In some usecases you just need the sequence lengths in an indexed
        fashion. If you access this attribute, we will make a hash map in memory.

def length_by_id(self):
        """In some usecases you just need the sequence lengths in an indexed
        fashion. If you access this attribute, we will make a hash map in memory."""
        hashmap = dict((seq.id, len(seq)) for seq in self)
        tmp = hashmap.copy()
        hashmap.update(tmp)
        return hashmap