import time

class Timer:
     def __init__(self):
       self.start_time = None

     def start(self):
       """开始计时"""
       self.start_time = time.time()

     def stop(self):
        """停止计时并返回持续时间"""
        if self.start_time is None:
         raise RuntimeError("Timer has not been started.")
        elapsed_time = time.time() - self.start_time
        self.start_time = None # 重置计时器
        return elapsed_time