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160 lines
4.2 KiB
ReStructuredText
160 lines
4.2 KiB
ReStructuredText
.. currentmodule:: asyncio
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Queues
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======
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**Source code:** :source:`Lib/asyncio/queues.py`
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Queues:
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* :class:`Queue`
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* :class:`PriorityQueue`
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* :class:`LifoQueue`
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asyncio queue API was designed to be close to classes of the :mod:`queue`
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module (:class:`~queue.Queue`, :class:`~queue.PriorityQueue`,
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:class:`~queue.LifoQueue`), but it has no *timeout* parameter. The
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:func:`asyncio.wait_for` function can be used to cancel a task after a timeout.
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Queue
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-----
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.. class:: Queue(maxsize=0, \*, loop=None)
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A queue, useful for coordinating producer and consumer coroutines.
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If *maxsize* is less than or equal to zero, the queue size is infinite. If
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it is an integer greater than ``0``, then ``yield from put()`` will block
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when the queue reaches *maxsize*, until an item is removed by :meth:`get`.
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Unlike the standard library :mod:`queue`, you can reliably know this Queue's
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size with :meth:`qsize`, since your single-threaded asyncio application won't
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be interrupted between calling :meth:`qsize` and doing an operation on the
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Queue.
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This class is :ref:`not thread safe <asyncio-multithreading>`.
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.. versionchanged:: 3.4.4
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New :meth:`join` and :meth:`task_done` methods.
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.. method:: empty()
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Return ``True`` if the queue is empty, ``False`` otherwise.
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.. method:: full()
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Return ``True`` if there are :attr:`maxsize` items in the queue.
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.. note::
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If the Queue was initialized with ``maxsize=0`` (the default), then
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:meth:`full()` is never ``True``.
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.. coroutinemethod:: get()
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Remove and return an item from the queue. If queue is empty, wait until
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an item is available.
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This method is a :ref:`coroutine <coroutine>`.
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.. seealso::
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The :meth:`empty` method.
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.. method:: get_nowait()
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Remove and return an item from the queue.
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Return an item if one is immediately available, else raise
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:exc:`QueueEmpty`.
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.. coroutinemethod:: join()
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Block 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
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queue. The count goes down whenever a consumer thread calls
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:meth:`task_done` to indicate that the item was retrieved and all work on
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it is complete. When the count of unfinished tasks drops to zero,
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:meth:`join` unblocks.
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This method is a :ref:`coroutine <coroutine>`.
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.. versionadded:: 3.4.4
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.. coroutinemethod:: put(item)
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Put an item into the queue. If the queue is full, wait until a free slot
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is available before adding item.
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This method is a :ref:`coroutine <coroutine>`.
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.. seealso::
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The :meth:`full` method.
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.. method:: put_nowait(item)
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Put an item into the queue without blocking.
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If no free slot is immediately available, raise :exc:`QueueFull`.
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.. method:: qsize()
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Number of items in the queue.
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.. method:: task_done()
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Indicate that a formerly enqueued task is complete.
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Used by queue consumers. For each :meth:`~Queue.get` used to fetch a task, a
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subsequent call to :meth:`task_done` tells the queue that the processing
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on the task is complete.
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If a :meth:`join` is currently blocking, it will resume when all items
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have been processed (meaning that a :meth:`task_done` call was received
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for every item that had been :meth:`~Queue.put` into the queue).
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Raises :exc:`ValueError` if called more times than there were items
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placed in the queue.
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.. versionadded:: 3.4.4
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.. attribute:: maxsize
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Number of items allowed in the queue.
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PriorityQueue
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-------------
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.. class:: PriorityQueue
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A subclass of :class:`Queue`; retrieves entries in priority order (lowest
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first).
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Entries are typically tuples of the form: (priority number, data).
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LifoQueue
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---------
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.. class:: LifoQueue
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A subclass of :class:`Queue` that retrieves most recently added entries
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first.
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Exceptions
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^^^^^^^^^^
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.. exception:: QueueEmpty
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Exception raised when the :meth:`~Queue.get_nowait` method is called on a
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:class:`Queue` object which is empty.
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.. exception:: QueueFull
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Exception raised when the :meth:`~Queue.put_nowait` method is called on a
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:class:`Queue` object which is full.
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