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5.10.1 Queue Objects


5.10.1 Queue Objects

Class Queue implements queue objects and has the methods described below. This class can be derived from in order to implement other queue organizations (e.g. stack) but the inheritable interface is not described here. See the source code for details. The public methods are:

qsize( )
Return the approximate size of the queue. Because of multithreading semantics, this number is not reliable.

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

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

put( item[, block[, timeout]])
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).

New in version 2.3: the timeout parameter.

put_nowait( item)
Equivalent to put(item, False).

get( [block[, timeout]])
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).

New in version 2.3: the timeout parameter.

get_nowait( )
Equivalent to get(False).

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

task_done( )
Indicate that a formerly enqueued task is complete. Used by queue consumer threads. For each get() used to fetch a task, a subsequent call to task_done() tells the queue that the processing on the task is complete.

If a join() is currently blocking, it will resume when all items have been processed (meaning that a task_done() call was received for every item that had been put() into the queue).

Raises a ValueError if called more times than there were items placed in the queue. New in version 2.5.

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 calls task_done() to indicate that the item was retrieved and all work on it is complete. When the count of unfinished tasks drops to zero, join() unblocks. New in version 2.5.

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

    def worker(): 
        while True: 
            item = q.get() 
            do_work(item) 
            q.task_done() 

    q = Queue() 
    for i in range(num_worker_threads): 
         t = Thread(target=worker)
         t.setDaemon(True)
         t.start() 

    for item in source():
        q.put(item) 

    q.join()       # block until all tasks are done
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