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.

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.

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).

3. LifoQueue

class asyncio.LifoQueue

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

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.