gh-127750: Improve singledispatchmethod caching

Author: vodik

Lib/functools.py

@@ -1027,9 +1027,7 @@ def __init__(self, func):
 
         self.dispatcher = singledispatch(func)
         self.func = func
-
-        import weakref # see comment in singledispatch function
-        self._method_cache = weakref.WeakKeyDictionary()
+        self._method_cache = {}
 
     def register(self, cls, method=None):
         """generic_method.register(cls, func) -> func
@@ -1041,13 +1039,12 @@ def register(self, cls, method=None):
     def __get__(self, obj, cls=None):
         if self._method_cache is not None:
             try:
-                _method = self._method_cache[obj]
-            except TypeError:
-                self._method_cache = None
+                _obj_ref, _method = self._method_cache[id(obj)]
             except KeyError:
                 pass
             else:
-                return _method
+                if _obj_ref() is obj:
+                    return _method
 
         dispatch = self.dispatcher.dispatch
         funcname = getattr(self.func, '__name__', 'singledispatchmethod method')
@@ -1062,7 +1059,14 @@ def _method(*args, **kwargs):
         update_wrapper(_method, self.func)
 
         if self._method_cache is not None:
-            self._method_cache[obj] = _method
+            obj_id = id(obj)
+
+            def _remove(_):
+                self._method_cache.pop(obj_id, None)
+
+            import weakref # see comment in singledispatch function
+            obj_ref = weakref.ref(obj, _remove)
+            self._method_cache[obj_id] = (obj_ref, _method)
 
         return _method
 

Misc/NEWS.d/next/Library/2024-12-08-21-03-55.gh-issue-127750.94rNtr.rst

@@ -0,0 +1,2 @@
+Improve caching of :class:`functools.singledispatchmethod` to fix issues
+where sometimes different object instances could collide with each other.