Add __set__ to functools.cached_property (#9762)

stdlib/functools.pyi

@@ -148,6 +148,8 @@ if sys.version_info >= (3, 8):
         @overload
         def __get__(self, instance: object, owner: type[Any] | None = None) -> _T: ...
         def __set_name__(self, owner: type[Any], name: str) -> None: ...
+        # __set__ is not defined at runtime, but @cached_property is designed to be settable
+        def __set__(self, instance: object, value: _T) -> None: ...
         if sys.version_info >= (3, 9):
             def __class_getitem__(cls, item: Any) -> GenericAlias: ...
 

test_cases/stdlib/check_functools.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import sys
+
+if sys.version_info >= (3, 8):
+    from functools import cached_property
+    from typing_extensions import assert_type
+
+    class A:
+        def __init__(self, x: int):
+            self.x = x
+
+        @cached_property
+        def x(self) -> int:
+            return 0
+
+    assert_type(A(x=1).x, int)
+
+    class B:
+        @cached_property
+        def x(self) -> int:
+            return 0
+
+    def check_cached_property_settable(x: int) -> None:
+        b = B()
+        assert_type(b.x, int)
+        b.x = x
+        assert_type(b.x, int)

tests/stubtest_allowlists/py310.txt

@@ -16,6 +16,7 @@ builtins.property.__set_name__  # Doesn't actually exist
 contextlib.AbstractAsyncContextManager.__class_getitem__
 contextlib.AbstractContextManager.__class_getitem__
 fractions.Fraction.__new__  # overload is too complicated for stubtest to resolve
+functools.cached_property.__set__  # Stub is a while lie; see comments in the stub
 gettext.install
 gettext.translation
 hmac.new  # Stub is a white lie; see comments in the stub

tests/stubtest_allowlists/py311.txt

@@ -20,6 +20,7 @@ enum.auto.__init__
 enum.auto.value
 fractions.Fraction.__new__  # overload is too complicated for stubtest to resolve
 ftplib.FTP.trust_server_pasv_ipv4_address
+functools.cached_property.__set__  # Stub is a while lie; see comments in the stub
 ipaddress.IPv4Interface.hostmask
 ipaddress.IPv6Interface.hostmask
 ipaddress._BaseNetwork.broadcast_address

tests/stubtest_allowlists/py38.txt

@@ -46,6 +46,7 @@ dummy_threading.Thread.native_id
 dummy_threading.local.__new__
 fractions.Fraction.__new__  # overload is too complicated for stubtest to resolve
 ftplib.FTP.trust_server_pasv_ipv4_address  # Dangerous to use, intentionally undocumented, intentionally missing from typeshed. #6154
+functools.cached_property.__set__  # Stub is a while lie; see comments in the stub
 gettext.install  # codeset default value is ['unspecified'] so can't be specified
 gettext.translation  # codeset default value is ['unspecified'] so can't be specified
 hmac.new  # Stub is a white lie; see comments in the stub

tests/stubtest_allowlists/py39.txt

@@ -40,6 +40,7 @@ distutils.core.extension_keywords
 distutils.core.gen_usage
 distutils.core.setup_keywords
 fractions.Fraction.__new__  # overload is too complicated for stubtest to resolve
+functools.cached_property.__set__  # Stub is a while lie; see comments in the stub
 gettext.install
 gettext.translation
 hmac.new  # Stub is a white lie; see comments in the stub