Add support for PEP 698 - override decorator

docs/source/class_basics.rst

@@ -208,6 +208,31 @@ override has a compatible signature:
    subtype such as ``list[int]``. Similarly, you can vary argument types
    **contravariantly** -- subclasses can have more general argument types.
 
+In order to ensure that your code remains correct when renaming methods,
+it can be helpful to explicitly mark a method as overriding a base
+method. This can be done with the ``@override`` decorator. If the base
+method is then renamed while the overriding method is not, mypy will
+show an error:
+
+.. code-block:: python
+
+   from typing import override
+
+   class Base:
+       def f(self, x: int) -> None:
+           ...
+       def g_renamed(self, y: str) -> None:
+           ...
+
+   class Derived1(Base):
+       @override
+       def f(self, x: int) -> None:   # OK
+           ...
+
+       @override
+       def g(self, y: str) -> None:   # Error: no corresponding base method found
+           ...
+
 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

mypy/checker.py

@@ -1760,25 +1760,35 @@ def expand_typevars(
         else:
             return [(defn, typ)]
 
-    def check_method_override(self, defn: FuncDef | OverloadedFuncDef | Decorator) -> None:
+    def check_method_override(self, defn: FuncDef | OverloadedFuncDef | Decorator) -> bool | None:
         """Check if function definition is compatible with base classes.
 
         This may defer the method if a signature is not available in at least one base class.
+        Return ``None`` if that happens.
+
+        Return ``True`` if an attribute with the method name was found in the base class.
         """
         # Check against definitions in base classes.
+        found_base_method = False
         for base in defn.info.mro[1:]:
-            if self.check_method_or_accessor_override_for_base(defn, base):
+            result = self.check_method_or_accessor_override_for_base(defn, base)
+            if result is None:
                 # Node was deferred, we will have another attempt later.
-                return
+                return None
+            found_base_method |= result
+        return found_base_method
 
     def check_method_or_accessor_override_for_base(
         self, defn: FuncDef | OverloadedFuncDef | Decorator, base: TypeInfo
-    ) -> bool:
+    ) -> bool | None:
         """Check if method definition is compatible with a base class.
 
-        Return True if the node was deferred because one of the corresponding
+        Return ``None`` if the node was deferred because one of the corresponding
         superclass nodes is not ready.
+
+        Return ``True`` if an attribute with the method name was found in the base class.
         """
+        found_base_method = False
         if base:
             name = defn.name
             base_attr = base.names.get(name)
@@ -1789,22 +1799,24 @@ def check_method_or_accessor_override_for_base(
                 # Second, final can't override anything writeable independently of types.
                 if defn.is_final:
                     self.check_if_final_var_override_writable(name, base_attr.node, defn)
+                found_base_method = True
 
             # Check the type of override.
             if name not in ("__init__", "__new__", "__init_subclass__"):
                 # Check method override
                 # (__init__, __new__, __init_subclass__ are special).
                 if self.check_method_override_for_base_with_name(defn, name, base):
-                    return True
+                    return None
                 if name in operators.inplace_operator_methods:
                     # Figure out the name of the corresponding operator method.
                     method = "__" + name[3:]
                     # An inplace operator method such as __iadd__ might not be
                     # always introduced safely if a base class defined __add__.
                     # TODO can't come up with an example where this is
                     #      necessary; now it's "just in case"
-                    return self.check_method_override_for_base_with_name(defn, method, base)
-        return False
+                    if self.check_method_override_for_base_with_name(defn, method, base):
+                        return None
+        return found_base_method
 
     def check_method_override_for_base_with_name(
         self, defn: FuncDef | OverloadedFuncDef | Decorator, name: str, base: TypeInfo
@@ -4638,7 +4650,9 @@ def visit_decorator(self, e: Decorator) -> None:
             self.check_incompatible_property_override(e)
         # For overloaded functions we already checked override for overload as a whole.
         if e.func.info and not e.func.is_dynamic() and not e.is_overload:
-            self.check_method_override(e)
+            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)
 
         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/messages.py

@@ -1407,6 +1407,13 @@ def cant_assign_to_method(self, context: Context) -> None:
     def cant_assign_to_classvar(self, name: str, context: Context) -> None:
         self.fail(f'Cannot assign to class variable "{name}" via instance', context)
 
+    def no_overridable_method(self, name: str, context: Context) -> None:
+        self.fail(
+            f'Method "{name}" is marked as an override, '
+            "but no base method with this name was found",
+            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/nodes.py

@@ -758,6 +758,7 @@ class FuncDef(FuncItem, SymbolNode, Statement):
         "deco_line",
         "is_trivial_body",
         "is_mypy_only",
+        "is_explicit_override",
     )
 
     __match_args__ = ("name", "arguments", "type", "body")
@@ -785,6 +786,8 @@ def __init__(
         self.deco_line: int | None = None
         # Definitions that appear in if TYPE_CHECKING are marked with this flag.
         self.is_mypy_only = False
+        # Decorated with @override
+        self.is_explicit_override = False
 
     @property
     def name(self) -> str:

mypy/semanal.py

@@ -241,6 +241,7 @@
     FINAL_TYPE_NAMES,
     NEVER_NAMES,
     OVERLOAD_NAMES,
+    OVERRIDE_DECORATOR_NAMES,
     PROTOCOL_NAMES,
     REVEAL_TYPE_NAMES,
     TPDICT_NAMES,
@@ -1490,6 +1491,10 @@ def visit_decorator(self, dec: Decorator) -> None:
                 dec.func.is_class = True
                 dec.var.is_classmethod = True
                 self.check_decorated_function_is_method("classmethod", dec)
+            elif refers_to_fullname(d, OVERRIDE_DECORATOR_NAMES):
+                removed.append(i)
+                dec.func.is_explicit_override = True
+                self.check_decorated_function_is_method("override", dec)
             elif refers_to_fullname(
                 d,
                 (

mypy/types.py

@@ -157,6 +157,8 @@
     "typing.dataclass_transform",
     "typing_extensions.dataclass_transform",
 )
+# Supported @override decorator names.
+OVERRIDE_DECORATOR_NAMES: Final = ("typing.override", "typing_extensions.override")
 
 # A placeholder used for Bogus[...] parameters
 _dummy: Final[Any] = object()

test-data/unit/check-functions.test

@@ -2725,3 +2725,154 @@ TS = TypeVar("TS", bound=str)
 f: Callable[[Sequence[TI]], None]
 g: Callable[[Union[Sequence[TI], Sequence[TS]]], None]
 f = g
+
+[case explicitOverride]
+from typing import override
+
+class A:
+    def f(self, x: int) -> str: pass
+    @override
+    def g(self, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class B(A):
+    @override
+    def f(self, x: int) -> str: pass
+    @override
+    def g(self, x: int) -> str: pass
+
+class C(A):
+    @override
+    def f(self, x: str) -> str: pass  # E: Argument 1 of "f" is incompatible with supertype "A"; supertype defines the argument type as "int" \
+                                      # N: This violates the Liskov substitution principle \
+                                      # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+    def g(self, x: int) -> str: pass
+
+class D(A): pass
+class E(D): pass
+class F(E):
+    @override
+    def f(self, x: int) -> str: pass
+[typing fixtures/typing-medium.pyi]
+
+[case explicitOverrideStaticmethod]
+from typing import override
+
+class A:
+    @staticmethod
+    def f(x: int) -> str: pass
+
+class B(A):
+    @staticmethod
+    @override
+    def f(x: int) -> str: pass
+    @override
+    @staticmethod
+    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class C(A):  # inverted order of decorators
+    @override
+    @staticmethod
+    def f(x: int) -> str: pass
+    @override
+    @staticmethod
+    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class D(A):
+    @staticmethod
+    @override
+    def f(x: str) -> str: pass  # E: Argument 1 of "f" is incompatible with supertype "A"; supertype defines the argument type as "int" \
+                                # N: This violates the Liskov substitution principle \
+                                # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+[typing fixtures/typing-medium.pyi]
+[builtins fixtures/callable.pyi]
+
+[case explicitOverrideClassmethod]
+from typing import override
+
+class A:
+    @classmethod
+    def f(cls, x: int) -> str: pass
+
+class B(A):
+    @classmethod
+    @override
+    def f(cls, x: int) -> str: pass
+    @override
+    @classmethod
+    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class C(A):  # inverted order of decorators
+    @override
+    @classmethod
+    def f(cls, x: int) -> str: pass
+    @override
+    @classmethod
+    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class D(A):
+    @classmethod
+    @override
+    def f(cls, x: str) -> str: pass  # E: Argument 1 of "f" is incompatible with supertype "A"; supertype defines the argument type as "int" \
+                                     # N: This violates the Liskov substitution principle \
+                                     # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
+[typing fixtures/typing-medium.pyi]
+[builtins fixtures/callable.pyi]
+
+[case explicitOverrideProperty]
+from typing import override
+
+class A:
+    @property
+    def f(self) -> str: pass
+
+class B(A):
+    @property
+    @override
+    def f(self) -> str: pass
+    @override
+    @property
+    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class C(A):  # inverted order of decorators
+    @override
+    @property
+    def f(self) -> str: pass
+    @override
+    @property
+    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+
+class D(A):
+    @property
+    @override
+    def f(self) -> int: pass  # E: Signature of "f" incompatible with supertype "A"
+[builtins fixtures/property.pyi]
+[typing fixtures/typing-medium.pyi]
+
+[case invalidExplicitOverride]
+from typing import override
+
+@override  # E: "override" used with a non-method
+def f(x: int) -> str: pass
+
+@override  # this should probably throw an error but the signature from typeshed should ensure this already
+class A: pass
+
+def g() -> None:
+    @override  # E: "override" used with a non-method
+    def h(b: bool) -> int: pass
+[typing fixtures/typing-medium.pyi]
+
+[case explicitOverrideSpecialMethods]
+from typing import override
+
+class A:
+    def __init__(self, a: int) -> None: pass
+
+class B(A):
+    @override
+    def __init__(self, b: str) -> None: pass
+
+class C:
+    @override
+    def __init__(self, a: int) -> None: pass
+[typing fixtures/typing-medium.pyi]

test-data/unit/fixtures/property.pyi

@@ -16,6 +16,7 @@ class classmethod: pass
 class list: pass
 class dict: pass
 class int: pass
+class float: pass
 class str: pass
 class bytes: pass
 class bool: pass

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

@@ -73,3 +73,4 @@ class _SpecialForm: pass
 TYPE_CHECKING = 1
 
 def dataclass_transform() -> Callable[[T], T]: ...
+def override(f: T) -> T: ...