From 49ad99ee66f1d4d51ddd89051404cf6e7b2ecc13 Mon Sep 17 00:00:00 2001
From: karthickkrishnasaisha <karthickkrishna51@gmail.com>
Date: Wed, 4 Feb 2026 12:54:46 -0800
Subject: [PATCH 1/2] Clarify Container[T] __contains__ parameter rules

---
 mypy/messages.py                    | 15 +++++++++++++++
 test-data/unit/check-protocols.test | 23 +++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/mypy/messages.py b/mypy/messages.py
index c5756a463894..d5cbfc93047f 100644
--- a/mypy/messages.py
+++ b/mypy/messages.py
@@ -2295,6 +2295,21 @@ 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.)
diff --git a/test-data/unit/check-protocols.test b/test-data/unit/check-protocols.test
index c5136415edd3..442e2003598e 100644
--- a/test-data/unit/check-protocols.test
+++ b/test-data/unit/check-protocols.test
@@ -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: ...

From e051f99d185281261e2710c6fbce3e6b25c9a0ae Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Thu, 5 Feb 2026 03:10:33 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/messages.py | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/mypy/messages.py b/mypy/messages.py
index d5cbfc93047f..96ff55fb4a1e 100644
--- a/mypy/messages.py
+++ b/mypy/messages.py
@@ -2296,11 +2296,10 @@ def report_protocol_problems(
                             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"}
-                    ):
+                    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" '