# 反例：一个类干太多事
class UserManager:
    def get_user_info(self, user_id):
        # 连接数据库，获取用户信息
        pass
    
    def export_user_to_excel(self, user_id):
        # 获取用户信息，生成Excel
        pass
    
    def send_welcome_email(self, user_id):
        # 获取用户信息，发送邮件
        pass

# 正例：职责单一
class UserRepository:
    def get_user_info(self, user_id):
        # 只负责数据获取
        pass

class UserExcelExporter:
    def export(self, user_info):
        # 只负责生成Excel
        pass

class EmailService:
    def send_email(self, user_info, subject, body):
        # 只负责发送邮件
        pass

# 反例：一个类干太多事
class UserManager:
    def get_user_info(self, user_id):
        # 连接数据库，获取用户信息
        pass
    
    def export_user_to_excel(self, user_id):
        # 获取用户信息，生成Excel
        pass
    
    def send_welcome_email(self, user_id):
        # 获取用户信息，发送邮件
        pass

# 正例：职责单一
class UserRepository:
    def get_user_info(self, user_id):
        # 只负责数据获取
        pass

class UserExcelExporter:
    def export(self, user_info):
        # 只负责生成Excel
        pass

class EmailService:
    def send_email(self, user_info, subject, body):
        # 只负责发送邮件
        pass