gh-85160: improve performance of functools.singledispatchmethod (#107148)

Co-authored-by: mental <m3nta1@yahoo.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/functools.py

@@ -928,11 +928,14 @@ class singledispatchmethod:
     """
 
     def __init__(self, func):
+        import weakref # see comment in singledispatch function
         if not callable(func) and not hasattr(func, "__get__"):
             raise TypeError(f"{func!r} is not callable or a descriptor")
 
         self.dispatcher = singledispatch(func)
         self.func = func
+        self._method_cache = weakref.WeakKeyDictionary()
+        self._all_weakrefable_instances = True
 
     def register(self, cls, method=None):
         """generic_method.register(cls, func) -> func
@@ -942,13 +945,27 @@ def register(self, cls, method=None):
         return self.dispatcher.register(cls, func=method)
 
     def __get__(self, obj, cls=None):
+        if self._all_weakrefable_instances:
+            try:
+                _method = self._method_cache[obj]
+            except TypeError:
+                self._all_weakrefable_instances = False
+            except KeyError:
+                pass
+            else:
+                return _method
+
+        dispatch = self.dispatcher.dispatch
         def _method(*args, **kwargs):
-            method = self.dispatcher.dispatch(args[0].__class__)
-            return method.__get__(obj, cls)(*args, **kwargs)
+            return dispatch(args[0].__class__).__get__(obj, cls)(*args, **kwargs)
 
         _method.__isabstractmethod__ = self.__isabstractmethod__
         _method.register = self.register
         update_wrapper(_method, self.func)
+
+        if self._all_weakrefable_instances:
+            self._method_cache[obj] = _method
+
         return _method
 
     @property

Lib/test/test_functools.py

@@ -2474,6 +2474,74 @@ def _(arg):
         self.assertTrue(A.t(''))
         self.assertEqual(A.t(0.0), 0.0)
 
+    def test_slotted_class(self):
+        class Slot:
+            __slots__ = ('a', 'b')
+            @functools.singledispatchmethod
+            def go(self, item, arg):
+                pass
+
+            @go.register
+            def _(self, item: int, arg):
+                return item + arg
+
+        s = Slot()
+        self.assertEqual(s.go(1, 1), 2)
+
+    def test_classmethod_slotted_class(self):
+        class Slot:
+            __slots__ = ('a', 'b')
+            @functools.singledispatchmethod
+            @classmethod
+            def go(cls, item, arg):
+                pass
+
+            @go.register
+            @classmethod
+            def _(cls, item: int, arg):
+                return item + arg
+
+        s = Slot()
+        self.assertEqual(s.go(1, 1), 2)
+        self.assertEqual(Slot.go(1, 1), 2)
+
+    def test_staticmethod_slotted_class(self):
+        class A:
+            __slots__ = ['a']
+            @functools.singledispatchmethod
+            @staticmethod
+            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)
+        self.assertTrue(a.t(0))
+        self.assertTrue(a.t(''))
+        self.assertEqual(a.t(0.0), 0.0)
+
+    def test_assignment_behavior(self):
+        # see gh-106448
+        class A:
+            @functools.singledispatchmethod
+            def t(arg):
+                return arg
+
+        a = A()
+        a.t.foo = 'bar'
+        a2 = A()
+        with self.assertRaises(AttributeError):
+            a2.t.foo
+
     def test_classmethod_register(self):
         class A:
             def __init__(self, arg):

Misc/NEWS.d/next/Library/2020-11-10-07-04-15.bpo-40988.5kBC-O.rst

@@ -0,0 +1,3 @@
+Improve performance of :class:`functools.singledispatchmethod` by caching the
+generated dispatch wrapper. Optimization suggested by frederico. Patch by
+@mental32, Alex Waygood and Pieter Eendebak.