If you want to call custom create method if method is not defined then follow the following code in your .py file.

 if values.get('acquirer_id'):
            acquirer = self.pool['payment.acquirer'].browse(cr, uid, values.get('acquirer_id'), context=context)

            # compute fees
            custom_method_name = '%s_compute_fees' % acquirer.provider
            if hasattr(Acquirer, custom_method_name):
                fees = getattr(Acquirer, custom_method_name)(
                    cr, uid, acquirer.id, values.get('amount', 0.0), values.get('currency_id'), values.get('country_id'), context=None)
                values['fees'] = float_round(fees, 2)

            # custom create
            custom_method_name = '%s_create' % acquirer.provider
            if hasattr(self, custom_method_name):
                values.update(getattr(self, custom_method_name)(cr, uid, values, context=context))

        return super(PaymentTransaction, self).create(cr, uid, values, context=context)

    def on_change_partner_id(self, cr, uid, ids, partner_id, context=None):
        partner = None
        if partner_id:
            partner = self.pool['res.partner'].browse(cr, uid, partner_id, context=context)
        return {'values': {
            'partner_name': partner and partner.name or False,
            'partner_lang': partner and partner.lang or 'en_US',
            'partner_email': partner and partner.email or False,
            'partner_zip': partner and partner.zip or False,
            'partner_address': _partner_format_address(partner and partner.street or '', partner and partner.street2 or ''),
            'partner_city': partner and partner.city or False,
            'partner_country_id': partner and partner.country_id.id or False,
            'partner_phone': partner and partner.phone or False,
        }}


    def form_feedback(self, cr, uid, data, acquirer_name, context=None):
        invalid_parameters, tx = None, None

        tx_find_method_name = '_%s_form_get_tx_from_data' % acquirer_name
        if hasattr(self, tx_find_method_name):
            tx = getattr(self, tx_find_method_name)(cr, uid, data, context=context)

        invalid_param_method_name = '_%s_form_get_invalid_parameters' % acquirer_name
        if hasattr(self, invalid_param_method_name):
            invalid_parameters = getattr(self, invalid_param_method_name)(cr, uid, tx, data, context=context)

        if invalid_parameters:
            _error_message = '%s: incorrect tx data:\n' % (acquirer_name)
            for item in invalid_parameters:
                _error_message += '\t%s: received %s instead of %s\n' % (item[0], item[1], item[2])
            _logger.error(_error_message)
            return False

        feedback_method_name = '_%s_form_validate' % acquirer_name
        if hasattr(self, feedback_method_name):
            return getattr(self, feedback_method_name)(cr, uid, tx, data, context=context)

        return True



    def s2s_create(self, cr, uid, values, cc_values, context=None):
        tx_id, tx_result = self.s2s_send(cr, uid, values, cc_values, context=context)
        self.s2s_feedback(cr, uid, tx_id, tx_result, context=context)
        return tx_id

    def s2s_send(self, cr, uid, values, cc_values, context=None):
        """ Create and send server-to-server transaction.

        :param dict values: transaction values
        :param dict cc_values: credit card values that are not stored into the
                               payment.transaction object. Acquirers should
                               handle receiving void or incorrect cc values.
                               Should contain :

                                - holder_name
                                - number
                                - cvc
                                - brand
                                - expiry_date
                                - expiry_date_yy
                                - expiry_date_mm
        """
        tx_id, result = None, None

        if values.get('acquirer_id'):
            acquirer = self.pool['payment.acquirer'].browse(cr, uid, values.get('acquirer_id'), context=context)
            custom_method_name = '_%s_s2s_send' % acquirer.provider
            if hasattr(self, custom_method_name):
                tx_id, result = getattr(self, custom_method_name)(cr, uid, values, cc_values, context=context)

        if tx_id is None and result is None:
            tx_id = super(PaymentTransaction, self).create(cr, uid, values, context=context)
        return (tx_id, result)

    def s2s_feedback(self, cr, uid, tx_id, data, context=None):
        """ Handle the feedback of a server-to-server transaction. """
        tx = self.browse(cr, uid, tx_id, context=context)
        invalid_parameters = None

        invalid_param_method_name = '_%s_s2s_get_invalid_parameters' % tx.acquirer_id.provider
        if hasattr(self, invalid_param_method_name):
            invalid_parameters = getattr(self, invalid_param_method_name)(cr, uid, tx, data, context=context)

        if invalid_parameters:
            _error_message = '%s: incorrect tx data:\n' % (tx.acquirer_id.name)
            for item in invalid_parameters:
                _error_message += '\t%s: received %s instead of %s\n' % (item[0], item[1], item[2])
            _logger.error(_error_message)
            return False

        feedback_method_name = '_%s_s2s_validate' % tx.acquirer_id.provider
        if hasattr(self, feedback_method_name):
            return getattr(self, feedback_method_name)(cr, uid, tx, data, context=context)

        return True

    def s2s_get_tx_status(self, cr, uid, tx_id, context=None):
        """ Get the tx status. """
        tx = self.browse(cr, uid, tx_id, context=context)

        invalid_param_method_name = '_%s_s2s_get_tx_status' % tx.acquirer_id.provider
        if hasattr(self, invalid_param_method_name):
            return getattr(self, invalid_param_method_name)(cr, uid, tx, context=context)

        return True