Longest Substring without using repeated characters in python

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        #k=[]
        a=0
        for j in range(len(s)):
            k=[]
            for i in range(j,len(s)):
                if s[i] not in k:
                    k.append(s[i]) 
                else:
                    break
            a=max(len(k),a)
        return a