Queue objects

The queue type provided by processing is a multi-producer, multi-consumer FIFO queue modelled on the Queue.Queue class in the standard library.

Queue(maxsize=0)

Returns a process shared queue implemented using a pipe and a few locks/semaphores. When a process first puts an item on the queue a feeder thread is started which transfers objects from a buffer into the pipe.

Queue.Queue implements all the methods of Queue.Queue except for qsize(), task_done() and join().

empty()

Return True if the queue is empty, False otherwise. Because of multithreading/multiprocessing semantics, this is not reliable.

full()

Return True if the queue is full, False otherwise. Because of multithreading/multiprocessing semantics, this is not reliable.

put(item, block=True, timeout=None)

Put item into the queue. If optional args block is true and timeout is None (the default), block if necessary until a free slot is available. If timeout is a positive number, it blocks at most timeout seconds and raises the Full exception if no free slot was available within that time. Otherwise (block is false), put an item on the queue if a free slot is immediately available, else raise the Full exception (timeout is ignored in that case).

put_nowait(item), putNoWait(item)

Equivalent to put(item, False).

get(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 positive 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).

get_nowait(), getNoWait()

Equivalent to get(False).

processing.Queue has a few additional methods not found in Queue.Queue which are usually unnecessary:

putMany(iterable)

If the queue has infinite size then this adds all items in the iterable to the queue's buffer. So q.putMany(X) is a faster alternative to for x in X: q.put(x). Raises an error if the queue has finite size.

close()

Indicates that no more data will be put on this queue by the current process. The background thread will quit once it has flushed all buffered data to the pipe. This is called automatically when the queue is garbage collected.

joinThread()

This joins the background thread and can only be used after close() has been called. This blocks until the background thread exits, ensuring that all data in the buffer has been flushed to the pipe.

By default if a process is not the creator of the queue then on exit it will attempt to join the queue's background thread. The process can call cancelJoin() to prevent this behaviour.

cancelJoin()

Prevents the background thread from being joined automatically when the process exits. Unnecessary if the current process created the queue.