True, the current idiom needs to still be used in those cases, although we could introduce another method to help with this situation as well:

# Could also be named use_accelerator to be less hostile-sounding.
def requires_accelerator(self, cls):
  if self.accelerated_module is None:
    raise SkipTest  # With proper message
  else:
    setattr(cls, self.module_name, self.accelerated_module)
    return cls

Then the idiom becomes:

  @pep399_tests.requires_accelerator
  class AcceleratorSpecificTests(unittest.TestCase): pass


This then extends out to also the current idiom if you don't want to have any "magical" classes:

  @pep399_tests.requires_accelerator
  class AcceleratedExampleTests(unittest.TestCase): pass

  # Can add another decorator for this if desired.
  class PyExampleTests(unittest.TestCase):
    module = pep399_tests.py_module


This also has the benefit of extracting out the module attribute name to minimize messing even that up.