mirror of https://github.com/python/cpython
179 lines
6.3 KiB
ReStructuredText
179 lines
6.3 KiB
ReStructuredText
:mod:`queue` --- A synchronized queue class
|
|
===========================================
|
|
|
|
.. module:: queue
|
|
:synopsis: A synchronized queue class.
|
|
|
|
|
|
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.
|
|
|
|
Implements three types of queue whose only difference is the order that
|
|
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:
|
|
|
|
.. class:: Queue(maxsize)
|
|
|
|
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.
|
|
|
|
|
|
.. class:: LifoQueue(maxsize)
|
|
|
|
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.
|
|
|
|
|
|
.. class:: PriorityQueue(maxsize)
|
|
|
|
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)``.
|
|
|
|
|
|
.. exception:: Empty
|
|
|
|
Exception raised when non-blocking :meth:`get` (or :meth:`get_nowait`) is called
|
|
on a :class:`Queue` object which is empty.
|
|
|
|
|
|
.. exception:: Full
|
|
|
|
Exception raised when non-blocking :meth:`put` (or :meth:`put_nowait`) is called
|
|
on a :class:`Queue` object which is full.
|
|
|
|
.. seealso::
|
|
|
|
:class:`collections.deque` is an alternative implementation of unbounded
|
|
queues with fast atomic :func:`append` and :func:`popleft` operations that
|
|
do not require locking.
|
|
|
|
|
|
.. _queueobjects:
|
|
|
|
Queue Objects
|
|
-------------
|
|
|
|
Queue objects (:class:`Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`)
|
|
provide the public methods described below.
|
|
|
|
|
|
.. method:: Queue.qsize()
|
|
|
|
Return the approximate size of the queue. Note, qsize() > 0 doesn't
|
|
guarantee that a subsequent get() will not block, nor will qsize() < maxsize
|
|
guarantee that put() will not block.
|
|
|
|
|
|
.. method:: Queue.empty()
|
|
|
|
Return ``True`` if the queue is empty, ``False`` otherwise. If empty()
|
|
returns ``True`` it doesn't guarantee that a subsequent call to put()
|
|
will not block. Similarly, if empty() returns ``False`` it doesn't
|
|
guarantee that a subsequent call to get() will not block.
|
|
|
|
|
|
.. method:: Queue.full()
|
|
|
|
Return ``True`` if the queue is full, ``False`` otherwise. If full()
|
|
returns ``True`` it doesn't guarantee that a subsequent call to get()
|
|
will not block. Similarly, if full() returns ``False`` it doesn't
|
|
guarantee that a subsequent call to put() will not block.
|
|
|
|
|
|
.. method:: Queue.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 :exc:`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 :exc:`Full` exception (*timeout* is
|
|
ignored in that case).
|
|
|
|
|
|
.. method:: Queue.put_nowait(item)
|
|
|
|
Equivalent to ``put(item, False)``.
|
|
|
|
|
|
.. method:: Queue.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 :exc:`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 :exc:`Empty` exception (*timeout* is ignored in that case).
|
|
|
|
|
|
.. method:: Queue.get_nowait()
|
|
|
|
Equivalent to ``get(False)``.
|
|
|
|
Two methods are offered to support tracking whether enqueued tasks have been
|
|
fully processed by daemon consumer threads.
|
|
|
|
|
|
.. method:: Queue.task_done()
|
|
|
|
Indicate that a formerly enqueued task is complete. Used by queue consumer
|
|
threads. For each :meth:`get` used to fetch a task, a subsequent call to
|
|
:meth:`task_done` tells the queue that the processing on the task is complete.
|
|
|
|
If a :meth:`join` is currently blocking, it will resume when all items have been
|
|
processed (meaning that a :meth:`task_done` call was received for every item
|
|
that had been :meth:`put` into the queue).
|
|
|
|
Raises a :exc:`ValueError` if called more times than there were items placed in
|
|
the queue.
|
|
|
|
|
|
.. method:: 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 calls :meth:`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, :meth:`join` unblocks.
|
|
|
|
|
|
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
|
|
|