[CheckIO/Scientific Expedition] Longest substring of unique characters

1. Problem : https://py.checkio.org/en/mission/longest-substring-of-unique-characters/
2. My solution

def longest_substr(s: str) -> int:
    # your code here
    
    no_repeat_list = [] 
    longest = 0
    for char in [*s]:
        if no_repeat_list.count(char) == 0: no_repeat_list.append(char)
        else: 
            index = no_repeat_list.index(char)+1
            no_repeat_list.append(char)
            no_repeat_list = no_repeat_list[index:]
        if len(no_repeat_list) > longest: longest = len(no_repeat_list)
            
    return longest