Queues
******

asyncio queues are designed to be similar to classes of the "queue"
module.  Although asyncio queues are not thread-safe, they are
designed to be used specifically in async/await code.

Note that methods of asyncio queues don't have a *timeout* parameter;
use "asyncio.wait_for()" function to do queue operations with a
timeout.

See also the Examples section below.


Queue
=====

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

   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.

   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".

   coroutine get()

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

   get_nowait()

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

   coroutine join()

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

      当条目添加到队列的时候，未完成任务的计数就会增加。每当消费协程调
      用 "task_done()" 表示这个条目已经被回收，该条目所有工作已经完成
      ，未完成计数就会减少。当未完成计数降到零的时候， "join()" 阻塞被
      解除。

   coroutine put(item)

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

   put_nowait(item)

      Put an item into the queue without blocking.

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

   qsize()

      Return the 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.


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*.


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())
