[ty] Make TypeIs invariant in its type argument #20428
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ericmarkmartin
requested review from
AlexWaygood,
carljm,
dcreager and
sharkdp
as code owners
Diagnostic diff on typing conformance testsNo changes detected when running ty on typing conformance tests ✅ |
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| ## TypeIs | ||
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| ```toml | ||
| [environment] | ||
| python-version = "3.13" | ||
| ``` | ||
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| `TypeIs[T]` is invariant in `T`. See the [typing spec][typeis-spec] for a justification. | ||
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| ```py | ||
| from typing import TypeIs | ||
| from ty_extensions import is_assignable_to, is_subtype_of, static_assert | ||
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| class A: | ||
| pass | ||
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| class B(A): | ||
| pass | ||
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| assert not is_subtype_of(TypeIs[B], TypeIs[A]) | ||
| assert not is_subtype_of(TypeIs[A], TypeIs[B]) | ||
| assert not is_assignable_to(TypeIs[B], TypeIs[A]) | ||
| assert not is_assignable_to(TypeIs[A], TypeIs[B]) | ||
| ``` |
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Isn't TypeIs's variance already tested in is_subtype_of.md?
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Oh this is interesting. The pre-existence of these tests implies the invariance was already being enforced? I'll have to look further
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Do the tests you added here fail before the code change in this PR?
It seems to me that what you are testing for here is assignability/subtyping of TypeIs types themselves (which I think was already correctly invariant), but the code change you made would not impact that, it would instead affect variance inference when you have e.g. TypeIs[T] as part of a method signature or as an attribute on a PEP695 generic class with typevar T. Which seems like a rare/unusual case, but still worth getting right.
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Yeah, I missed that there was existing logic for the in variance of TypeIs. Changed the tests, and also fixed assert -> static_assert 🙈
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Summary
What it says on the tin. See the typing spec for justification.
Test Plan
Add more tests to PEP 695
variance.mdsuite.