Preserve narrowing in unreachable code #20710
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hauntsaninja
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
JukkaL
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JukkaL
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Not a full review, but I'm interested if this affects performance.
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| # The second pass narrows down the types and type checks bodies. | ||
| unmatched_types: TypeMap = None | ||
| unmatched_types: TypeMap = {s.subject: UninhabitedType()} |
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Now we construct some extra temporary dictionaries and UninhabitedType objects. Can you measure if this has measurable performance impact?
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There is maybe some perf impact, something like 0.3% on self check I think it's well worth the cost. Not checking unreachable code is one of the most surprising mypy behaviours and it's a thing that makes accurately improving narrowing right now quite scary (see e.g. #20660 (comment) or fixing #12535 ) I also do think I can maybe claw some of it back in future refactors |
A5rocks
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A5rocks
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Makes sense!
If perf really matters, maybe we can replace the dictionaries with an associative list or something like that. Or even a 2-tuple if a function is guaranteed to return at most 1 result! But IMO it's worth it to take a slight perf hit in order to provide new functionality.
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Yes, I was experimenting with 2-tuples and this is promising (but a larger change). Esp since we actually probably want to combine keyed by I also recently landed several percentage point improvements, so I feel it is okay if I spend 0.3% :-) |
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In the future, it would be nice to have an option to continue to check unreachable code. See #18707 from A5rocks. This PR lays some semantics preserving groundwork for that.
This is effectively a subset of #18707 , but I go just a little bit further and change the type of TypeMap to exclude None
Btw A5rocks if you are willing to rebase your PR, that would be great. I think
--check-unreachablewill be a very useful feature.Co-authored-by: A5rocks