Python 元类执行过程

class XMeta(type):

    def __new__(mcs, *args, **kwargs):
        """
        【第一步】
        元类用__new__创建类。只会调用一次

        __new__ 方法始终都是类的静态方法，即使没有被加上静态方法装饰器

        """
        print('[X-meta new]', mcs, args, kwargs)
        return type.__new__(mcs, *args, **kwargs)

    def __init__(cls, *args, **kwargs):
        """
        【第二步】
        元类创建类时调用该构造方法，不需要返回值。只会调用一次

        """

        print('[X-meta init]', cls, args, kwargs)
        super().__init__(*args, **kwargs)

    def __call__(cls, *args, **kwargs):
        """
        【第三步】
        类实例化时会调用。每次都会调用

        该方法里会依次调用类的 __new__, __init__

        """
        print('[X-meta call]', cls, args, kwargs)
        return super().__call__(*args, **kwargs)


class A(metaclass=XMeta):

    def __new__(cls, *args, **kwargs):
        """
        __new__ 方法始终都是类的静态方法，即使没有被加上静态方法装饰器
        __new__ 必须要有返回值

        ！！！！！！注意：
        return super().__new__(cls, *args, **kwargs)
        报错：
            TypeError: object.__new__() takes exactly one argument (the type to instantiate)
        解决：
            执行 object.__ new__ 或 super().__new__ 时，只能传cls参数，否则会报错

        参考：
            https://blog.csdn.net/LaoYuanPython/article/details/93642448
            https://blog.csdn.net/weixin_30954607/article/details/95152484
            https://ask.csdn.net/questions/1061620
        """

        print('[A new]', cls, args, kwargs)
        # return object.__new__(cls)  # 执行父类（不是元类！！）的创建方法
        return super().__new__(cls)

    def __init__(self, *args, **kwargs):
        """
        构造方法，不需要返回值
        """
        print('[A init]', self, args, kwargs)

    def __call__(self, args, kwargs):
        """
        该方法使类的实例对象成为一个可调用对象
        """
        print('[A call]', self, args, kwargs)

    def echo(self, s='yes'):
        print('[A echo]', self, s)


a = A()


# 输出
[X-meta new] <class '__main__.XMeta'> ('A', (), {'__module__': '__main__', '__qualname__': 'A', '__new__': <function A.__new__ at 0x10ad16b90>, '__init__': <function A.__init__ at 0x10ad16c20>, '__call__': <function A.__call__ at 0x10ad16cb0>, 'echo': <function A.echo at 0x10ad16d40>, '__classcell__': <cell at 0x10ad2a650: empty>}) {}
[X-meta init] <class '__main__.A'> ('A', (), {'__module__': '__main__', '__qualname__': 'A', '__new__': <function A.__new__ at 0x10ad16b90>, '__init__': <function A.__init__ at 0x10ad16c20>, '__call__': <function A.__call__ at 0x10ad16cb0>, 'echo': <function A.echo at 0x10ad16d40>, '__classcell__': <cell at 0x10ad2a650: XMeta object at 0x7fc485717050>}) {}
[X-meta call] <class '__main__.A'> () {}
[A new] <class '__main__.A'> () {}
[A init] <__main__.A object at 0x10ad2a850> () {}