18.5.8. Queues
**************

Queues:

* "Queue"

* "PriorityQueue"

* "LifoQueue"

asyncio queue API was designed to be close to classes of the "queue"
module ("Queue", "PriorityQueue", "LifoQueue"), but it has no
*timeout* parameter. The "asyncio.wait_for()" function can be used to
cancel a task after a timeout.


18.5.8.1. Queue
===============

class asyncio.Queue(maxsize=0, *, loop=None)

   A queue, useful for coordinating producer and consumer coroutines.

   If *maxsize* is less than or equal to zero, the queue size is
   infinite. If it is an integer greater than "0", then "yield from
   put()" will block when the queue reaches *maxsize*, until an item
   is removed by "get()".

   Unlike the standard library "queue", you can reliably know this
   Queue’s size with "qsize()", since your single-threaded asyncio
   application won’t be interrupted between calling "qsize()" and
   doing an operation on the Queue.

   This class is not thread safe.

   Changed in version 3.4.4: New "join()" and "task_done()" methods.

   empty()

      Return "True" if the queue is empty, "False" otherwise.

   full()

      Return "True" if there are "maxsize" items in the queue.

      Note: If the Queue was initialized with "maxsize=0" (the
        default), then "full()" is never "True".

   coroutine get()

      Remove and return an item from the queue. If queue is empty,
      wait until an item is available.

      This method is a coroutine.

      See also: The "empty()" method.

   get_nowait()

      Remove and return an item from the queue.

      Return an item if one is immediately available, else raise
      "QueueEmpty".

   coroutine join()

      Block 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.

      This method is a coroutine.

      New in version 3.4.4.

   coroutine put(item)

      Put an item into the queue. If the queue is full, wait until a
      free slot is available before adding item.

      This method is a coroutine.

      See also: The "full()" method.

   put_nowait(item)

      Put an item into the queue without blocking.

      If no free slot is immediately available, raise "QueueFull".

   qsize()

      Number of items in the queue.

   task_done()

      Indicate that a formerly enqueued task is complete.

      Used by queue consumers. 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 "ValueError" if called more times than there were items
      placed in the queue.

      New in version 3.4.4.

   maxsize

      Number of items allowed in the queue.


18.5.8.2. PriorityQueue
=======================

class asyncio.PriorityQueue

   A subclass of "Queue"; retrieves entries in priority order (lowest
   first).

   Entries are typically tuples of the form: (priority number, data).


18.5.8.3. LifoQueue
===================

class asyncio.LifoQueue

   A subclass of "Queue" that retrieves most recently added entries
   first.


18.5.8.3.1. Exceptions
----------------------

exception asyncio.QueueEmpty

   Exception raised when the "get_nowait()" method is called on a
   "Queue" object which is empty.

exception asyncio.QueueFull

   Exception raised when the "put_nowait()" method is called on a
   "Queue" object which is full.
