     Output:
>>
>>     > $ python display_class_attributes.py
>>     >
>>     > self.attribute == 2: True
>>     >
>>     > self.attribute == 2: True
>>     >
>>     > self.attribute == 1: False
>>     >
>>     > $
>>
>>     Thanks.
>>
>>     - dpb
>>
>>
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