def DF(s, k):
    """
        Return a set in a list shape
        of distinct factors of length 
        k of the string s
        --------
        Parameters:
            s (str)
            k (int)
        
        eg:
        DF("BANANA", 2)
        Output:
        ['NA', 'BA', 'AN']
    """
    n=len(s)
    return list(set([s[i:i+k] for i in range(0, max(0, n-k+1))]))