Given a string, determine the length of the longest substring with no repeating characters

substring.py

def find_len_substr(string):
    start = 0
    cur = start

    cur_max = 0

    seen = {}
    while start < len(string):
        deja_vu = False
        for x in range(start, len(string)):
            if string[x] in seen and seen[string[x]] >= start:
                # Log upto the spot we have now
                length = x - start
                if length > cur_max:
                    cur_max = length
                # Move the start pointer up one and keep calculating
                start = seen[string[x]] + 1
                # Flag variable we'll need once we exit
                deja_vu = True
                # Short circuit if we can't find a longer substring
                if len(string) - start < cur_max:
                    return cur_max
            seen[string[x]] = x
        if not deja_vu:
            return len(string) - start
    return cur_max

assert find_len_substr('abc') == 3
assert find_len_substr('abab') == 2
assert find_len_substr('aaaaabbbbc') == 2
assert find_len_substr('abcbadda') == 4