Python利用元类来控制实例创建

问题：

改变实例创建方式，以此来实现单例模式，缓存或者其他类似的特性。

解决方法：

如果想定制化创建实例的过程，可以通过定制一个元类并以某种方式重新实现它的__call__()方法。

使用元类的单例模式实现：

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)
c = Spam()
print(a is c)

运行结果：

Creating Spam
True
True

不使用元类的单例模式实现：

class _Spam:
    def __init__(self):
        print("Creating Spam")

_spam_instance = None

def Spam():
    global _spam_instance
    if _spam_instance is not None:
        return _spam_instance
    else:
        _spam_instance = _Spam()
        return _spam_instance

a = Spam()
b = Spam()
print(a is b)
c = Spam()
print(a is c)

运行结果：

Creating Spam
True
True

创建缓存实例：（不使用元类方法链接：https://www.cnblogs.com/weswes/p/10007794.html）

import weakref

class Cached(type):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__cache = weakref.WeakValueDictionary()

    def __call__(self, *args, **kwargs):
        if args in self.__cache:
            return self.__cache[args]
        else:
            obj = super().__call__(*args, **kwargs)
            self.__cache[args] = obj
            return obj

class Spam(metaclass=Cached):
    def __init__(self, name):
        print("Creating Spam({!r})".format(name))
        self.name = name

    def __call__(self, *args, **kwargs):
        print(999)

a = Spam("Guido")
b = Spam("Diana")
c = Spam("Guido")
print(a is b)
print(a is c)

运行结果：

Creating Spam('Guido')
Creating Spam('Diana')
False
True