D'oh. I just realized why the -M option wasn't being recognized - I had passed it after the actual test name, so it was being treated as another test instead of an option. Sorry for the confusion :/ As for the actual test results, test_huge_bytes_(32|64)b both pass, but test_huge_str fails with this traceback: ====================================================================== FAIL: test_huge_str (test.test_pickle.InMemoryPickleTests) ---------------------------------------------------------------------- Traceback (most recent call last): File "/usr/local/google/home/nadeemvawda/code/cpython/3.2/Lib/test/support.py", line 1108, in wrapper return f(self, maxsize) File "/usr/local/google/home/nadeemvawda/code/cpython/3.2/Lib/test/pickletester.py", line 1151, in test_huge_str self.dumps(data, protocol=proto) AssertionError: (<class 'ValueError'>, <class 'OverflowError'>) not raised The same error occurs on the default branch.