字符串的替换函数replace有一个坑，

a = "123456"

a.replace("6","7")

print a

结果还是"123456"

看看replace函数的介绍，

Return a copy of string S with all occurrences of substring old replaced by new.  If the optional argument count is given, only the first count occurrences are replaced. 替换之后原来是返回一个新的copy，正确的做法是：

a = "123456"

b = a.replace("6","7")

print b