*使用元类控制实例的创建

你想通过改变实例创建方式来实现单例、缓存或其他类似的特性。
如果你想自定义这个步骤，你可以定义一个元类并自己实现 call() 方法。

1. 假设你不想任何人创建这个类的实例

class NoInstances(type):
    def __call__(self, *args, **kwargs):
        raise TypeError("Can't instantuate directly")
class Spam(metaclass=NoInstances):
    @staticmethod
    def grok(x):
        print('Spam.grok')
Spam.grok(42)
# Spam.grok
s = Spam()
# TypeError: Can't instantuate directly

2. 假如你想实现单例模式 [链接]

class Singleton(type):
    def __init__(self, *args, **kwargs):
        self.__instance = None
        super().__init__(*args, **kwargs)
    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super().__call__(*args, **kwargs)
            return self.__instance
        else:
            return self.__instance
class Spam(metaclass=Singleton):
    def __init__(self):
        print('Creating Spam')
a = Spam()
b = Spam()
print(a is b)
# True
c = Spam()
print(a is c)
# True

3. 元类实现缓存实例

import weakref
class Cached(type):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__cache = weakref.WeakValueDictionary()
    def __call__(self, *args):
        if args in self.__cache:
            return self.__cache[args]
        else:
            obj = super().__call__(*args)
            self.__cache[args] = obj
            return obj
class Spam(metaclass=Cached):
    def __init__(self, name):
        print('Creating Spam({!r})'.format(name))
        self.name = name
a = Spam('Guido')
# Creating Spam('Guido')
b = Spam('Diana')
# Creating Spam('Diana')
c = Spam('Guido')
# Cached
print(a is b)
# False
print(a is c)
# True