As part of PEP 399 <https://www.python.org/dev/peps/pep-0399/>, an idiom for testing both C and pure Python versions of a library is suggested making use if import_fresh_module.
Unfortunately, I'm finding that this is not amazingly robust. We have this issue: https://bugs.python.org/issue40058, where the tester for datetime needs to do some funky manipulations <https://github.com/python/cpython/blob/302e5a8f79514fd84bafbc44b7c97ec636302322/Lib/test/test_datetime.py#L14-L23>to the state of sys.modules for reasons that are now somewhat unclear, and still sys.modules is apparently left in a bad state. When implementing PEP 615, I ran into similar issues and found it very difficult to get two independent instances of the same module – one with the C extension blocked and one with it intact. I ended up manually importing the C and Python extensions and grafting them onto two "fresh" imports with nothing blocked <https://github.com/pganssle/zoneinfo/blob/ffd21a6d065e04725e04b37bb430c2559fefd5fa/tests/_support.py#L14>. This seems to work most of the time in my repo, but when I import it into CPython, I'm now seeing failures due to this issue. The immediate symptom is that assertRaises is seeing a mismatch between the exception raised by the module and the exception *on* the module. Here's the Travis error <https://travis-ci.org/github/python/cpython/jobs/683080079#L3154-L3186> (ignore the part about `tzdata`, that needs to be removed as misleading), and here's the test <https://github.com/python/cpython/pull/19909/files#diff-885914f7d01a0c33a25e41809f812068R220-R222>. Evidently calling module.ZoneInfo("Bad_Zone") is raising a different module's ZoneInfoNotFoundError in some cases and I have no idea why. Is anyone familiar more familiar with the import system willing to take a look at these issues? Thanks, Paul
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