def assert_trigger(self, session, protocol):
        """Asserts software or hardware trigger.

        Corresponds to viAssertTrigger function of the VISA library.

        :param session: Unique logical identifier to a session.
        :param protocol: Trigger protocol to use during assertion. (Constants.PROT*)
        :return: return value of the library call.
        :rtype: :class:`pyvisa.constants.StatusCode`
        """
        try:
            return self.sessions[session].assert_trigger(protocol)

        except KeyError:
            return constants.StatusCode.error_invalid_object