关于python:python-中是先运行-metaclass-还是先有类属性

答案：先有 “类属性”，再有 “运行 metaclass”

# 定义一个元类class CustomMetaclass(type):    def __new__(cls, name, bases, attrs):        print('> cls', cls)        print('> name', name)        print('> attrs', attrs)        print('> cls dict', cls.__dict__)        # 在创立类时批改属性        new_attrs = {}        for attr_name, attr_value in attrs.items():            if isinstance(attr_value, str):                new_attrs[attr_name] = attr_value.upper()            else:                new_attrs[attr_name] = attr_value        obj = super().__new__(cls, name, bases, new_attrs)        print(obj.__dict__)                print(type(obj))                return obj# 应用元类创立类class MyClass(metaclass=CustomMetaclass):    name = 'John'    age = 30    greeting = 'Hello'    def say_hello(self):        print(self.greeting)# 创立类的实例并调用办法obj = MyClass()print(obj.name)         # 输入: 'JOHN'print(obj.age)          # 输入: 30obj.say_hello()         # 输入: 'Hello'

输入后果如下：

> cls <class '__main__.CustomMetaclass'>> name MyClass> attrs {'__module__': '__main__', '__qualname__': 'MyClass', 'name': 'John', 'age': 30, 'greeting': 'Hello', 'say_hello': <function MyClass.say_hello at 0x1025c2200>}> cls dict {'__module__': '__main__', '__new__': <staticmethod(<function CustomMetaclass.__new__ at 0x1025c2290>)>, '__doc__': None}{'__module__': '__MAIN__', 'name': 'JOHN', 'age': 30, 'greeting': 'HELLO', 'say_hello': <function MyClass.say_hello at 0x1025c2200>, '__dict__': <attribute '__dict__' of 'MyClass' objects>, '__weakref__': <attribute '__weakref__' of 'MyClass' objects>, '__doc__': None}<class '__main__.CustomMetaclass'>JOHN30