Python 元类(中)：拦截类的创建方法

class SimpleMeta(type):
    def __new__(mcs, name, bases, namespace):
        print(f"  正在创建类: {name}")
        cls = super().__new__(mcs, name, bases, namespace)
        return cls
class MyClass(metaclass=SimpleMeta):
    pass
# 输出: 正在创建类: MyClass
print(type(MyClass))  # <class '__main__.SimpleMeta'> —— 不再是 type

class Another(metaclass=SimpleMeta):
    pass
class AndAnother(metaclass=SimpleMeta):
    pass
# 输出:
#   正在创建类: Another
#   正在创建类: AndAnother

class DebugMeta(type):
    def __new__(mcs, name, bases, namespace):
        print(f"  namespace = {namespace}")
        return super().__new__(mcs, name, bases, namespace)
class Sample(metaclass=DebugMeta):
    x = 10
    def hello(self):
        return "hi"
# 输出:
# namespace = {'__module__': '__main__', '__qualname__': 'Sample', 'x': 10, 'hello': <function Sample.hello at 0x...>}

class AutoAttrMeta(type):
    def __new__(mcs, name, bases, namespace):
        cls = super().__new__(mcs, name, bases, namespace)
        if not hasattr(cls, 'version'):
            cls.version = "1.0"
        return cls
class Service(metaclass=AutoAttrMeta):
    pass
class AdvancedService(Service):
    version = "2.0"  # 自己定义了，不会被覆盖
class BasicService(metaclass=AutoAttrMeta):
    pass
print(Service.version)         # 1.0  ← 自动加上了
print(AdvancedService.version) # 2.0  ← 自己定义的，没被覆盖
print(BasicService.version)    # 1.0  ← 也自动加上了

class LowercaseMeta(type):
    def __new__(mcs, name, bases, namespace):
        for attr_name in namespace:
            if not attr_name.startswith('_') and attr_name != attr_name.lower():
                raise TypeError(
                    f"类 '{name}' 中的属性 '{attr_name}' 必须全小写！"
                )
        return super().__new__(mcs, name, bases, namespace)
class GoodClass(metaclass=LowercaseMeta):
    my_field = 1       # ✓ 小写
    another_field = 2  # ✓ 小写
print(GoodClass.my_field)  # 1 —— 没问题
try:
    class BadClass(metaclass=LowercaseMeta):
        BadField = 1   # ✗ 大写了！
except TypeError as e:
    print(e)  # 类 'BadClass' 中的属性 'BadField' 必须全小写！

class SingletonMeta(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            print(f"  创建 {cls.__name__} 的唯一实例")
            cls._instances[cls] = super().__call__(*args, **kwargs)
        else:
            print(f"  返回已有实例")
        return cls._instances[cls]
class Database(metaclass=SingletonMeta):
    def __init__(self):
        self.connection_id = id(self)
db1 = Database()  # 创建 Database 的唯一实例
db2 = Database()  # 返回已有实例
print(db1 is db2)                        # True —— 同一个对象
print(db1.connection_id == db2.connection_id)  # True

Database()
    │
    ▼
SingletonMeta.__call__(Database)   ← 元类拦截
    │
    ├─ 第一次 → super().__call__() → Database.__new__() → Database.__init__()
    │
    └─ 之后 → 直接返回缓存的实例

class PluginMeta(type):
    registry = {}
    def __new__(mcs, name, bases, namespace):
        cls = super().__new__(mcs, name, bases, namespace)
        if name != "Base":          # 跳过基类本身
            mcs.registry[name] = cls
        return cls
class Base(metaclass=PluginMeta):
    pass
class AuthPlugin(Base):
    def authenticate(self):
        return "authenticated"
class LogPlugin(Base):
    def log(self, msg):
        return f"LOG: {msg}"
# 不需要手动注册，定义类的时候就完成了
print(list(PluginMeta.registry.keys()))  # ['AuthPlugin', 'LogPlugin']
# 可以通过名字动态实例化
plugin = PluginMeta.registry["AuthPlugin"]()
print(plugin.authenticate())  # authenticated

class TraceMeta(type):
    def __new__(mcs, name, bases, namespace):
        print(f"  [元类] 正在创建 {name}")
        return super().__new__(mcs, name, bases, namespace)
print("开始定义类...")
class A(metaclass=TraceMeta):
    pass
print("类定义完成")
# 输出:
# 开始定义类...
#   [元类] 正在创建 A
# 类定义完成