<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def setup_assertion(self, authn, sp_entity_id, in_response_to, consumer_url, name_id, policy, _issuer, authn_statement, identity, best_effort, sign_response, farg=None, session_not_on_or_after=None, **kwargs):
 """ Construct and return the Assertion :param authn: Authentication information :param sp_entity_id: :param in_response_to: The ID of the request this is an answer to :param consumer_url: The recipient of the assertion :param name_id: The NameID of the subject :param policy: Assertion policies :param _issuer: Issuer of the statement :param authn_statement: An AuthnStatement instance :param identity: Identity information about the Subject :param best_effort: Even if not the SPs demands can be met send a response. :param sign_response: Sign the response, only applicable if ErrorResponse :param kwargs: Extra keyword arguments :return: An Assertion instance """


        ast = Assertion(identity)
        ast.acs = self.config.getattr("attribute_converters", "idp")
        if policy is None:
            policy = Policy()
        try:
            ast.apply_policy(sp_entity_id, policy, self.metadata)
        except MissingValue as exc:
            if not best_effort:
                return self.create_error_response(in_response_to, consumer_url,
                                                  exc, sign_response)

        farg = self.update_farg(in_response_to, consumer_url, farg)

        if authn:  # expected to be a dictionary
            # Would like to use dict comprehension but ...
            authn_args = dict(
                [(AUTHN_DICT_MAP[k], v) for k, v in authn.items() if
                 k in AUTHN_DICT_MAP])
            authn_args.update(kwargs)

            assertion = ast.construct(
                sp_entity_id, self.config.attribute_converters, policy,
                issuer=_issuer, farg=farg['assertion'], name_id=name_id,
                session_not_on_or_after=session_not_on_or_after,
                **authn_args)

        elif authn_statement:  # Got a complete AuthnStatement
            assertion = ast.construct(
                sp_entity_id, self.config.attribute_converters, policy,
                issuer=_issuer, authn_statem=authn_statement,
                farg=farg['assertion'], name_id=name_id,
                **kwargs)
        else:
            assertion = ast.construct(
                sp_entity_id, self.config.attribute_converters, policy,
                issuer=_issuer, farg=farg['assertion'], name_id=name_id,
                session_not_on_or_after=session_not_on_or_after,
                **kwargs)
        return assertion