Find largest substring without repeating characters

find_largest_substring.py

def find_substring(string):
    """ Function that returns the longest 
        substring without repetition
    """
    max_length = 0
    init_sub = -1
    letters_dict = {}

    for idx, letter in enumerate(string):
        if letter in letters_dict:
            init_sub = max(init_sub, letters_dict[letter])
        max_length = max(max_length, idx - init_sub)
        letters_dict[letter] = idx

    return max_length