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