gh-106292: restore checking __dict__ in cached_property.__get__ (#106380)

* gh-106292: restore checking __dict__ in cached_property.__get__

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>

1 files changed, 13 insertions, 10 deletions

diff --git a/Lib/functools.py b/Lib/functools.py
index 4d5e2709007..8518450a8d4 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -957,9 +957,10 @@ class singledispatchmethod:
 
 
 ################################################################################
-### cached_property() - computed once per instance, cached as attribute
+### cached_property() - property result cached as instance attribute
 ################################################################################
 
+_NOT_FOUND = object()
 
 class cached_property:
     def __init__(self, func):
@@ -990,15 +991,17 @@ class cached_property:
                 f"instance to cache {self.attrname!r} property."
             )
             raise TypeError(msg) from None
-        val = self.func(instance)
-        try:
-            cache[self.attrname] = val
-        except TypeError:
-            msg = (
-                f"The '__dict__' attribute on {type(instance).__name__!r} instance "
-                f"does not support item assignment for caching {self.attrname!r} property."
-            )
-            raise TypeError(msg) from None
+        val = cache.get(self.attrname, _NOT_FOUND)
+        if val is _NOT_FOUND:
+            val = self.func(instance)
+            try:
+                cache[self.attrname] = val
+            except TypeError:
+                msg = (
+                    f"The '__dict__' attribute on {type(instance).__name__!r} instance "
+                    f"does not support item assignment for caching {self.attrname!r} property."
+                )
+                raise TypeError(msg) from None
         return val
 
     __class_getitem__ = classmethod(GenericAlias)