I'd call the descriptor methods directly. You are unit testing the descriptors, not how Python uses descriptors in general.
That way, you also have far more control over what exactly gets passed in; you can mock out the type and instance arguments to your hearts content.
import unittest
class MockClass(object):
# add any methods to assert access here
class DescriptorTests(unittest.TestCase):
def _make_one(self, *args, **kw):
from original_module import DescriptorClass
return DescriptorClass(*args, **kw)
def test_class_access(self):
# only a type is passed in, no instance
desc = self._make_one()
res = desc.__get__(None, MockClass)
self.assertEqual(res.someattribute, 'somevalue')
# etc.
if __name__ == '__main__':
unittest.main()