Fix partially defined in the case of missing type maps #15995
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Thanks @AlexWaygood for sending me on this adventure. This took me a while to debug! It turns out when we don't need to warn about unreachable code, we don't end up calling `self.is_noop_for_reachability(s)` (which is meant to tell us whether the code should be warned about or is `raise AssertionError` or `typing.assert_never(never)` or something. https://github.com/python/mypy/blob/6f650cff9ab21f81069e0ae30c92eae94219ea63/mypy/checker.py#L2748 This innocuous check has a side effect that turns out to be important for the partially undefined checks. These checks work by reaching into the type map populated by the checker. But if we never actually ended up analysing the code, we never populate the type map. This therefore changes things to assume that if we couldn't find the expression in the type map, it's probably because it was unreachable.
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
Thanks so much for digging into this! :D |
AlexWaygood
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Aug 30, 2023
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| [case testUnreachableCausingMissingTypeMap] | ||
| # flags: --enable-error-code possibly-undefined --enable-error-code used-before-def --no-warn-unreachable | ||
| # Regression test for https://github.com/python/mypy/issues/15958 | ||
| from typing import Union, NoReturn | ||
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| def assert_never(__x: NoReturn) -> NoReturn: ... | ||
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| def foo(x: Union[int, str]) -> None: | ||
| if isinstance(x, str): | ||
| f = "foo" | ||
| elif isinstance(x, int): | ||
| f = "bar" | ||
| else: | ||
| assert_never(x) | ||
| f # OK | ||
| [builtins fixtures/tuple.pyi] |
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Would it be worth adding a match/case test as well, or would that just be unnecessarily duplicative?
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Good q, I think just duplicative. (fwiw I'd changed to if since that runs on all Python versions)
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Thanks @AlexWaygood for sending me on this adventure. This took me a while for me to debug!
When we don't need to warn about unreachable code, we don't end up calling
self.is_noop_for_reachability(s)(which is meant to tell us whether the code should be warned about or israise AssertionErrorortyping.assert_never(never)or something).mypy/mypy/checker.py
Line 2748 in 6f650cf
This innocuous check has a side effect that turns out to be important for the partially undefined checks. These checks work by reaching into the type map populated by the checker. But if we never actually ended up analysing the code, we never populate the type map.
This therefore changes things to assume that if we couldn't find the expression in the type map, it's probably because it was unreachable.