class Spam:
def bar(self, x:int, y:int):
print('Bar 1:', x, y)
def bar(self, s:str, n:int = 0):
print('Bar 2:', s, n)
s = Spam()
s.bar(2, 3) # Prints Bar 1: 2 3
s.bar('hello') # Prints Bar 2: hello 0
下面是我们第一步的尝试，使用到了一个元类和描述器：
# multiple.py
import inspect
import types
class MultiMethod:
'''
Represents a single multimethod.
'''
def __init__(self, name):
self._methods = {}
self.__name__ = name
def register(self, meth):
'''
Register a new method as a multimethod
'''
sig = inspect.signature(meth)
# Build a type signature from the method's annotations
types = []
for name, parm in sig.parameters.items():
if name == 'self':
continue
if parm.annotation is inspect.Parameter.empty:
raise TypeError(
'Argument {} must be annotated with a type'.format(name)
)
if not isinstance(parm.annotation, type):
raise TypeError(
'Argument {} annotation must be a type'.format(name)
)
if parm.default is not inspect.Parameter.empty:
self._methods[tuple(types)] = meth
types.append(parm.annotation)
self._methods[tuple(types)] = meth
def __call__(self, *args):
'''
Call a method based on type signature of the arguments
'''
types = tuple(type(arg) for arg in args[1:])
meth = self._methods.get(types, None)
if meth:
return meth(*args)
else:
raise TypeError('No matching method for types {}'.format(types))
def __get__(self, instance, cls):
'''
Descriptor method needed to make calls work in a class
'''
if instance is not None:
return types.MethodType(self, instance)
else:
return self
class MultiDict(dict):
'''
Special dictionary to build multimethods in a metaclass
'''
def __setitem__(self, key, value):
if key in self:
# If key already exists, it must be a multimethod or callable
current_value = self[key]
if isinstance(current_value, MultiMethod):
current_value.register(value)
else:
mvalue = MultiMethod(key)
mvalue.register(current_value)
mvalue.register(value)
super().__setitem__(key, mvalue)
else:
super().__setitem__(key, value)
class MultipleMeta(type):
'''
Metaclass that allows multiple dispatch of methods
'''
def __new__(cls, clsname, bases, clsdict):
return type.__new__(cls, clsname, bases, dict(clsdict))
@classmethod
def __prepare__(cls, clsname, bases):
return MultiDict()
为了使用这个类，你可以像下面这样写：
class Spam(metaclass=MultipleMeta):
def bar(self, x:int, y:int):
print('Bar 1:', x, y)
def bar(self, s:str, n:int = 0):
print('Bar 2:', s, n)
# Example: overloaded __init__
import time
class Date(metaclass=MultipleMeta):
def __init__(self, year: int, month:int, day:int):
self.year = year
self.month = month
self.day = day
def __init__(self):
t = time.localtime()
self.__init__(t.tm_year, t.tm_mon, t.tm_mday)
下面是一个交互示例来验证它能正确的工作：
>>> s = Spam()
>>> s.bar(2, 3)
Bar 1: 2 3
>>> s.bar('hello')
Bar 2: hello 0
>>> s.bar('hello', 5)
Bar 2: hello 5
>>> s.bar(2, 'hello')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "multiple.py", line 42, in __call__
raise TypeError('No matching method for types {}'.format(types))
TypeError: No matching method for types (<class 'int'>, <class 'str'>)
>>> # Overloaded __init__
>>> d = Date(2012, 12, 21)
>>> # Get today's date
>>> e = Date()
>>> e.year
2012
>>> e.month
12
>>> e.day
3
>>> b = s.bar
>>> b
<bound method Spam.bar of <__main__.Spam object at 0x1006a46d0>>
>>> b.__self__
<__main__.Spam object at 0x1006a46d0>
>>> b.__func__
<__main__.MultiMethod object at 0x1006a4d50>
>>> b(2, 3)
Bar 1: 2 3
>>> b('hello')
Bar 2: hello 0
不过本节的实现还有一些限制，其中一个是它不能使用关键字参数。例如：
>>> s.bar(x=2, y=3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __call__() got an unexpected keyword argument 'y'
>>> s.bar(s='hello')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __call__() got an unexpected keyword argument 's'