import importlib
from abc import ABC, abstractmethod

# 抽象主题
class Subject(ABC):
    @abstractmethod
    def request(self):
        pass

# 真实对象 1
class RealSubjectA(Subject):
    def request(self):
        print("RealSubjectA: Handling request.")

# 真实对象 2
class RealSubjectB(Subject):
    def request(self):
        print("RealSubjectB: Handling request.")

# 默认代理（备用对象）
class DefaultSubject(Subject):
    def request(self):
        print("DefaultSubject: Handling request as fallback.")

# 代理对象
class Proxy(Subject):
    def __init__(self, real_class_name: str):
        self._real_subject = self._create_real_subject(real_class_name)

    def request(self):
        print("Proxy: Additional logic before delegating.")
        self._real_subject.request()
        print("Proxy: Additional logic after delegating.")

    @staticmethod
    def _create_real_subject(class_name: str) -> Subject:
        # 动态加载类
        try:
            cls = globals().get(class_name)  # 从当前全局作用域中获取类
            if cls is None:
                raise KeyError
            if not issubclass(cls, Subject):
                raise TypeError(f"{class_name} is not a subclass of Subject.")
            return cls()
        except (KeyError, TypeError):
            print(f"Warning: Class '{class_name}' not found or invalid. Using DefaultSubject.")
            return DefaultSubject()

# 客户端代码
def client_code(proxy: Proxy):
    proxy.request()

# 使用代理，通过字符串指定真实类
print("Client: Using RealSubjectA:")
proxy_a = Proxy("RealSubjectA")
client_code(proxy_a)

print("\nClient: Using RealSubjectB:")
proxy_b = Proxy("RealSubjectB")
client_code(proxy_b)

print("\nClient: Attempting to use a non-existent class:")
proxy_invalid = Proxy("NonExistentClass")
client_code(proxy_invalid)