Implement @cached_property attribute inference (#292)

django-stubs/utils/functional.pyi

@@ -1,16 +1,21 @@
-from typing import Any, Callable, Dict, List, Optional, Tuple, Type, Union
+from typing import Any, Callable, Dict, List, Optional, Tuple, Type, Union, TypeVar, Generic, overload
 from functools import wraps as wraps  # noqa: F401
 
 from django.db.models.base import Model
 
 def curry(_curried_func: Any, *args: Any, **kwargs: Any): ...
 
-class cached_property:
-    func: Callable = ...
+_T = TypeVar("_T")
+
+class cached_property(Generic[_T]):
+    func: Callable[..., _T] = ...
     __doc__: Any = ...
     name: str = ...
-    def __init__(self, func: Callable, name: Optional[str] = ...) -> None: ...
-    def __get__(self, instance: Any, cls: Type[Any] = ...) -> Any: ...
+    def __init__(self, func: Callable[..., _T], name: Optional[str] = ...): ...
+    @overload
+    def __get__(self, instance: None, cls: Type[Any] = ...) -> "cached_property[_T]": ...
+    @overload
+    def __get__(self, instance: object, cls: Type[Any] = ...) -> _T: ...
 
 class Promise: ...
 

scripts/enabled_test_modules.py

@@ -457,7 +457,7 @@ IGNORED_ERRORS = {
         'No overload variant of "join" matches argument types "str", "None"',
         'Argument 1 to "Archive" has incompatible type "None"; expected "str"',
         'Argument 1 to "to_path" has incompatible type "int"; expected "Union[Path, str]"',
-
+        'Cannot infer type argument 1 of "cached_property"',
     ],
     'view_tests': [
         "Module 'django.views.debug' has no attribute 'Path'",

test-data/typecheck/utils/test_functional.yml

@@ -0,0 +1,18 @@
+-   case: cached_property_class_vs_instance_attributes
+    main: |
+      from django.utils.functional import cached_property
+      from typing import List
+
+      class Foo:
+          @cached_property
+          def attr(self) -> List[str]: ...
+
+          reveal_type(attr)      # N: Revealed type is 'django.utils.functional.cached_property[builtins.list*[builtins.str]]'
+          reveal_type(attr.name) # N: Revealed type is 'builtins.str'
+
+      reveal_type(Foo.attr)      # N: Revealed type is 'django.utils.functional.cached_property[builtins.list*[builtins.str]]'
+      reveal_type(Foo.attr.func) # N: Revealed type is 'def (*Any, **Any) -> builtins.list*[builtins.str]'
+
+      f = Foo()
+      reveal_type(f.attr)        # N: Revealed type is 'builtins.list*[builtins.str]'
+      f.attr.name                # E: "List[str]" has no attribute "name"