python字符串表达式游戏

《Python核心编程》中第11章中一个示例：算术游戏（easyMath.py）

from operator import add, sub
from random import randint, choice

ops = {'+': add, '-': sub}

MAXTRIES = 2

def doprob():
    op = choice('+-')
    nums = [randint(1, 10) for i in range(2)]
    nums.sort(reverse = True) # reverse nums by sorted
    ans = ops[op](*nums) # calculate the right result
    
    pr = '%d %s %d = ' % (nums[0], op, nums[1])
    oops = 0 # record calculate times
    while True:
        try:
            if int(input(pr)) == ans:
                print('Correct!')
                break
            if oops == MAXTRIES:
                print('answer\n%s%d' % (pr, ans))
            else:
                print('incorrect... try again')
                oops += 1
        except (KeyboardInterrupt, EOFError, ValueError):
            print('invalid input... try again')
                
def main():
    while True:
        doprob()
        try:
            opt = input('Again? [y|n]').lower()
            if opt and opt[0] == 'n':
                break
        except (KeyboardInterrupt, EOFError):
            break
            
if __name__ == '__main__':
    main()