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

This commit is contained in:
Ethan Smith 2018-05-26 16:38:33 -04:00 committed by Łukasz Langa
parent 09c4a7dee2
commit c651275afe
5 changed files with 205 additions and 1 deletions

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@ -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

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@ -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)

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@ -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 = (

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@ -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

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@ -0,0 +1,2 @@
Create functools.singledispatchmethod to support generic single dispatch on
descriptors and methods.