def starts_in_seq(seq):
    """ Find the indexes of ATG codons in a sequence.
    Args:
        seq (Seq): sequence to scan
    Return:
        iterator: indexes of ATG occurences in sequence
    """
    start_codon="ATG"
    i = seq.find(start_codon)
    if(i == -1): return None
    while i != -1:
        yield i
        i = seq.find(start_codon, i+3)