为什么Python的queue.get()函数要检查获取not_empty锁的with块内队列是否不为空？

Question

我试图更好地理解队列库，所以这里的代码让我摸不着头脑：

def get(self, block=True, timeout=None):
        '''Remove and return an item from the queue.
        If optional args 'block' is true and 'timeout' is None (the default),
        block if necessary until an item is available. If 'timeout' is
        a non-negative number, it blocks at most 'timeout' seconds and raises
        the Empty exception if no item was available within that time.
        Otherwise ('block' is false), return an item if one is immediately
        available, else raise the Empty exception ('timeout' is ignored
        in that case).
        '''
        with self.not_empty: # <-------- notice this lock guarantees the queue is not empty
            if not block:
                if not self._qsize():
                    raise Empty
            elif timeout is None:
                while not self._qsize(): # <---------- so then why is this necessary?
                    self.not_empty.wait()
            elif timeout < 0:
                raise ValueError("'timeout' must be a non-negative number")
            else:
                endtime = time() + timeout
                while not self._qsize():
                    remaining = endtime - time()
                    if remaining <= 0.0:
                        raise Empty
                    self.not_empty.wait(remaining)
            item = self._get()
            self.not_full.notify()
            return item

根据上面的注释，我想知道为什么有必要检查队列是否不为空，如果我们已经获取了保证队列不为空的锁？我对“with”语句的理解是，它在锁 self.not_empty 上调用 .acquire() ，并且 .acquire() 会阻塞，直到锁可用。那么超时不应该在那里处理吗？