def _alt_ac_for_tx_ac(self, tx_ac):
alt_acs = [<EOL>e["<STR_LIT>"] for e in self.hdp.get_tx_mapping_options(tx_ac) if<EOL>e["<STR_LIT>"] == self.alt_aln_method and e["<STR_LIT>"] in self._assembly_accessions<EOL>]<EOL>if not alt_acs:<EOL><INDENT>raise HGVSDataNotAvailableError("<STR_LIT>".format(<EOL>tx_ac=tx_ac, an=self.assembly_name, am=self.alt_aln_method))<EOL><DEDENT>if len(alt_acs) > <NUM_LIT:1>:<EOL><INDENT>names = set(self._assembly_map[ac] for ac in alt_acs)<EOL>if names != set("<STR_LIT>"):<EOL><INDENT>alts = "<STR_LIT:U+002CU+0020>".join(<EOL>["<STR_LIT>".format(ac=ac, n=self._assembly_map[ac]) for ac in alt_acs])<EOL>raise HGVSError("<STR_LIT>"<EOL>"<STR_LIT>".format(<EOL>tx_ac=tx_ac,<EOL>an=self.assembly_name,<EOL>am=self.alt_aln_method,<EOL>alts=alts))<EOL><DEDENT>if self.in_par_assume is None:<EOL><INDENT>raise HGVSError("<STR_LIT>"<EOL>"<STR_LIT>".format(<EOL>tx_ac=tx_ac, an=self.assembly_name, am=self.alt_aln_method))<EOL><DEDENT>alt_acs = [ac for ac in alt_acs if self._assembly_map[ac] == self.in_par_assume]<EOL>if len(alt_acs) != <NUM_LIT:1>:<EOL><INDENT>raise HGVSError("<STR_LIT>"<EOL>"<STR_LIT>".format(<EOL>tx_ac=tx_ac,<EOL>an=self.assembly_name,<EOL>am=self.alt_aln_method,<EOL>ipa=self.in_par_assume,<EOL>n=len(alt_acs)))<EOL><DEDENT><DEDENT>assert len(alt_acs) == <NUM_LIT:1>, "<STR_LIT>"<EOL>return alt_acs[<NUM_LIT:0>]<EOL>
return chromosomal accession for given transcript accession (and
        the_assembly and aln_method setting used to instantiate this
        AssemblyMapper)