Narrow is with final types correctly
#15646
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Problems looks real, checking. |
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mypy soon will have special narrowing for `@final` singleton types: python/mypy#15646 So, it is a good idea to use `@final` for such types.
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| if isinstance(item, (CallableType, TypeType)) and item.is_singleton_type(): |
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What if the RHS of the is operator is a specialized generic class like list[int]? The is conditional will always evaluate to False then.
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| x: Union[Type[Mark], Type[Other], Type[None], int] | ||
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| if x is Mark: |
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What about M = NewType("M", Mark")? This acts as a subtype of Mark (and is allowed even though Mark is marked @final), but x is Mark will evaluate to false if x is M. Maybe that's an edge case that can be ignored, but it does represent a small hole.
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I guess we can ignore this edge case. I'd be surprised if users are passing the NewType definition somewhere as a value, and even more in this particular situation combined with a @final. The benefit from this feature outweighs supporting the edge case.
And even then, isn't the issue only in the opposite case, when we have if x is not Marc:? A situation where at runtime we do enter the block, but we narrowed incorrectly.
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Why can you create a NewType of a final type? That seems inconsistent - NewType creates a subtype, but final means that the type isn't allowed to have subclasses. I don't see this mentioned anywhere in the docs/PEPs. I guess it could mean that you have a subset of values, but they all will be the same type in reality. It's still a little contradictory though.
| x: Union[Type[Mark], Type[None], int] | ||
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| reveal_type(x) # N: Revealed type is "Union[Type[__main__.Mark], Type[None], builtins.int]" |
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Why does this not narrow the type to Type[Mark] in the positive (if) case?
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Diff from mypy_primer, showing the effect of this PR on open source code: trio (https://github.com/python-trio/trio)
+ src/trio/_core/_local.py:54: error: Unused "type: ignore" comment [unused-ignore]
+ src/trio/_core/_local.py:57: error: Unused "type: ignore" comment [unused-ignore]
steam.py (https://github.com/Gobot1234/steam.py)
+ steam/ext/commands/converters.py:532: error: Unused "type: ignore" comment [unused-ignore]
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Now we can narrow
@finaltypes withisandis not.Closes #15553