Python元类下单例模式的使用介绍（代码示例）

# -*- coding: utf8 -*-

class Singleton(object):
    def __init__(self):
        print 'entrance of __init__'

    def __new__(cls, *args, **kwargs):
        if not hasattr(cls, '_instance'):
            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
        return cls._instance

if __name__ == '__main__':
    s1 = Singleton()
    s2 = Singleton()

    print s1, s2

# -*- coding: utf8 -*-

# 单例模式
class SingletonMetaClass(type):
    def __init__(cls, *args, **kwargs):
        """
        初始化类
        :param args:
        :param kwargs:
        """
        print 'MetaClass.__init__ called'
        print cls._instance
        super(SingletonMetaClass, cls).__init__(*args, **kwargs)


    def __new__(cls, name, bases, attrs):
        """
        创建类，负责类的行为和属性的创建
        :param name:
        :param bases:
        :param attrs:
        :return:
        """
        print 'MetaClass.__new__ called'

        # 单例模式
        cls._instance = None

        return type.__new__(cls, name, bases, attrs)

    # __call__方法其实和类的创建过程和实例化没有多大关系了，定义了__call__方法才能被使用函数的方式执行。
    # 被该元类创建的类，属于该元类的一个实例。因此创建其实例的时候，会调用元类的__call_方法
    def __call__(cls, *args, **kwargs):
        """
        使类变为可调用对象
        :param args:
        :param kwargs:
        :return:
        """
        print 'MetaClass.__call__ called'
        if cls._instance is None:
            # 这里会去调用类的__new__函数
            cls._instance = super.__call__(SingletonMetaClass, cls).__call__(*args, **kwargs)
        return cls._instance

class A():
    __metaclass__ = SingletonMetaClass

    def __new__(cls, *args, **kwargs):
        print 'A.__new__ called'
        return super(A, cls).__new__(cls, *args, **kwargs)

if __name__ == '__main__':
    # 因为类A是SingletonMetaClass的一个实例，执行A()时会调用元类SingletonMetaClass的__call__方法
    a1 = A()
    print a1
    a2 = A()
    print a2

MetaClass.__new__ called
MetaClass.__init__ called
None
MetaClass.__call__ called
A.__new__ called
<__main__.A object at 0x00000000036D2EB8>
MetaClass.__call__ called
<__main__.A object at 0x00000000036D2EB8>