18.5.8. Queues — Python documentation
18.5.8. Queues
Source code: :source:`Lib/asyncio/queues.py`
Queues:
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
, thenyield from put()
will block when the queue reaches maxsize, until an item is removed byget()
.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 neverTrue
.
- get_nowait()
Remove and return an item from the queue.
Return an item if one is immediately available, else raise QueueEmpty.
- 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 beenput()
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.