unit testing class hierarchies

[Terry Reedy]

On 10/3/2012 5:33 AM, Oscar Benjamin wrote:
> On 3 October 2012 02:20, Steven D'Aprano
> <steve+comp.lang.python at pearwood.info> wrote:
>>
>> But surely, regardless of where that functionality is defined, you still
>> need to test that both D1 and D2 exhibit the correct behaviour? Otherwise
>> D2 (say) may break that functionality and your tests won't notice.
>>
>> Given a class hierarchy like this:
>>
>> class AbstractBaseClass:
>>      spam = "spam"
>>
>> class D1(AbstractBaseClass): pass
>> class D2(D1): pass
>>
>>
>> I write tests like this:
>>
>> class TestD1CommonBehaviour(unittest.TestCase):
>>      cls = D1
>>      def testSpam(self):
>>           self.assertTrue(self.cls.spam == "spam")
>>      def testHam(self):
>>           self.assertFalse(hasattr(self.cls, 'ham'))
>>
>> class TestD2CommonBehaviour(TestD1CommonBehaviour):
>>      cls = D2
>
> That's an excellent idea. I wanted a convenient way to run the same
> tests on two classes in order to test both a pure python and a
> cython-accelerator module implementation of the same class.

Python itself has same issue with testing Python and C coded modules. It 
has the additional issue that the Python class by default import the C 
version, so additional work is needed to avoid that and actually test 
the python code.

For instance, heapq.test_heapq.py has

...
py_heapq = support.import_fresh_module('heapq', blocked=['_heapq'])
c_heapq = support.import_fresh_module('heapq', fresh=['_heapq'])
...
class TestHeap(TestCase):
     module = None
... <multiple test methods for functions module.xxx>

class TestHeapPython(TestHeap):
     module = py_heapq

@skipUnless(c_heapq, 'requires _heapq')
class TestHeapC(TestHeap):
     module = c_heapq
...
def test_main(verbose=None):
     test_classes = [TestModules, TestHeapPython, TestHeapC,

# TestHeap is omitted from the list and not run directly

-- 
Terry Jan Reedy