'Set as ready with value; the value may be retrived via the `value`
property getter
:raises AssertionError: if result was already set'

def set_value_once(self, *args, **kwargs):

    self.signal_once()
    try:
        self._values = (self._value_class(*args, **kwargs),)
    except Exception:
        LOGGER.error('set_value_once    failed:    value_class=%r;    args=%r;    kwargs=%r', self._value_class, args, kwargs)
        raise
