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cpython_sandbox / Doc / library / queue.rst

:mod:`Queue` --- A synchronized queue class

Note

The :mod:`Queue` module has been renamed to :mod:`queue` in Python 3. The :term:`2to3` tool will automatically adapt imports when converting your sources to Python 3.

Source code: :source:`Lib/Queue.py`


The :mod:`Queue` module implements multi-producer, multi-consumer queues. It is especially useful in threaded programming when information must be exchanged safely between multiple threads. The :class:`Queue` class in this module implements all the required locking semantics. It depends on the availability of thread support in Python; see the :mod:`threading` module.

The module implements three types of queue, which differ only in the order in which the entries are retrieved. In a FIFO queue, the first tasks added are the first retrieved. In a LIFO queue, the most recently added entry is the first retrieved (operating like a stack). With a priority queue, the entries are kept sorted (using the :mod:`heapq` module) and the lowest valued entry is retrieved first.

The :mod:`Queue` module defines the following classes and exceptions:

Constructor for a FIFO queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.

Constructor for a LIFO queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.

Constructor for a priority queue. maxsize is an integer that sets the upperbound limit on the number of items that can be placed in the queue. Insertion will block once this size has been reached, until queue items are consumed. If maxsize is less than or equal to zero, the queue size is infinite.

The lowest valued entries are retrieved first (the lowest valued entry is the one returned by sorted(list(entries))[0]). A typical pattern for entries is a tuple in the form: (priority_number, data).

Queue Objects

Queue objects (:class:`Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`) provide the public methods described below.

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

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.daemon = True
     t.start()

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

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