def _get (self, timeout):
        """Non thread-safe utility function of self.get() doing the real
        work."""
        if timeout is None:
            while self._empty():
                self.not_empty.wait()
        else:
            if timeout < 0:
                raise ValueError("'timeout' must be a positive number")
            endtime = _time() + timeout
            while self._empty():
                remaining = endtime - _time()
                if remaining <= 0.0:
                    raise Empty()
                self.not_empty.wait(remaining)
        self.in_progress += 1
        return self.queue.popleft()
Non thread-safe utility function of self.get() doing the real
        work.