Add strict_override_decorator option (PEP 698)

docs/source/class_basics.rst

@@ -233,6 +233,12 @@ show an error:
        def g(self, y: str) -> None:   # Error: no corresponding base method found
            ...
 
+.. note::
+
+   Use ``--strict-override-decorator`` or
+   :confval:`strict_override_decorator = True <strict_override_decorator>` to require
+   methods overrides use the ``@override`` decorator. Emit an error if it is missing.
+
 You can also override a statically typed method with a dynamically
 typed one. This allows dynamically typed code to override methods
 defined in library classes without worrying about their type

docs/source/config_file.rst

@@ -714,6 +714,13 @@ section of the command line docs.
    Prohibit equality checks, identity checks, and container checks between
    non-overlapping types.
 
+.. confval:: strict_override_decorator
+
+    :type: boolean
+    :default: False
+
+    Require ``override`` decorator if method is overriding a base class method.
+
 .. confval:: strict
 
     :type: boolean

mypy/checker.py

@@ -645,6 +645,12 @@ def _visit_overloaded_func_def(self, defn: OverloadedFuncDef) -> None:
             found_base_method = self.check_method_override(defn)
             if defn.is_explicit_override and found_base_method is False:
                 self.msg.no_overridable_method(defn.name, defn)
+            elif (
+                found_base_method
+                and self.options.strict_override_decorator
+                and not defn.is_explicit_override
+            ):
+                self.msg.override_decorator_missing(defn.name, defn.impl or defn)
             self.check_inplace_operator_method(defn)
         if not defn.is_property:
             self.check_overlapping_overloads(defn)
@@ -971,7 +977,13 @@ def _visit_func_def(self, defn: FuncDef) -> None:
                 # overload, the legality of the override has already
                 # been typechecked, and decorated methods will be
                 # checked when the decorator is.
-                self.check_method_override(defn)
+                found_base_method = self.check_method_override(defn)
+                if (
+                    found_base_method
+                    and self.options.strict_override_decorator
+                    and defn.name not in ("__init__", "__new__")
+                ):
+                    self.msg.override_decorator_missing(defn.name, defn)
             self.check_inplace_operator_method(defn)
         if defn.original_def:
             # Override previous definition.
@@ -4742,6 +4754,12 @@ def visit_decorator(self, e: Decorator) -> None:
             found_base_method = self.check_method_override(e)
             if e.func.is_explicit_override and found_base_method is False:
                 self.msg.no_overridable_method(e.func.name, e.func)
+            elif (
+                found_base_method
+                and self.options.strict_override_decorator
+                and not e.func.is_explicit_override
+            ):
+                self.msg.override_decorator_missing(e.func.name, e.func)
 
         if e.func.info and e.func.name in ("__init__", "__new__"):
             if e.type and not isinstance(get_proper_type(e.type), (FunctionLike, AnyType)):

mypy/main.py

@@ -825,6 +825,14 @@ def add_invertible_flag(
         group=strictness_group,
     )
 
+    add_invertible_flag(
+        "--strict-override-decorator",
+        default=False,
+        strict_flag=False,
+        help="Require override decorator if method is overriding a base class method.",
+        group=strictness_group,
+    )
+
     add_invertible_flag(
         "--extra-checks",
         default=False,

mypy/messages.py

@@ -1524,6 +1524,13 @@ def no_overridable_method(self, name: str, context: Context) -> None:
             context,
         )
 
+    def override_decorator_missing(self, name: str, context: Context) -> None:
+        self.fail(
+            f'Method "{name}" is not marked as override '
+            "but is overriding a method in a base class",
+            context,
+        )
+
     def final_cant_override_writable(self, name: str, ctx: Context) -> None:
         self.fail(f'Cannot override writable attribute "{name}" with a final one', ctx)
 

mypy/options.py

@@ -52,6 +52,7 @@ class BuildType:
     "strict_concatenate",
     "strict_equality",
     "strict_optional",
+    "strict_override_decorator",
     "warn_no_return",
     "warn_return_any",
     "warn_unreachable",
@@ -201,6 +202,9 @@ def __init__(self) -> None:
         # This makes 1 == '1', 1 in ['1'], and 1 is '1' errors.
         self.strict_equality = False
 
+        # Require override decorator. Strict mode for PEP 698.
+        self.strict_override_decorator = False
+
         # Deprecated, use extra_checks instead.
         self.strict_concatenate = False
 

test-data/unit/check-functions.test

@@ -3007,3 +3007,75 @@ class C(A):
     def f(self, y: int | str) -> str: pass
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/tuple.pyi]
+
+[case requireExplicitOverrideMethod]
+# flags: --strict-override-decorator --python-version 3.12
+from typing import override
+
+class A:
+    def f(self, x: int) -> str: pass
+
+class B(A):
+    @override
+    def f(self, y: int) -> str: pass
+
+class C(A):
+    def f(self, y: int) -> str: pass  # E: Method "f" is not marked as override but is overriding a method in a base class
+[typing fixtures/typing-override.pyi]
+
+[case requireExplicitOverrideSpecialMethod]
+# flags: --strict-override-decorator --python-version 3.12
+from typing import Self, override
+
+# Don't require override decorator for __init__ and __new__
+# See: https://github.com/python/typing/issues/1376
+class A:
+    def __init__(self) -> None: pass
+    def __new__(cls) -> Self: pass
+[typing fixtures/typing-override.pyi]
+
+[case requireExplicitOverrideProperty]
+# flags: --strict-override-decorator --python-version 3.12
+from typing import override
+
+class A:
+    @property
+    def prop(self) -> int: pass
+
+class B(A):
+    @override
+    @property
+    def prop(self) -> int: pass
+
+class C(A):
+    @property
+    def prop(self) -> int: pass  # E: Method "prop" is not marked as override but is overriding a method in a base class
+[typing fixtures/typing-override.pyi]
+[builtins fixtures/property.pyi]
+
+[case requireExplicitOverrideOverload]
+# flags: --strict-override-decorator --python-version 3.12
+from typing import overload, override
+
+class A:
+    @overload
+    def f(self, x: int) -> str: ...
+    @overload
+    def f(self, x: str) -> str: ...
+    def f(self, x): pass
+
+class B(A):
+    @overload
+    def f(self, y: int) -> str: ...
+    @overload
+    def f(self, y: str) -> str: ...
+    @override
+    def f(self, y): pass
+
+class C(A):
+    @overload
+    def f(self, y: int) -> str: ...
+    @overload
+    def f(self, y: str) -> str: ...
+    def f(self, y): pass  # E: Method "f" is not marked as override but is overriding a method in a base class
+[typing fixtures/typing-override.pyi]

test-data/unit/fixtures/typing-override.pyi

@@ -0,0 +1,24 @@
+TypeVar = 0
+Generic = 0
+Any = 0
+overload = 0
+Type = 0
+Literal = 0
+Optional = 0
+Self = 0
+Tuple = 0
+ClassVar = 0
+
+T = TypeVar('T')
+T_co = TypeVar('T_co', covariant=True)
+KT = TypeVar('KT')
+
+class Iterable(Generic[T_co]): pass
+class Iterator(Iterable[T_co]): pass
+class Sequence(Iterable[T_co]): pass
+class Mapping(Iterable[KT], Generic[KT, T_co]):
+    def keys(self) -> Iterable[T]: pass  # Approximate return type
+    def __getitem__(self, key: T) -> T_co: pass
+
+
+def override(__arg: T) -> T: ...