首先官方解释
S.index(sub[, start[, end]]) -> int Like S.find() but raise ValueError when the substring is not found.
S.find(sub[, start[, end]]) -> int Return the lowest index in S where substring sub is found, such that sub is contained within S[start:end]. Optional arguments start and end are interpreted as in slice notation. Return -1 on failure.
可以看到S.index()与S.find()类似，不过索引字符串中的子串没找到会报错。
而S.find()在找不到substring时，不会报错，而会返回-1
总结：
s.index(x)：返回字符串中出现x的最左端的索引值，如果不在则抛出valueError异常
s.find(x) ：返回字符串中出现x的最左端字符的索引值，如果不在则返回-1
