Python 关于with_metaclass

def with_metaclass(meta, *bases):
    class metaclass(type):
        print('enter mataclass')

        def __new__(metacls, name, this_bases, d):
            """
            修改了{name}类的父类
            """
            print('[metaclass new]', metacls, name, this_bases, d)
            return meta(name, bases, d)

        def __init__(cls, *args, **kwargs):
            print('[metaclass init]', cls, args, kwargs)

    return type.__new__(metaclass, "temporary_class", (), {"x": lambda s: print(s)})


class A:
    @staticmethod
    def echo(s=None):
        print(s)


class B(type):
    def __init__(cls, *args, **kwargs):
        """
        不会执行metaclass构造方法
        """
        print('[B init]', cls, args, kwargs)


M = with_metaclass(B, A)  # 创建的临时类
print('创建M的元类 ', M.__class__)
print('M的父类 ', M.__mro__)
print('>' * 20)


class N(M):  # 创建类N时候才去执行metaclass中的方法
    pass


print('创建N的元类 ', N.__class__)
print('N的父类 ', N.__mro__)