def METHOD_NAME(self, parameters, value):
    """
    This gets called with the value of our ``--priv-launch-key``
    if it is specified.  It needs to determine if the path
    provided is valid and, if it is, it stores it in the instance
    variable ``_key_path`` for use by the decrypt routine.
    """
    if value:
        path = os.path.expandvars(value)
        path = os.path.expanduser(path)
        if os.path.isfile(path):
            self._key_path = path
            endpoint_prefix = \
                self._operation_model.service_model.endpoint_prefix
            event = 'after-call.%s.%s' % (endpoint_prefix,
                                          self._operation_model.name)
            self._session.register(event, self._decrypt_password_data)
        else:
            msg = ('priv-launch-key should be a path to the '
                   'local SSH private key file used to launch '
                   'the instance.')
            raise ValueError(msg)