Reuse functools.cached_property definition instead of defining our own (typeddjango#1771)

Mypy has special handling for functools.cached_property, making it compatible with classvars and the @Property decorator (mypy-play example). But this handling did not extend to our django.utils.functional.cached_property.

By simply re-exporting functools.cached_property, we can piggyback on the already existing infrastructure in typeshed/mypy.

Note that typeshed did not define functools.cached_property on Python 3.7 and older. So this will break for users of older Python versions. But I think that's fine since we advertise 3.8 as the minimal Python version.

Author: intgr

django-stubs/utils/functional.pyi

@@ -1,21 +1,14 @@
 from collections.abc import Callable, Sequence
+
+# Mypy has special handling for functools.cached_property, reuse typeshed's definition instead of defining our own
+from functools import cached_property as cached_property
 from typing import Any, Generic, Protocol, SupportsIndex, TypeVar, overload
 
 from django.db.models.base import Model
 from typing_extensions import Self, TypeAlias
 
 _T = TypeVar("_T")
 
-class cached_property(Generic[_T]):
-    func: Callable[[Any], _T]
-    name: str | None
-    def __init__(self, func: Callable[[Any], _T], name: str | None = ...) -> None: ...
-    @overload
-    def __get__(self, instance: None, cls: type[Any] | None = ...) -> Self: ...
-    @overload
-    def __get__(self, instance: object, cls: type[Any] | None = ...) -> _T: ...
-    def __set_name__(self, owner: type[Any], name: str) -> None: ...
-
 # Promise is only subclassed by a proxy class defined in the lazy function
 # so it makes sense for it to have all the methods available in that proxy class
 class Promise:

scripts/stubtest/allowlist.txt

@@ -143,3 +143,46 @@ django.middleware.csrf.REASON_BAD_TOKEN
 
 # RemovedInDjango41
 django.core.cache.backends.memcached.MemcachedCache
+
+# We re-export `functools.cached_property` which has different semantics
+django.utils.functional.cached_property.__class_getitem__
+django.utils.functional.cached_property.__init__
+django.utils.functional.cached_property.__set__
+django.utils.functional.cached_property.name
+
+# Ignore @cached_property error "cannot reconcile @property on stub with runtime object"
+django.db.migrations.RenameField.new_name_lower
+django.db.migrations.RenameField.old_name_lower
+django.db.migrations.RenameIndex.new_name_lower
+django.db.migrations.RenameIndex.old_name_lower
+django.db.migrations.RenameModel.new_name_lower
+django.db.migrations.RenameModel.old_name_lower
+django.db.migrations.operations.RenameField.new_name_lower
+django.db.migrations.operations.RenameField.old_name_lower
+django.db.migrations.operations.RenameIndex.new_name_lower
+django.db.migrations.operations.RenameIndex.old_name_lower
+django.db.migrations.operations.RenameModel.new_name_lower
+django.db.migrations.operations.RenameModel.old_name_lower
+django.db.migrations.operations.fields.FieldOperation.model_name_lower
+django.db.migrations.operations.fields.FieldOperation.name_lower
+django.db.migrations.operations.fields.RenameField.new_name_lower
+django.db.migrations.operations.fields.RenameField.old_name_lower
+django.db.migrations.operations.models.AlterTogetherOptionOperation.option_value
+django.db.migrations.operations.models.IndexOperation.model_name_lower
+django.db.migrations.operations.models.ModelOperation.name_lower
+django.db.migrations.operations.models.RenameIndex.new_name_lower
+django.db.migrations.operations.models.RenameIndex.old_name_lower
+django.db.migrations.operations.models.RenameModel.new_name_lower
+django.db.migrations.operations.models.RenameModel.old_name_lower
+django.db.migrations.state.ModelState.name_lower
+django.db.migrations.state.ProjectState.apps
+django.middleware.csrf.CsrfViewMiddleware.allowed_origin_subdomains
+django.middleware.csrf.CsrfViewMiddleware.allowed_origins_exact
+django.middleware.csrf.CsrfViewMiddleware.csrf_trusted_origins_hosts
+django.urls.URLPattern.lookup_str
+django.urls.URLResolver.url_patterns
+django.urls.URLResolver.urlconf_module
+django.urls.resolvers.URLPattern.lookup_str
+django.urls.resolvers.URLResolver.url_patterns
+django.urls.resolvers.URLResolver.urlconf_module
+django.utils.connection.BaseConnectionHandler.settings

tests/typecheck/utils/test_functional.yml

@@ -1,23 +1,36 @@
 -   case: cached_property_class_vs_instance_attributes
     main: |
       from django.utils.functional import cached_property
-      from typing import List
+      from typing import List, ClassVar
 
       class Foo:
           @cached_property
           def attr(self) -> List[str]: ...
-          @cached_property  # E: Argument 1 to "cached_property" has incompatible type "Callable[[Foo, str], List[str]]"; expected "Callable[[Any], List[str]]"  [arg-type]
+
+          @cached_property  # E: Too many arguments for property  [misc]
           def attr2(self, arg2: str) -> 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 "Union[builtins.str, None]"
+          reveal_type(attr)  # N: Revealed type is "def (self: main.Foo) -> builtins.list[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) -> builtins.list[builtins.str]"
+      reveal_type(Foo.attr)  # N: Revealed type is "def (self: main.Foo) -> 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"  [attr-defined]
+      reveal_type(f.attr)  # N: Revealed type is "builtins.list[builtins.str]"
+      f.attr.func  # E: "List[str]" has no attribute "func"  [attr-defined]
+
+      # May be overridden by @property
+      class Bar(Foo):
+          @property
+          def attr(self) -> List[str]: ...
+
+      # May be overridden by ClassVar
+      class Quux(Foo):
+          attr: ClassVar[List[str]] = []
+
+      # ClassVar may not be overridden by cached_property
+      class Baz(Quux):
+          @cached_property
+          def attr(self) -> List[str]: ...  # E: Cannot override writeable attribute with read-only property  [override]
 
 -   case: str_promise_proxy
     main:  |