timer_game.py

import time, random

class StopWatch():
    def __init__(self):
        self.started = 0.0
        self.stopped = 0.0
        self.elapsed = 0.0
    def reset(self):
        self.__init__()
    def start(self):
        self.reset()
        self.started = time.time()
        self.stopped = 0.0
    def stop(self):
        self.stopped = time.time()
        self.elapsed =  self.stopped - self.started
        if self.elapsed <= 0.0:
            self.elapsed = 0.0
        return  self.elapsed

class Game():
    def __init__(self):
        self.seconds = random.randint(1,10)
        self.watch = StopWatch()
    def play(self):
        print("This game tests your internal clock. How accurate are you?")
        print("Press Enter to start, then press Enter again after {} seconds.".format(self.seconds))
        _ = input()
        self.watch.start()
        _ = input()
        self.elapsed = self.watch.stop()
        print("You stopped at {0:.3f} seconds.".format(self.elapsed))
        if self.elapsed > self.seconds:
            print("You were {0:.3f} seconds slow.".format(self.elapsed - self.seconds))
        elif self.elapsed < self.seconds:
            print("You were {0:.3f} seconds fast".format(self.seconds - self.elapsed))
        elif self.elapsed == self.seconds:
            print("You were exact! That's not really possible and you should never see this.")     

if __name__ == '__main__':
    game = Game()
    game.play()
    again = input("Play again? Y|n  ")
    while again in ('','Y','y'):
        game = Game()
        game.play()
        again = input("Play again? Y|n  ")