Installs handlers for event callbacks.

        :param session: Unique logical identifier to a session.
        :param event_type: Logical event identifier.
        :param handler: Interpreted as a valid reference to a handler to be installed by a client application.
        :param user_handle: A value specified by an application that can be used for identifying handlers
                            uniquely for an event type.
        :returns: user handle (a ctypes object)

def install_visa_handler(self, session, event_type, handler, user_handle=None):
        """Installs handlers for event callbacks.

        :param session: Unique logical identifier to a session.
        :param event_type: Logical event identifier.
        :param handler: Interpreted as a valid reference to a handler to be installed by a client application.
        :param user_handle: A value specified by an application that can be used for identifying handlers
                            uniquely for an event type.
        :returns: user handle (a ctypes object)
        """
        try:
            new_handler = self.install_handler(session, event_type, handler, user_handle)
        except TypeError as e:
            raise errors.VisaTypeError(str(e))

        self.handlers[session].append(new_handler + (event_type,))
        return new_handler[1]