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

Doc/library/functools.rst

@@ -383,6 +383,52 @@ The :mod:`functools` module defines the following functions:
       The :func:`register` attribute supports using type annotations.
 
 
+.. class:: singledispatchmethod(func)
+
+   Transform a method into a :term:`single-dispatch <single
+   dispatch>` :term:`generic function`.
+
+   To define a generic method, decorate it with the ``@singledispatchmethod``
+   decorator. Note that the dispatch happens on the type of the first non-self
+   or non-cls argument, create your function accordingly::
+
+    class Negator:
+        @singledispatchmethod
+        def neg(self, arg):
+            raise NotImplementedError("Cannot negate a")
+
+        @neg.register
+        def _(self, arg: int):
+            return -arg
+
+        @neg.register
+        def _(self, arg: bool):
+            return not arg
+
+   ``@singledispatchmethod`` supports nesting with other decorators such as
+   ``@classmethod``. Note that to allow for ``dispatcher.register``,
+   ``singledispatchmethod`` must be the *outer most* decorator. Here is the
+   ``Negator`` class with the ``neg`` methods being class bound::
+
+    class Negator:
+        @singledispatchmethod
+        @classmethod
+        def neg(cls, arg):
+            raise NotImplementedError("Cannot negate a")
+
+        @neg.register
+        @classmethod
+        def _(cls, arg: int):
+            return -arg
+
+        @neg.register
+        @classmethod
+        def _(cls, arg: bool):
+            return not arg
+
+   The same pattern can be used for other similar decorators: ``staticmethod``,
+   ``abstractmethod``, and others.
+
 .. function:: update_wrapper(wrapper, wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)
 
    Update a *wrapper* function to look like the *wrapped* function. The optional

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)

Lib/test/test_functools.py

@@ -2147,6 +2147,124 @@ class TestSingleDispatch(unittest.TestCase):
                 return self.arg == other
         self.assertEqual(i("str"), "str")
 
+    def test_method_register(self):
+        class A:
+            @functools.singledispatchmethod
+            def t(self, arg):
+                self.arg = "base"
+            @t.register(int)
+            def _(self, arg):
+                self.arg = "int"
+            @t.register(str)
+            def _(self, arg):
+                self.arg = "str"
+        a = A()
+
+        a.t(0)
+        self.assertEqual(a.arg, "int")
+        aa = A()
+        self.assertFalse(hasattr(aa, 'arg'))
+        a.t('')
+        self.assertEqual(a.arg, "str")
+        aa = A()
+        self.assertFalse(hasattr(aa, 'arg'))
+        a.t(0.0)
+        self.assertEqual(a.arg, "base")
+        aa = A()
+        self.assertFalse(hasattr(aa, 'arg'))
+
+    def test_staticmethod_register(self):
+        class 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)
+
+    def test_classmethod_register(self):
+        class A:
+            def __init__(self, arg):
+                self.arg = arg
+
+            @functools.singledispatchmethod
+            @classmethod
+            def t(cls, arg):
+                return cls("base")
+            @t.register(int)
+            @classmethod
+            def _(cls, arg):
+                return cls("int")
+            @t.register(str)
+            @classmethod
+            def _(cls, arg):
+                return cls("str")
+
+        self.assertEqual(A.t(0).arg, "int")
+        self.assertEqual(A.t('').arg, "str")
+        self.assertEqual(A.t(0.0).arg, "base")
+
+    def test_callable_register(self):
+        class A:
+            def __init__(self, arg):
+                self.arg = arg
+
+            @functools.singledispatchmethod
+            @classmethod
+            def t(cls, arg):
+                return cls("base")
+
+        @A.t.register(int)
+        @classmethod
+        def _(cls, arg):
+            return cls("int")
+        @A.t.register(str)
+        @classmethod
+        def _(cls, arg):
+            return cls("str")
+
+        self.assertEqual(A.t(0).arg, "int")
+        self.assertEqual(A.t('').arg, "str")
+        self.assertEqual(A.t(0.0).arg, "base")
+
+    def test_abstractmethod_register(self):
+        class Abstract(abc.ABCMeta):
+
+            @functools.singledispatchmethod
+            @abc.abstractmethod
+            def add(self, x, y):
+                pass
+
+        self.assertTrue(Abstract.add.__isabstractmethod__)
+
+    def test_type_ann_register(self):
+        class A:
+            @functools.singledispatchmethod
+            def t(self, arg):
+                return "base"
+            @t.register
+            def _(self, arg: int):
+                return "int"
+            @t.register
+            def _(self, arg: str):
+                return "str"
+        a = A()
+
+        self.assertEqual(a.t(0), "int")
+        self.assertEqual(a.t(''), "str")
+        self.assertEqual(a.t(0.0), "base")
+
     def test_invalid_registrations(self):
         msg_prefix = "Invalid first argument to `register()`: "
         msg_suffix = (

Misc/ACKS

@@ -1510,6 +1510,7 @@ Václav Šmilauer
 Allen W. Smith
 Christopher Smith
 Eric V. Smith
+Ethan H. Smith
 Gregory P. Smith
 Mark Smith
 Nathaniel J. Smith

Misc/NEWS.d/next/Library/2018-03-29-03-09-22.bpo-32380.NhuGig.rst

+Create functools.singledispatchmethod to support generic single dispatch on
+descriptors and methods.