无法弄清楚如何在Python类中修复此错误

Question

我是Python的初学者，并试图找出错误以及如何修复这个简短的python代码：

from time import *
class Stopwatch:
    def __init__(self):
        self.reset()
    def start(self):
        if not self.running:
            self.start_time = clock()
            self.running = True
        else:
            print('Stopwatch already running')
    def stop(self):
        if self.running:
            self.elapsed += clock() - self.start_time
            self.running = False
        else:
            print('stopwatch not running')
    def reset(self):
        self.start_time = self.elapsed = 0
        self.running = False
    def elapsed(self):
        if not self.running:
            return self.elapsed
        else:
            print("stopwatch must be stopped")
            return None

timer = Stopwatch()
timer.start()
sleep(2)
print('I am awake \n')
timer.stop()
print(timer.elapsed())

---＆GT;我得到这个错误说 TypeError：'float'对象不可调用 但为什么'浮'不可赎回？ 谢谢你的帮助 霍华德