Hi,

> I don't think the decorator approach would work for the doctests, it looks 
> like it could be an interesting approach though. I have a feeling that 
> it's going to have to be done in some kind of ugly subclass though, I'll 
> dig into unittest deeper this weekend to see how that might be done.

Doctests will be annoying indeed. I never use doctests so I can't
suggest you anything.
As for standard unit tests, the common idiom is something like:

class JSONEncodingTests:
    def test_encode1(self):
        self.assertEquals(self.encode("foo"), "bar")
    # etc.

class CJSONEncodingTests(JSONEncodingTests, unittest.TestCase):
    encode = json.c_encode

class PyJSONEncodingTests(JSONEncodingTests, unittest.TestCase):
    encode = json.py_encode

(I'm CC'ing you since bugs.python.org looks down)

Regards

Antoine.