Consider property access from class objects (now for real)

mypy/checker.py

@@ -182,6 +182,7 @@
     ANY_STRATEGY,
     MYPYC_NATIVE_INT_NAMES,
     OVERLOAD_NAMES,
+    PROPERTY_DECORATOR_NAMES,
     AnyType,
     BoolTypeQuery,
     CallableType,
@@ -2335,6 +2336,29 @@ def check_method_override_for_base_with_name(
             )
         return False
 
+    def get_property_instance(self, method: Decorator | OverloadedFuncDef) -> Instance | None:
+        if method.type is None:
+            return None
+        deco = method if isinstance(method, Decorator) else method.items[0]
+        if not isinstance(deco, Decorator):
+            return None
+        property_deco_name = next(
+            (
+                name
+                for d in deco.original_decorators
+                for name in PROPERTY_DECORATOR_NAMES
+                if refers_to_fullname(d, name)
+            ),
+            None,
+        )
+        if property_deco_name is not None:
+            # Extra attr preserves the underlying node to support alias assignments
+            # (see testPropertyAliasInClassBody)
+            return self.named_type(property_deco_name).copy_with_extra_attr(
+                "__mypy-wrapped-property", method.type
+            )
+        return None
+
     def get_op_other_domain(self, tp: FunctionLike) -> Type | None:
         if isinstance(tp, CallableType):
             if tp.arg_kinds and tp.arg_kinds[0] == ARG_POS:
@@ -4394,6 +4418,13 @@ def set_inferred_type(self, var: Var, lvalue: Lvalue, type: Type) -> None:
         refers to the variable (lvalue). If var is None, do nothing.
         """
         if var and not self.current_node_deferred:
+            p_type = get_proper_type(type)
+            if (
+                isinstance(p_type, Instance)
+                and p_type.extra_attrs
+                and "__mypy-wrapped-property" in p_type.extra_attrs.attrs
+            ):
+                type = p_type.extra_attrs.attrs["__mypy-wrapped-property"]
             var.type = type
             var.is_inferred = True
             var.is_ready = True

mypy/checker_shared.py

@@ -15,11 +15,13 @@
 from mypy.nodes import (
     ArgKind,
     Context,
+    Decorator,
     Expression,
     FuncItem,
     LambdaExpr,
     MypyFile,
     Node,
+    OverloadedFuncDef,
     RefExpr,
     SymbolNode,
     TypeInfo,
@@ -277,6 +279,10 @@ def checking_await_set(self) -> Iterator[None]:
     def get_precise_awaitable_type(self, typ: Type, local_errors: ErrorWatcher) -> Type | None:
         raise NotImplementedError
 
+    @abstractmethod
+    def get_property_instance(self, method: Decorator | OverloadedFuncDef) -> Instance | None:
+        raise NotImplementedError
+
     @abstractmethod
     def is_defined_in_stub(self, typ: Instance, /) -> bool:
         raise NotImplementedError

mypy/checkexpr.py

@@ -384,14 +384,19 @@ def analyze_ref_expr(self, e: RefExpr, lvalue: bool = False) -> Type:
             if isinstance(result, PartialType):
                 result = self.chk.handle_partial_var_type(result, lvalue, node, e)
         elif isinstance(node, Decorator):
-            result = self.analyze_var_ref(node.var, e)
+            property_type = self.chk.get_property_instance(node)
+            if property_type is not None:
+                result = property_type
+            else:
+                result = self.analyze_var_ref(node.var, e)
         elif isinstance(node, OverloadedFuncDef):
-            if node.type is None:
+            result = node.type
+            if result is None:
                 if self.chk.in_checked_function() and node.items:
                     self.chk.handle_cannot_determine_type(node.name, e)
                 result = AnyType(TypeOfAny.from_error)
-            else:
-                result = node.type
+            elif property_type := self.chk.get_property_instance(node):
+                result = property_type
         elif isinstance(node, (FuncDef, TypeInfo, TypeAlias, MypyFile, TypeVarLikeExpr)):
             result = self.analyze_static_reference(node, e, e.is_alias_rvalue or lvalue)
         else:

mypy/checkmember.py

@@ -1226,6 +1226,16 @@ def analyze_class_attribute_access(
                     erase_vars.add(itype.type.self_type)
                 t = erase_typevars(t, {tv.id for tv in erase_vars})
 
+        if (
+            isinstance(node.node, Decorator)
+            and node.node.func.is_property
+            or isinstance(node.node, OverloadedFuncDef)
+            and node.node.is_property
+        ):
+            property_type = mx.chk.get_property_instance(node.node)
+            if property_type is not None:
+                return property_type
+
         is_classmethod = (
             (is_decorated and cast(Decorator, node.node).func.is_class)
             or (isinstance(node.node, SYMBOL_FUNCBASE_TYPES) and node.node.is_class)

mypy/semanal.py

@@ -260,6 +260,7 @@
     NEVER_NAMES,
     OVERLOAD_NAMES,
     OVERRIDE_DECORATOR_NAMES,
+    PROPERTY_DECORATOR_NAMES,
     PROTOCOL_NAMES,
     REVEAL_TYPE_NAMES,
     TPDICT_NAMES,
@@ -1691,16 +1692,7 @@ def visit_decorator(self, dec: Decorator) -> None:
                 removed.append(i)
                 dec.func.is_explicit_override = True
                 self.check_decorated_function_is_method("override", dec)
-            elif refers_to_fullname(
-                d,
-                (
-                    "builtins.property",
-                    "abc.abstractproperty",
-                    "functools.cached_property",
-                    "enum.property",
-                    "types.DynamicClassAttribute",
-                ),
-            ):
+            elif refers_to_fullname(d, PROPERTY_DECORATOR_NAMES):
                 removed.append(i)
                 dec.func.is_property = True
                 dec.var.is_property = True

mypy/types.py

@@ -181,6 +181,15 @@
 # Supported @override decorator names.
 OVERRIDE_DECORATOR_NAMES: Final = ("typing.override", "typing_extensions.override")
 
+# Supported property decorators
+PROPERTY_DECORATOR_NAMES: Final = (
+    "builtins.property",
+    "abc.abstractproperty",
+    "functools.cached_property",
+    "enum.property",
+    "types.DynamicClassAttribute",
+)
+
 ELLIPSIS_TYPE_NAMES: Final = ("builtins.ellipsis", "types.EllipsisType")
 
 # A placeholder used for Bogus[...] parameters

test-data/unit/check-callable.test

@@ -628,6 +628,7 @@ class A:
 
     g2 = f2
 
+reveal_type(A().f)  # N: Revealed type is "builtins.int"
 reveal_type(A().g)  # N: Revealed type is "builtins.int"
 reveal_type(A().g2)  # N: Revealed type is "builtins.int"
 A().g = 1  # E: Property "g" defined in "A" is read-only

test-data/unit/check-classes.test

@@ -1621,7 +1621,7 @@ class A:
         self.x = 1
         self.x = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
         return ''
-[builtins fixtures/property.pyi]
+[builtins fixtures/property-full.pyi]
 
 [case testPropertyNameIsChecked]
 class A:
@@ -1706,7 +1706,7 @@ class A:
 a = A()
 a.f.xx
 a.f = '' # E: Property "f" defined in "A" is read-only
-[builtins fixtures/property.pyi]
+[builtins fixtures/property-full.pyi]
 
 [case testPropertyWithSetter]
 import typing
@@ -1723,7 +1723,7 @@ a.f.x # E: "int" has no attribute "x"
 a.f = '' # E: Incompatible types in assignment (expression has type "str", variable has type "int")
 a.f = 1
 reveal_type(a.f)  # N: Revealed type is "builtins.int"
-[builtins fixtures/property.pyi]
+[builtins fixtures/property-full.pyi]
 
 [case testPropertyWithDeleterButNoSetter]
 import typing
@@ -1737,7 +1737,60 @@ class A:
 a = A()
 a.f = a.f # E: Property "f" defined in "A" is read-only
 a.f.x # E: "int" has no attribute "x"
-[builtins fixtures/property.pyi]
+[builtins fixtures/property-full.pyi]
+
+[case testPropertyAccessOnClass]
+class Foo:
+    @property
+    def bar(self) -> bool:
+        return True
+
+    reveal_type(bar)  # N: Revealed type is "builtins.property"
+
+reveal_type(Foo.bar)  # N: Revealed type is "builtins.property"
+reveal_type(Foo.bar(Foo()))  # E: "property" not callable \
+                             # N: Revealed type is "Any"
+reveal_type(Foo.bar.fget(Foo()))  # E: "None" not callable \
+                                  # N: Revealed type is "Any"
+
+class Bar:
+    @property
+    def bar(self) -> bool:
+        return True
+    @bar.setter
+    def bar(self, bar: bool) -> None:
+        pass
+
+    reveal_type(bar)  # N: Revealed type is "builtins.property"
+
+reveal_type(Bar.bar)  # N: Revealed type is "builtins.property"
+reveal_type(Bar.bar(Bar()))  # E: "property" not callable \
+                             # N: Revealed type is "Any"
+reveal_type(Bar.bar.fget(Bar()))  # E: "None" not callable \
+                                  # N: Revealed type is "Any"
+[builtins fixtures/property-full.pyi]
+
+[case testPropertyAccessOnClass2]
+import functools
+from functools import cached_property
+
+class Foo:
+    @cached_property
+    def foo(self) -> bool:
+        return True
+
+    @functools.cached_property
+    def bar(self) -> bool:
+        return True
+
+    reveal_type(foo)  # N: Revealed type is "functools.cached_property[Any]"
+    reveal_type(bar)  # N: Revealed type is "functools.cached_property[Any]"
+
+reveal_type(Foo.foo)  # N: Revealed type is "functools.cached_property[Any]"
+reveal_type(Foo.bar)  # N: Revealed type is "functools.cached_property[Any]"
+Foo.foo(Foo())  # E: "cached_property[Any]" not callable
+Foo.bar(Foo())  # E: "cached_property[Any]" not callable
+[builtins fixtures/property-full.pyi]
 
 -- Descriptors
 -- -----------

test-data/unit/check-functions.test

@@ -3321,7 +3321,7 @@ class A:
     @decorator
     def f(self) -> int: ...
 
-reveal_type(A.f)  # N: Revealed type is "__main__.something_callable"
+reveal_type(A.f)  # N: Revealed type is "builtins.property"
 reveal_type(A().f)  # N: Revealed type is "builtins.str"
 [builtins fixtures/property.pyi]
 

test-data/unit/check-protocols.test

@@ -3346,7 +3346,7 @@ def test(arg: P) -> None: ...
 # TODO: skip type mismatch diagnostics in this case.
 test(B)  # E: Argument 1 to "test" has incompatible type "type[B]"; expected "P" \
          # N: Following member(s) of "B" have conflicts: \
-         # N:     foo: expected "int", got "Callable[[B], int]" \
+         # N:     foo: expected "int", got "property" \
          # N: Only class variables allowed for class object access on protocols, foo is an instance variable of "B"
 test(C)  # E: Argument 1 to "test" has incompatible type "type[C]"; expected "P" \
          # N: Only class variables allowed for class object access on protocols, foo is an instance variable of "C"

test-data/unit/fixtures/property-full.pyi

@@ -0,0 +1,43 @@
+from typing import Any, Callable, Generic, TypeVar
+
+_T = TypeVar('_T')
+
+class object:
+    def __init__(self) -> None: pass
+
+class type:
+    def __init__(self, x: Any) -> None: pass
+
+class function: pass
+
+class property:
+    fget: Callable[[Any], Any] | None
+    fset: Callable[[Any, Any], None] | None
+    fdel: Callable[[Any], None] | None
+    __isabstractmethod__: bool
+
+    def __init__(
+        self,
+        fget: Callable[[Any], Any] | None = ...,
+        fset: Callable[[Any, Any], None] | None = ...,
+        fdel: Callable[[Any], None] | None = ...,
+        doc: str | None = ...,
+    ) -> None: ...
+    def getter(self, fget: Callable[[Any], Any], /) -> property: ...
+    def setter(self, fset: Callable[[Any, Any], None], /) -> property: ...
+    def deleter(self, fdel: Callable[[Any], None], /) -> property: ...
+    def __get__(self, instance: Any, owner: type | None = None, /) -> Any: ...
+    def __set__(self, instance: Any, value: Any, /) -> None: ...
+    def __delete__(self, instance: Any, /) -> None: ...
+class classmethod: pass
+
+class list: pass
+class dict: pass
+class int: pass
+class float: pass
+class str: pass
+class bytes: pass
+class bool: pass
+class ellipsis: pass
+
+class tuple(Generic[_T]): pass

test-data/unit/fixtures/property.pyi

@@ -10,7 +10,7 @@ class type:
 
 class function: pass
 
-property = object()  # Dummy definition
+class property: pass  # Dummy definition
 class classmethod: pass
 
 class list(typing.Generic[_T]): pass