练习1：定义一个类描述数字时钟

from time import sleep
class Clock():
    def __init__(self,hour,minute,seconds):
        self._hour = hour
        self._minute = minute
        self._seconds = seconds

    def run(self):
        self._seconds += 1
        if self._seconds == 60:
            self._seconds = 0
            self._minute += 1
            if self._minute == 60:
                self._minute = 0
                self._hour += 1
                if self._hour == 24:
                    self._hour =0

    def show(self):
        return ("%02d:%02d:%02d"%(self._hour,self._minute,self._seconds))

def main():
    clock = Clock(23,59,58)
    while True:
        print(clock.show())
        sleep(1)
        clock.run()



if __name__=="__main__":
    main()