python - Clear all items from the queue

64
votes

How can I clear a queue. For example I have datas in a queue, but for some reason I don't need the existing data, and just want to clear the queue.

Is there any way? Will this work:

oldQueue = Queue.Queue()
3
pythonqueue
If you read the documentation, it refers you to the Queue module's latest source, and there you could see the that the internal representation of a FIFO queue is a deque. In the documentation for deque you'd notice it has a clear() method, which is what you want. You'd probably also want to wrap that call with a q.mutex.acquire() and q.mutex.release() as the rest of the code does around such operations to make it thread safe. - martineau

3 Answers

111
votes
q = Queue.Queue()
q.queue.clear()

EDIT I omitted the issue of thread safety for clarity and brevity, but @Dan D is quite correct, the following is better.

q = Queue.Queue()
with q.mutex:
    q.queue.clear()
38
votes

You just can not clear the queue, because every put also add the unfinished_tasks member. The join method depends on this value. And all_tasks_done needs to be notified also.

q.mutex.acquire()
q.queue.clear()
q.all_tasks_done.notify_all()
q.unfinished_tasks = 0
q.mutex.release()

or in decent way, use get and task_done pair to safely clear the tasks.

while not q.empty():
    try:
        q.get(False)
    except Empty:
        continue
    q.task_done()

or just create a new Queue and delete old one.

6
votes

This seems to do it pretty well for me. I welcome comments/additions in case I missed anything important.

class Queue(queue.Queue):
  '''
  A custom queue subclass that provides a :meth:`clear` method.
  '''

  def clear(self):
    '''
    Clears all items from the queue.
    '''

    with self.mutex:
      unfinished = self.unfinished_tasks - len(self.queue)
      if unfinished <= 0:
        if unfinished < 0:
          raise ValueError('task_done() called too many times')
        self.all_tasks_done.notify_all()
      self.unfinished_tasks = unfinished
      self.queue.clear()
      self.not_full.notify_all()