gh-85160: improve performance of singledispatchmethod

Lib/functools.py

@@ -933,6 +933,7 @@ def __init__(self, func):
 
         self.dispatcher = singledispatch(func)
         self.func = func
+        self._dispatch_method = None
 
     def register(self, cls, method=None):
         """generic_method.register(cls, func) -> func
@@ -942,13 +943,17 @@ def register(self, cls, method=None):
         return self.dispatcher.register(cls, func=method)
 
     def __get__(self, obj, cls=None):
+        if self._dispatch_method:
+            return self._dispatch_method
+
         def _method(*args, **kwargs):
             method = self.dispatcher.dispatch(args[0].__class__)
             return method.__get__(obj, cls)(*args, **kwargs)
 
         _method.__isabstractmethod__ = self.__isabstractmethod__
         _method.register = self.register
         update_wrapper(_method, self.func)
+        self._dispatch_method = _method
         return _method
 
     @property

Lib/test/test_functools.py

@@ -2474,6 +2474,26 @@ def _(arg):
         self.assertTrue(A.t(''))
         self.assertEqual(A.t(0.0), 0.0)
 
+    def test_staticmethod__slotted_class(self):
+        class A:
+            __slots__ = ['a']
+            @functools.singledispatchmethod
+            def t(arg):
+                return arg
+            @t.register(int)
+            @staticmethod
+            def _(arg):
+                return isinstance(arg, int)
+            @t.register(str)
+            @staticmethod
+            def _(arg):
+                return isinstance(arg, str)
+        a = A()
+
+        self.assertTrue(A.t(0))
+        self.assertTrue(A.t(''))
+        self.assertEqual(A.t(0.0), 0.0)
+
     def test_classmethod_register(self):
         class A:
             def __init__(self, arg):

Misc/NEWS.d/next/Library/2023-07-05-11-28-57.gh-issue-85160.t925jm.rst

@@ -0,0 +1 @@
+Improve performance of :class:`functools.singledispatchmethod`.