#!/usr/bin/env pythonimport randomimport sysdef add(x,y):   return x + ydef sub(x,y):    return  x - ydef probe():    CMDs = {‘+‘:add,‘-‘:sub}    alist = [random.randint(1,50) for i in range(2)]    alist.sort(reverse=True)    op = random.choice(CMDs.keys())    answer = CMDs[op](*alist)    prompt = """%s %s %s:""" %(alist[0],op,alist[1])    i = 0    while i < 3:        try:            result = int(raw_input(prompt))        except (KeyboardInterrupt,EOFError),e:            print "\nUser cancelled"            sys.exit()        except (ValueError,IndexError,UnboundLocalError):            print "Invaild"        if answer == result:            print "very good"            break        print "input wrong still %d chance" % (2-i)        i += 1    else:        print ("%s%s") % (prompt,answer)if __name__ == ‘__main__‘:    while True:        probe()        yn = raw_input("continue(y/n)?>:".strip()[0])        if yn in ‘Nn‘:            print "bye-bye"            break