AP polnum minus 'polnum' prefix if polnum else ``None``.

def get_polnum(self, obj):
        """AP polnum minus 'polnum' prefix if polnum else ``None``."""
        ap_id = obj.candidate_election.candidate.ap_candidate_id
        if 'polnum-' in ap_id:
            return ap_id.replace('polnum-', '')
        return None