Python metaclass

初步认知

• 深刻理解Python中的元类(metaclass)

• python当中metaclass探讨

类和元类中的 __new__ , __init__, __call__ 调用顺序

见栗子:

#!/usr/bin/env python
#-*-coding:utf-8-*-


class Meta_A(type):
    def __new__(cls, name, bases, attrs):
        print("Meta: __new__")
        return  super(Meta_A,cls).__new__(cls,name,bases,attrs)

    def __init__(self, *args, **kwargs):
        print("Meta: __init__")
        super(Meta_A, self).__init__(*args,**kwargs)

    def __call__(self,*args,**kwargs):
        print("Meta: __call__")
        return super(Meta_A, self).__call__(*args,**kwargs)

    
class A(metaclass=Meta_A):
    def __new__(cls, *args, **kwargs):
        print('A: __new__')
        return super(A, cls).__new__(cls, *args, **kwargs)

    def __init__(self, *args, **kwargs):
        print('A: __init__')

    def __call__(self, *args, **kwargs):
        print('A: __call__')
        

if __name__ == "__main__":
    a = A()    
    a()

its output

Meta: __new__
Meta: __init__
Meta: __call__ 
A: __new__ 
A: __init__
A: __call__

正如 a()会调用class A中的 __call__() 方法一样, A() 也将调用元类 Meta_A 中的 __call__() 方法，然后才是回到原 class A中并调用 __new__(), __init__().

单例模式

• 1 __new__() 方法

class Singleton(object):
    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 is s2)          # True

不能用 __init__() 方法，那是因为类的实例对象在 __new__() 中创建，在 __init__() 只是初始化实例对象而已。

• 2 元类中的 __call__() / __init__()

class Singleton(type):
    def __init__(self, *args, **kwargs):
        self.__instance = None
        super(Singleton,self).__init__(*args, **kwargs)

    def __call__(self, *args, **kwargs):
        if self.__instance is None:
            self.__instance = super(Singleton,self).__call__(*args, **kwargs)
        return self.__instance


class Foo(metaclass=Singleton):
    pass

if __name__ == '__main__':
    foo1 = Foo()
    foo2 = Foo()

    print(foo1 is foo2)        # True

• 3 元类中的 __call__() / __new__()

class Singleton(type):
    def __new__(cls, name,bases,attrs):
        attrs["_instance"] = None
        return  super(Singleton,cls).__new__(cls,name,bases,attrs)

    def __call__(self, *args, **kwargs):
        if self._instance is None:
            self._instance = super(Singleton,self).__call__(*args, **kwargs)
        return self._instance

class Foo(metaclass=Singleton):
    pass

if __name__ == '__main__':
    foo1 = Foo()
    foo2 = Foo()

    print(foo1 is foo2)  # True