find the longest subtring

longest_substr.py

def longest_substr(s):
    """Find the longest substring in a string.
    
    This should be O(n), return value is the span 
    of the longest substring. In case there is
    multiple longest substrings, the first one wins.
    """
    start = end = 0
    longest = (0, 0)
    chars = set()
    while True:
        try:
            next_ = s[end]
        except IndexError:
            return longest
        if next_ in chars:
            chars.remove(s[start])
            start += 1
        else:
            chars.add(next_)
            end += 1  # increment now already to get proper span indices 
            if (end-start) > (longest[1]-longest[0]):
                longest = (start, end)
  
# >>> longest_substr('')
# (0, 0)
# >>> longest_substr('a')
# (0, 1)
# >>> longest_substr('aaaaaaaaaaaaaaaa')
# (0, 1)
# >>> longest_substr('aaaaaabaaaaaaaaa')
# (5, 7)
# >>> longest_substr('aaaaaabaaaabaaaa')
# (5, 7)
# >>> longest_substr('aaaaaabaaaabcaaa')
# (10, 13)
# >>> longest_substr('abcdefghijklmnop')
# (0, 16)
# >>> longest_substr('aaaaaaaaaaaaaaab')
# (15, 16)