bpo-32380: Create functools.singledispatchmethod (#6306)

Lib/functools.py

@@ -11,7 +11,7 @@
 
 __all__ = ['update_wrapper', 'wraps', 'WRAPPER_ASSIGNMENTS', 'WRAPPER_UPDATES',
            'total_ordering', 'cmp_to_key', 'lru_cache', 'reduce', 'partial',
-           'partialmethod', 'singledispatch']
+           'partialmethod', 'singledispatch', 'singledispatchmethod']
 
 try:
     from _functools import reduce
@@ -826,3 +826,40 @@ def singledispatch(func):
     wrapper._clear_cache = dispatch_cache.clear
     update_wrapper(wrapper, func)
     return wrapper
+
+
+# Descriptor version
+class singledispatchmethod:
+    """Single-dispatch generic method descriptor.
+
+    Supports wrapping existing descriptors and handles non-descriptor
+    callables as instance methods.
+    """
+
+    def __init__(self, func):
+        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
+
+    def register(self, cls, method=None):
+        """generic_method.register(cls, func) -> func
+
+        Registers a new implementation for the given *cls* on a *generic_method*.
+        """
+        return self.dispatcher.register(cls, func=method)
+
+    def __get__(self, obj, cls):
+        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)
+        return _method
+
+    @property
+    def __isabstractmethod__(self):
+        return getattr(self.func, '__isabstractmethod__', False)