Do not handle NotImplementedType as Any anymore.

misc/typeshed_patches/0001-Stop-subclassing_NotImplemented-from-Any.patch

@@ -0,0 +1,25 @@
+From 9b584b626fc18881055a3f48080fe3afcf9bb583 Mon Sep 17 00:00:00 2001
+From: Christoph Tyralla <c.tyralla@bjoernsen.de>
+Date: Sat, 25 Oct 2025 09:39:03 +0200
+Subject: [PATCH] modify typeshed instead of hacking `calculate_mro`
+
+---
+ mypy/typeshed/stdlib/builtins.pyi | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/mypy/typeshed/stdlib/builtins.pyi b/mypy/typeshed/stdlib/builtins.pyi
+index ddf81db18..933a06640 100644
+--- a/mypy/typeshed/stdlib/builtins.pyi
++++ b/mypy/typeshed/stdlib/builtins.pyi
+@@ -1269,7 +1269,7 @@ class property:
+
+ @final
+ @type_check_only
+-class _NotImplementedType(Any):
++class _NotImplementedType:
+     __call__: None
+
+ NotImplemented: _NotImplementedType
+-- 
+2.45.1.windows.1
+

mypy/checker.py

@@ -178,6 +178,7 @@
     coerce_to_literal,
     custom_special_method,
     erase_def_to_union_or_bound,
+    erase_notimplemented,
     erase_to_bound,
     erase_to_union_or_bound,
     false_only,
@@ -4895,6 +4896,7 @@ def infer_context_dependent(
         return typ
 
     def check_return_stmt(self, s: ReturnStmt) -> None:
+
         defn = self.scope.current_function()
         if defn is not None:
             if defn.is_generator:
@@ -4942,17 +4944,11 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                             s.expr, return_type, allow_none_return=allow_none_func_call
                         )
                     )
-                # Treat NotImplemented as having type Any, consistent with its
-                # definition in typeshed prior to python/typeshed#4222.
-                if (
-                    isinstance(typ, Instance)
-                    and typ.type.fullname == "builtins._NotImplementedType"
-                ):
-                    typ = AnyType(TypeOfAny.special_form)
 
                 if defn.is_async_generator:
                     self.fail(message_registry.RETURN_IN_ASYNC_GENERATOR, s)
                     return
+
                 # Returning a value of type Any is always fine.
                 if isinstance(typ, AnyType):
                     # (Unless you asked to be warned in that case, and the
@@ -4961,10 +4957,6 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                         self.options.warn_return_any
                         and not self.current_node_deferred
                         and not is_proper_subtype(AnyType(TypeOfAny.special_form), return_type)
-                        and not (
-                            defn.name in BINARY_MAGIC_METHODS
-                            and is_literal_not_implemented(s.expr)
-                        )
                         and not (
                             isinstance(return_type, Instance)
                             and return_type.type.fullname == "builtins.object"
@@ -4983,9 +4975,12 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                         return
                     self.fail(message_registry.NO_RETURN_VALUE_EXPECTED, s)
                 else:
+                    typ_: Type = typ
+                    if defn.name in BINARY_MAGIC_METHODS or defn.name == "__subclasshook__":
+                        typ_ = erase_notimplemented(typ)
                     self.check_subtype(
                         subtype_label="got",
-                        subtype=typ,
+                        subtype=typ_,
                         supertype_label="expected",
                         supertype=return_type,
                         context=s.expr,
@@ -5098,22 +5093,15 @@ def type_check_raise(self, e: Expression, s: RaiseStmt, optional: bool = False)
             # where we allow `raise e from None`.
             expected_type_items.append(NoneType())
 
-        self.check_subtype(
-            typ, UnionType.make_union(expected_type_items), s, message_registry.INVALID_EXCEPTION
-        )
+        message = message_registry.INVALID_EXCEPTION
+        if isinstance(typ, Instance) and typ.type.fullname == "builtins._NotImplementedType":
+            message = message.with_additional_msg('; did you mean "NotImplementedError"?')
+        self.check_subtype(typ, UnionType.make_union(expected_type_items), s, message)
 
         if isinstance(typ, FunctionLike):
             # https://github.com/python/mypy/issues/11089
             self.expr_checker.check_call(typ, [], [], e)
 
-        if isinstance(typ, Instance) and typ.type.fullname == "builtins._NotImplementedType":
-            self.fail(
-                message_registry.INVALID_EXCEPTION.with_additional_msg(
-                    '; did you mean "NotImplementedError"?'
-                ),
-                s,
-            )
-
     def visit_try_stmt(self, s: TryStmt) -> None:
         """Type check a try statement."""
 

mypy/checkexpr.py

@@ -132,6 +132,7 @@
 from mypy.typeops import (
     callable_type,
     custom_special_method,
+    erase_notimplemented,
     erase_to_union_or_bound,
     false_only,
     fixup_partial_type,
@@ -3554,7 +3555,7 @@ def visit_op_expr(self, e: OpExpr) -> Type:
             else:
                 assert_never(use_reverse)
             e.method_type = method_type
-            return result
+            return erase_notimplemented(result)
         else:
             raise RuntimeError(f"Unknown operator {e.op}")
 
@@ -3705,7 +3706,7 @@ def visit_comparison_expr(self, e: ComparisonExpr) -> Type:
                 result = join.join_types(result, sub_result)
 
         assert result is not None
-        return result
+        return erase_notimplemented(result)
 
     def find_partial_type_ref_fast_path(self, expr: Expression) -> Type | None:
         """If expression has a partial generic type, return it without additional checks.
@@ -4228,15 +4229,16 @@ def check_op(
             # callable types.
             results_final = make_simplified_union(all_results)
             inferred_final = self.combine_function_signatures(get_proper_types(all_inferred))
-            return results_final, inferred_final
+            return erase_notimplemented(results_final), inferred_final
         else:
-            return self.check_method_call_by_name(
+            result, inferred = self.check_method_call_by_name(
                 method=method,
                 base_type=base_type,
                 args=[arg],
                 arg_kinds=[ARG_POS],
                 context=context,
             )
+            return erase_notimplemented(result), inferred
 
     def check_boolean_op(self, e: OpExpr) -> Type:
         """Type check a boolean operation ('and' or 'or')."""

mypy/mro.py

@@ -17,6 +17,7 @@ def calculate_mro(info: TypeInfo, obj_type: Callable[[], Instance] | None = None
     info.mro = mro
     # The property of falling back to Any is inherited.
     info.fallback_to_any = any(baseinfo.fallback_to_any for baseinfo in info.mro)
+
     type_state.reset_all_subtype_caches_for(info)
 
 

mypy/typeops.py

@@ -999,6 +999,21 @@ def is_singleton_type(typ: Type) -> bool:
     return typ.is_singleton_type()
 
 
+def is_notimplemented(t: ProperType) -> bool:
+    return isinstance(t, Instance) and t.type.fullname == "builtins._NotImplementedType"
+
+
+def erase_notimplemented(t: Type) -> Type:
+    t = get_proper_type(t)
+    if is_notimplemented(t):
+        return AnyType(TypeOfAny.special_form)
+    if isinstance(t, UnionType):
+        return UnionType.make_union(
+            [i for i in t.items if not is_notimplemented(get_proper_type(i))]
+        )
+    return t
+
+
 def try_expanding_sum_type_to_union(typ: Type, target_fullname: str) -> Type:
     """Attempts to recursively expand any enum Instances with the given target_fullname
     into a Union of all of its component LiteralTypes.

mypy/typeshed/stdlib/builtins.pyi

@@ -1269,7 +1269,7 @@ class property:
 
 @final
 @type_check_only
-class _NotImplementedType(Any):
+class _NotImplementedType:
     __call__: None
 
 NotImplemented: _NotImplementedType

test-data/unit/check-overloading.test

@@ -6852,3 +6852,96 @@ if isinstance(headers, dict):
 
 reveal_type(headers)  # N: Revealed type is "Union[__main__.Headers, typing.Iterable[tuple[builtins.bytes, builtins.bytes]]]"
 [builtins fixtures/isinstancelist.pyi]
+
+[case testReturnNotImplementedInBinaryMagicMethods]
+# flags: --warn-return-any
+from typing import Union
+
+class A:
+    def __add__(self, other: object) -> int:
+        return NotImplemented
+    def __radd__(self, other: object) -> Union[int, NotImplementedType]:
+        return NotImplemented
+    def __sub__(self, other: object) -> Union[int, NotImplementedType]:
+        return 1
+    def __isub__(self, other: object) -> int:
+        x: Union[int, NotImplementedType]
+        return x
+    def __mul__(self, other: object) -> Union[int, NotImplementedType]:
+        x: Union[int, NotImplementedType]
+        return x
+[builtins fixtures/notimplemented.pyi]
+
+[case testReturnNotImplementedABCSubclassHookMethod]
+# flags: --warn-return-any
+class A:
+    @classmethod
+    def __subclasshook__(cls, t: type[object], /) -> bool:
+        return NotImplemented
+[builtins fixtures/notimplemented.pyi]
+
+[case testReturnNotImplementedInNormalMethods]
+# flags: --warn-return-any
+from typing import Union
+
+class A:
+    def f(self) -> bool: return NotImplemented  # E: Incompatible return value type (got "_NotImplementedType", expected "bool")
+    def g(self) -> NotImplementedType: return True # E: Incompatible return value type (got "bool", expected "_NotImplementedType")
+    def h(self) -> NotImplementedType: return NotImplemented
+    def i(self) -> Union[bool, NotImplementedType]: return NotImplemented
+    def j(self) -> Union[bool, NotImplementedType]: return True
+[builtins fixtures/notimplemented.pyi]
+
+[case testNotImplementedReturnedFromBinaryMagicMethod]
+# flags: --warn-return-any
+from typing import Union
+
+class A:
+    def __add__(self, x: A) -> Union[int, NotImplementedType]: ...
+    def __sub__(self, x: A) -> NotImplementedType: ...
+    def __imul__(self, x: A) -> Union[A, NotImplementedType]: ...
+    def __itruediv__(self, x: A) -> Union[A, NotImplementedType]: ...
+    def __ifloordiv__(self, x: A) -> Union[int, NotImplementedType]: ...
+    def __eq__(self, x: object) -> Union[bool, NotImplementedType]: ...
+    def __le__(self, x: int) -> Union[bool, NotImplementedType]: ...
+    def __lt__(self, x: int) -> NotImplementedType: ...
+    def __and__(self, x: object) -> NotImplementedType: ...
+class B(A):
+    def __radd__(self, x: A) -> Union[int, NotImplementedType]: ...
+    def __rsub__(self, x: A) -> NotImplementedType: ...
+    def __itruediv__(self, x: A) -> Union[A, NotImplementedType]: ...
+    def __ror__(self, x: object) -> NotImplementedType: ...
+
+a: A
+b: B
+
+reveal_type(a.__add__(a))  # N: Revealed type is "Union[builtins.int, builtins._NotImplementedType]"
+reveal_type(a.__sub__(a))  # N: Revealed type is "builtins._NotImplementedType"
+reveal_type(a.__imul__(a))  # N: Revealed type is "Union[__main__.A, builtins._NotImplementedType]"
+reveal_type(a.__eq__(a))  # N: Revealed type is "Union[builtins.bool, builtins._NotImplementedType]"
+reveal_type(a.__le__(1))  # N: Revealed type is "Union[builtins.bool, builtins._NotImplementedType]"
+
+reveal_type(a + a)  # N: Revealed type is "builtins.int"
+reveal_type(a - a)  # N: Revealed type is "Any"
+reveal_type(a + b)  # N: Revealed type is "builtins.int"
+reveal_type(a - b)  # N: Revealed type is "Any"
+def f1(a: A) -> None:
+    a += a  # E: Incompatible types in assignment (expression has type "int", variable has type "A")
+def f2(a: A) -> None:
+    a -= a
+    reveal_type(a)  # N: Revealed type is "__main__.A"
+def f3(a: A) -> None:
+    a *= a
+    reveal_type(a)  # N: Revealed type is "__main__.A"
+def f4(a: A) -> None:
+    a /= a
+    reveal_type(a)  # N: Revealed type is "__main__.A"
+def f5(a: A) -> None:
+    a //= a  # E: Result type of // incompatible in assignment
+reveal_type(a == a)  # N: Revealed type is "builtins.bool"
+reveal_type(a == 1)  # N: Revealed type is "builtins.bool"
+reveal_type(a <= 1)  # N: Revealed type is "builtins.bool"
+reveal_type(a < 1)  # N: Revealed type is "Any"
+reveal_type(a and int())  # N: Revealed type is "Union[__main__.A, builtins.int]"
+reveal_type(int() or a)  # N: Revealed type is "Union[builtins.int, __main__.A]"
+[builtins fixtures/notimplemented.pyi]

test-data/unit/check-warnings.test

@@ -178,21 +178,6 @@ def f() -> int: return g()
 [out]
 main:4: error: Returning Any from function declared to return "int"
 
-[case testReturnAnyForNotImplementedInBinaryMagicMethods]
-# flags: --warn-return-any
-class A:
-    def __eq__(self, other: object) -> bool: return NotImplemented
-[builtins fixtures/notimplemented.pyi]
-[out]
-
-[case testReturnAnyForNotImplementedInNormalMethods]
-# flags: --warn-return-any
-class A:
-    def some(self) -> bool: return NotImplemented
-[builtins fixtures/notimplemented.pyi]
-[out]
-main:3: error: Returning Any from function declared to return "bool"
-
 [case testReturnAnyFromTypedFunctionWithSpecificFormatting]
 # flags: --warn-return-any
 from typing import Any, Tuple

test-data/unit/fixtures/notimplemented.pyi

@@ -9,10 +9,15 @@ class function: pass
 class bool: pass
 class int: pass
 class str: pass
+class tuple: pass
 class dict: pass
+class classmethod: pass
+class ellipsis: pass
 
-class _NotImplementedType(Any):
+class _NotImplementedType:
     __call__: NotImplemented  # type: ignore
 NotImplemented: _NotImplementedType
 
+NotImplementedType = _NotImplementedType
+
 class BaseException: pass