chaines-valide.py

def valide(seq):
    """Retourne True si séquence ADN."""
    ret = len(seq) != 0
    for c in seq:
        ret = (ret and True) if (c in "atgc") else False
    return ret

if __name__=="__main__": 
    assert valide("atgc")
    assert not valide("atgcz")