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179 lines
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ReStructuredText
179 lines
6.3 KiB
ReStructuredText
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:mod:`queue` --- A synchronized queue class
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===========================================
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.. module:: queue
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:synopsis: A synchronized queue class.
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The :mod:`queue` module implements multi-producer, multi-consumer queues.
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It is especially useful in threaded programming when information must be
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exchanged safely between multiple threads. The :class:`Queue` class in this
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module implements all the required locking semantics. It depends on the
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availability of thread support in Python; see the :mod:`threading`
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module.
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Implements three types of queue whose only difference is the order that
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the entries are retrieved. In a FIFO queue, the first tasks added are
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the first retrieved. In a LIFO queue, the most recently added entry is
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the first retrieved (operating like a stack). With a priority queue,
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the entries are kept sorted (using the :mod:`heapq` module) and the
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lowest valued entry is retrieved first.
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The :mod:`queue` module defines the following classes and exceptions:
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.. class:: Queue(maxsize)
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Constructor for a FIFO queue. *maxsize* is an integer that sets the upperbound
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limit on the number of items that can be placed in the queue. Insertion will
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block once this size has been reached, until queue items are consumed. If
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*maxsize* is less than or equal to zero, the queue size is infinite.
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.. class:: LifoQueue(maxsize)
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Constructor for a LIFO queue. *maxsize* is an integer that sets the upperbound
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limit on the number of items that can be placed in the queue. Insertion will
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block once this size has been reached, until queue items are consumed. If
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*maxsize* is less than or equal to zero, the queue size is infinite.
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.. class:: PriorityQueue(maxsize)
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Constructor for a priority queue. *maxsize* is an integer that sets the upperbound
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limit on the number of items that can be placed in the queue. Insertion will
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block once this size has been reached, until queue items are consumed. If
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*maxsize* is less than or equal to zero, the queue size is infinite.
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The lowest valued entries are retrieved first (the lowest valued entry is the
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one returned by ``sorted(list(entries))[0]``). A typical pattern for entries
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is a tuple in the form: ``(priority_number, data)``.
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.. exception:: Empty
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Exception raised when non-blocking :meth:`get` (or :meth:`get_nowait`) is called
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on a :class:`Queue` object which is empty.
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.. exception:: Full
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Exception raised when non-blocking :meth:`put` (or :meth:`put_nowait`) is called
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on a :class:`Queue` object which is full.
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.. seealso::
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:class:`collections.deque` is an alternative implementation of unbounded
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queues with fast atomic :func:`append` and :func:`popleft` operations that
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do not require locking.
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.. _queueobjects:
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Queue Objects
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-------------
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Queue objects (:class:`Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`)
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provide the public methods described below.
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.. method:: Queue.qsize()
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Return the approximate size of the queue. Note, qsize() > 0 doesn't
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guarantee that a subsequent get() will not block, nor will qsize() < maxsize
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guarantee that put() will not block.
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.. method:: Queue.empty()
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Return ``True`` if the queue is empty, ``False`` otherwise. If empty()
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returns ``True`` it doesn't guarantee that a subsequent call to put()
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will not block. Similarly, if empty() returns ``False`` it doesn't
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guarantee that a subsequent call to get() will not block.
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.. method:: Queue.full()
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Return ``True`` if the queue is full, ``False`` otherwise. If full()
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returns ``True`` it doesn't guarantee that a subsequent call to get()
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will not block. Similarly, if full() returns ``False`` it doesn't
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guarantee that a subsequent call to put() will not block.
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.. method:: Queue.put(item[, block[, timeout]])
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Put *item* into the queue. If optional args *block* is true and *timeout* is
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None (the default), block if necessary until a free slot is available. If
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*timeout* is a positive number, it blocks at most *timeout* seconds and raises
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the :exc:`Full` exception if no free slot was available within that time.
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Otherwise (*block* is false), put an item on the queue if a free slot is
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immediately available, else raise the :exc:`Full` exception (*timeout* is
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ignored in that case).
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.. method:: Queue.put_nowait(item)
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Equivalent to ``put(item, False)``.
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.. method:: Queue.get([block[, timeout]])
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Remove and return an item from the queue. If optional args *block* is true and
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*timeout* is None (the default), block if necessary until an item is available.
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If *timeout* is a positive number, it blocks at most *timeout* seconds and
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raises the :exc:`Empty` exception if no item was available within that time.
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Otherwise (*block* is false), return an item if one is immediately available,
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else raise the :exc:`Empty` exception (*timeout* is ignored in that case).
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.. method:: Queue.get_nowait()
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Equivalent to ``get(False)``.
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Two methods are offered to support tracking whether enqueued tasks have been
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fully processed by daemon consumer threads.
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.. method:: Queue.task_done()
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Indicate that a formerly enqueued task is complete. Used by queue consumer
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threads. For each :meth:`get` used to fetch a task, a subsequent call to
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:meth:`task_done` tells the queue that the processing on the task is complete.
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If a :meth:`join` is currently blocking, it will resume when all items have been
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processed (meaning that a :meth:`task_done` call was received for every item
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that had been :meth:`put` into the queue).
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Raises a :exc:`ValueError` if called more times than there were items placed in
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the queue.
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.. method:: Queue.join()
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Blocks until all items in the queue have been gotten and processed.
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The count of unfinished tasks goes up whenever an item is added to the queue.
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The count goes down whenever a consumer thread calls :meth:`task_done` to
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indicate that the item was retrieved and all work on it is complete. When the
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count of unfinished tasks drops to zero, :meth:`join` unblocks.
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Example of how to wait for enqueued tasks to be completed::
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def worker():
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while True:
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item = q.get()
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do_work(item)
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q.task_done()
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q = Queue()
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for i in range(num_worker_threads):
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t = Thread(target=worker)
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t.daemon = True
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t.start()
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for item in source():
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q.put(item)
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q.join() # block until all tasks are done
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