Queue¶

class asyncio.Queue(maxsize=0)¶

A first in, first out (FIFO) queue.

If maxsize is less than or equal to zero, the queue size is infinite. If it is an integer greater than 0, then await put() blocks when the queue reaches maxsize until an item is removed by get().

Unlike the standard library threading queue, the size of the queue is always known and can be returned by calling the qsize() method.

Changed in version 3.10: Removed the loop parameter.

This class is not thread safe.

maxsize¶

Number of items allowed in the queue.

empty()¶

Return True if the queue is empty, False otherwise.

full()¶

Return True if there are maxsize items in the queue.

If the queue was initialized with maxsize=0 (the default), then full() never returns True.

async get()¶

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

Raises QueueShutDown if the queue has been shut down and is empty, or if the queue has been shut down immediately.

get_nowait()¶

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

Raises QueueShutDown if the queue has been shut down and is empty.

async join()¶

Block until all items in the queue have been received and processed.

The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer coroutine 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.

async put(item)¶

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

Raises QueueShutDown if the queue has been shut down.

put_nowait(item)¶

Put an item into the queue without blocking.

If no free slot is immediately available, raise QueueFull.

Raises QueueShutDown if the queue has been shut down.

qsize()¶

Return the number of items in the queue.

shutdown(immediate=False)¶

Put a Queue instance into a shutdown mode.

The queue can no longer grow. Future calls to put() raise QueueShutDown. Currently blocked callers of put() will be unblocked and will raise QueueShutDown in the formerly awaiting task.

If immediate is false (the default), the queue can be wound down normally with get() calls to extract tasks that have already been loaded.

And if task_done() is called for each remaining task, a pending join() will be unblocked normally.

Once the queue is empty, future calls to get() will raise QueueShutDown.

If immediate is true, the queue is terminated immediately. The queue is drained to be completely empty and the count of unfinished tasks is reduced by the number of tasks drained. If unfinished tasks is zero, callers of join() are unblocked. Also, blocked callers of get() are unblocked and will raise QueueShutDown because the queue is empty.

Use caution when using join() with immediate set to true. This unblocks the join even when no work has been done on the tasks, violating the usual invariant for joining a queue.

Added in version 3.13.

task_done()¶

Indicate that a formerly enqueued work item is complete.

Used by queue consumers. For each get() used to fetch a work item, a subsequent call to task_done() tells the queue that the processing on the work item 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.

Priority Queue¶

class asyncio.PriorityQueue¶

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

Entries are typically tuples of the form (priority_number, data).

LIFO Queue¶

class asyncio.LifoQueue¶

A variant of Queue that retrieves most recently added entries first (last in, first out).

Exceptions¶

exception asyncio.QueueEmpty¶

This exception is raised when the get_nowait() method is called on an empty queue.

exception asyncio.QueueFull¶

Exception raised when the put_nowait() method is called on a queue that has reached its maxsize.

exception asyncio.QueueShutDown¶

Exception raised when put(), put_nowait(), get() or get_nowait() is called on a queue which has been shut down.

Added in version 3.13.

Examples¶

Queues can be used to distribute workload between several concurrent tasks:

import asyncio
import random
import time


async def worker(name, queue):
    while True:
        # Get a "work item" out of the queue.
        sleep_for = await queue.get()

        # Sleep for the "sleep_for" seconds.
        await asyncio.sleep(sleep_for)

        # Notify the queue that the "work item" has been processed.
        queue.task_done()

        print(f'{name} has slept for {sleep_for:.2f} seconds')


async def main():
    # Create a queue that we will use to store our "workload".
    queue = asyncio.Queue()

    # Generate random timings and put them into the queue.
    total_sleep_time = 0
    for _ in range(20):
        sleep_for = random.uniform(0.05, 1.0)
        total_sleep_time += sleep_for
        queue.put_nowait(sleep_for)

    # Create three worker tasks to process the queue concurrently.
    tasks = []
    for i in range(3):
        task = asyncio.create_task(worker(f'worker-{i}', queue))
        tasks.append(task)

    # Wait until the queue is fully processed.
    started_at = time.monotonic()
    await queue.join()
    total_slept_for = time.monotonic() - started_at

    # Cancel our worker tasks.
    for task in tasks:
        task.cancel()
    # Wait until all worker tasks are cancelled.
    await asyncio.gather(*tasks, return_exceptions=True)

    print('====')
    print(f'3 workers slept in parallel for {total_slept_for:.2f} seconds')
    print(f'total expected sleep time: {total_sleep_time:.2f} seconds')


asyncio.run(main())