def protein_ids(self, contig=None, strand=None):
        """
        What are all the protein IDs
        (optionally restrict to a given chromosome and/or strand)
        """
        protein_ids = self._all_feature_values(
            column="protein_id",
            feature="CDS",
            contig=contig,
            strand=strand,
            distinct=True)
        # drop None values
        return [protein_id for protein_id in protein_ids if protein_id]
What are all the protein IDs
        (optionally restrict to a given chromosome and/or strand)