longest substring with non repeating chars

lnr_substring.py

def lnrss(s):
	"""
	 Returns the length of longest substring of input string with no repeated letter.
	 Assumes input string to be in lower case and ignores any other characters.
	"""
	l = 0
	for i in range(len(s)):
		for j in range(len(s) - i):
			ss = s[j:j+i+1]
			if (not has_rep(ss)) and len(ss)>=l:
				l = len(ss)
	return l

def has_rep(ss):
        memo = {chr(k+97):0 for k in range(26)}
	for l in list(ss):
		try:
			memo[l] += 1 
			if memo[l] >1 :
				return True
		except KeyError as e:
			pass
	return False