Queue Objects¶

Queue objects (Queue,LifoQueue, orPriorityQueue)provide the public methods described below.

Queue.qsize()¶

Return the approximate size of the queue. Note, qsize() > 0 doesn’tguarantee that a subsequent get() will not block, nor will qsize() < maxsizeguarantee that put() will not block.

Queue.empty()¶

ReturnTrue if the queue is empty,False otherwise. If empty()returnsTrue it doesn’t guarantee that a subsequent call to put()will not block. Similarly, if empty() returnsFalse it doesn’tguarantee that a subsequent call to get() will not block.

Queue.full()¶

ReturnTrue if the queue is full,False otherwise. If full()returnsTrue it doesn’t guarantee that a subsequent call to get()will not block. Similarly, if full() returnsFalse it doesn’tguarantee that a subsequent call to put() will not block.

Queue.put(item,block=True,timeout=None)¶

Putitem into the queue. If optional argsblock is true andtimeout isNone (the default), block if necessary until a free slot is available. Iftimeout is a positive number, it blocks at mosttimeout seconds and raisestheFull exception if no free slot was available within that time.Otherwise (block is false), put an item on the queue if a free slot isimmediately available, else raise theFull exception (timeout isignored in that case).

RaisesShutDown if the queue has been shut down.

Queue.put_nowait(item)¶

Equivalent toput(item,block=False).

Queue.get(block=True,timeout=None)¶

Remove and return an item from the queue. If optional argsblock is true andtimeout isNone (the default), block if necessary until an item is available.Iftimeout is a positive number, it blocks at mosttimeout seconds andraises theEmpty exception if no item was available within that time.Otherwise (block is false), return an item if one is immediately available,else raise theEmpty exception (timeout is ignored in that case).

Prior to 3.0 on POSIX systems, and for all versions on Windows, ifblock is true andtimeout isNone, this operation goes intoan uninterruptible wait on an underlying lock. This means that no exceptionscan occur, and in particular a SIGINT will not trigger aKeyboardInterrupt.

RaisesShutDown if the queue has been shut down and is empty, or ifthe queue has been shut down immediately.

Queue.get_nowait()¶

Equivalent toget(False).

Two methods are offered to support tracking whether enqueued tasks have beenfully processed by daemon consumer threads.

Queue.task_done()¶

Indicate that a formerly enqueued task is complete. Used by queue consumerthreads. For eachget() used to fetch a task, a subsequent call totask_done() tells the queue that the processing on the task is complete.

If ajoin() is currently blocking, it will resume when all items have beenprocessed (meaning that atask_done() call was received for every itemthat had beenput() into the queue).

shutdown(immediate=True) callstask_done() for each remaining itemin the queue.

Raises aValueError if called more times than there were items placed inthe queue.

Queue.join()¶

Blocks until all items in the queue have been gotten and processed.

The count of unfinished tasks goes up whenever an item is added to the queue.The count goes down whenever a consumer thread callstask_done() toindicate that the item was retrieved and all work on it is complete. When thecount of unfinished tasks drops to zero,join() unblocks.

Example of how to wait for enqueued tasks to be completed:

importthreadingimportqueueq=queue.Queue()defworker():whileTrue:item=q.get()print(f'Working on{item}')print(f'Finished{item}')q.task_done()# Turn-on the worker thread.threading.Thread(target=worker,daemon=True).start()# Send thirty task requests to the worker.foriteminrange(30):q.put(item)# Block until all tasks are done.q.join()print('All work completed')

SimpleQueue Objects¶

SimpleQueue objects provide the public methods described below.

SimpleQueue.qsize()¶

Return the approximate size of the queue. Note, qsize() > 0 doesn’tguarantee that a subsequent get() will not block.

SimpleQueue.empty()¶

ReturnTrue if the queue is empty,False otherwise. If empty()returnsFalse it doesn’t guarantee that a subsequent call to get()will not block.

SimpleQueue.put(item,block=True,timeout=None)¶

Putitem into the queue. The method never blocks and always succeeds(except for potential low-level errors such as failure to allocate memory).The optional argsblock andtimeout are ignored and only providedfor compatibility withQueue.put().

CPython implementation detail: This method has a C implementation which is reentrant. That is, aput() orget() call can be interrupted by anotherput()call in the same thread without deadlocking or corrupting internalstate inside the queue. This makes it appropriate for use indestructors such as__del__ methods orweakref callbacks.

SimpleQueue.put_nowait(item)¶

Equivalent toput(item,block=False), provided for compatibility withQueue.put_nowait().

SimpleQueue.get(block=True,timeout=None)¶

Remove and return an item from the queue. If optional argsblock is true andtimeout isNone (the default), block if necessary until an item is available.Iftimeout is a positive number, it blocks at mosttimeout seconds andraises theEmpty exception if no item was available within that time.Otherwise (block is false), return an item if one is immediately available,else raise theEmpty exception (timeout is ignored in that case).

SimpleQueue.get_nowait()¶

Equivalent toget(False).