Add note explaining Container[T] __contains__ parameter rules #20741
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A5rocks
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Feb 4, 2026
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I don't think this is actually a common misunderstanding.
test-data/unit/check-protocols.test
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| def __contains__(self, item: str) -> bool: | ||
| return True | ||
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| c: Container[str] = StringContainer() |
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This test seems incomplete?
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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This PR clarifies a common point of confusion around Container[T] and the
required signature of contains.
Container[T] does not restrict the argument type of contains, which
must accept object to remain substitutable. The type parameter T is used
for membership checks and narrowing, not for the method signature.
The change adds a short explanatory note to the protocol mismatch
diagnostic and includes a regression test covering this case.
Fixes #20740