#!/usr/bin/env python
from operator import add, sub 
from random import randint, choice
ops = {'+': add, '-':sub}
#定义一个字典
MAXTRIES = 2 
def doprob():
  op = choice('+-')
  #用choice从'+-'中随意选择操作符 
  nums = [randint(1,10) for i in range(2)]
  #用randint(1,10)随机生成一个1到10的数,随机两次使用range(2) 
  nums.sort(reverse=True)
  #按升序排序
  ans = ops[op](*nums)
  #利用函数
  pr = '%d %s %d = ' % (nums[0], op, nums[1])
  oops = 0 
  #oops用来计算failure测试,当三次时自动给出答案
  while True:
    try:
      if int(raw_input(pr)) == ans:
        print 'correct'
        break
      if oops == MAXTRIES:
        print 'answer\n %s%d' % (pr, ans)
        break
      else:
        print 'incorrect... try again'
        oops += 1
    except (KeyboardInterrupt, EOFError, ValueError):
      print 'invalid ipnut... try again'
def main():
  while True:
    doprob()
    try:
      opt = raw_input('Again? [y]').lower()
      if opt and opt[0] == 'n':
        break
    except (KeyboardInterrupt, EOFError):
      break
if __name__ == '__main__':
  main()

8 - 1 = 7
correct
Again? [y]y
7 - 1 = 6
correct
Again? [y]y
9 + 4 = 0
incorrect... try again
9 + 4 =