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Add note explaining Container[T] __contains__ parameter rules #20741

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Add note explaining Container[T] __contains__ parameter rules #20741

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49ad99e
Clarify Container[T] __contains__ parameter rules
KarthickKrishnasaisha Feb 4, 2026
e051f99
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Feb 5, 2026
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14 changes: 14 additions & 0 deletions mypy/messages.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2295,6 +2295,20 @@ def report_protocol_problems(
parent_error=parent_error,
skip_self=class_obj or is_module,
)
# Clarify common confusion around Container[T] and __contains__.
if name == "__contains__" and supertype.type.fullname in {
"typing.Container",
"collections.abc.Container",
}:
self.note(
'Note: "Container[T]" does not restrict the argument type of '
'"__contains__" (it must accept "object"). The type parameter "T" '
"is used for membership checks (e.g. flagging non-overlapping "
"checks under --strict).",
context,
offset=OFFSET,
parent_error=parent_error,
)
self.print_more(conflict_types, context, OFFSET, MAX_ITEMS, code=parent_error.code)

# Report flag conflicts (i.e. settable vs read-only etc.)
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23 changes: 23 additions & 0 deletions test-data/unit/check-protocols.test
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Expand Up @@ -4693,3 +4693,26 @@ class Impl:
pass

x: Proto = Impl()

[case testContainerContainsAcceptsStr]
from collections.abc import Container

class StringContainer:
def __contains__(self, item: str) -> bool:
return True

c: Container[str] = StringContainer() # E: Incompatible types in assignment (expression has type "StringContainer", variable has type "Container[str]") \
# N: Following member(s) of "StringContainer" have conflicts: \
# N: Expected: \
# N: def __contains__(self, object, /) -> bool \
# N: Got: \
# N: def __contains__(self, str, /) -> bool \
# N: Note: "Container[T]" does not restrict the argument type of "__contains__" (it must accept "object"). The type parameter "T" is used for membership checks (e.g. flagging non-overlapping checks under --strict).
[file collections/__init__.pyi]
[file collections/abc.pyi]
from typing import Protocol, TypeVar

T_co = TypeVar("T_co", covariant=True)

class Container(Protocol[T_co]):
def __contains__(self, x: object) -> bool: ...