（s [i：j]）的目的？

Question

def longestPalindrome(s):
        if (s=="") or (len(set(list(s)))==1):
            return s
        m = s[0]
        for i in range(len(s)):
            # print( range(len(s)))
            for j in range(len(s), i,-1):
                # print(range(len(s),i,-1))
                print(s[i:j])
                if s[i:j]==s[i:j][::-1]:
                    if (j-i)>len(m):
                        m=s[i:j]
                        break

        return m