如何从队列中获取元素块？

Question

我有一个 queue，我需要从中获取 10 个条目的块并将它们放入列表中，然后进一步处理。下面的代码有效（在示例中，"processed further" 只是打印列表）。

import multiprocessing

# this is an example of the actual queue
q = multiprocessing.Queue()
for i in range(22):
    q.put(i)
q.put("END")

counter = 0
mylist = list()
while True:
    v = q.get()
    if v == "END":
        # outputs the incomplete (< 10 elements) list
        print(mylist)
        break
    else:
        mylist.append(v)
        counter += 1
        if counter % 10 == 0:
            print(mylist)
            # empty the list
            mylist = list()

# this outputs
# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
# [20, 21]

这段代码很丑。我不知道如何改进它 - 我前一段时间读过 how to use iter with a sentinel 但没看到我的问题如何利用它。

有没有更好的（=更多elegant/pythonic）解决问题的方法？

Answer 1

您可以使用 iter 两次：iter(q.get, 'END') returns 一个迭代器，它可以迭代队列中的值，直到 'END' 被 q.get() 返回.

那么你可以使用the grouper recipe

iter(lambda: list(IT.islice(iterator, 10)), [])

将迭代器分组为 10 个项目的块。

import itertools as IT
import multiprocessing as mp

q = mp.Queue()
for i in range(22):
    q.put(i)
q.put("END")

iterator = iter(q.get, 'END')
for chunk in iter(lambda: list(IT.islice(iterator, 10)), []):
    print(chunk)

产量

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[20, 21]

Answer 2

在 Python 3.8 中，您可以使用海象运算符。

import threading
from itertools import islice
from queue import Queue

def producer(queue, _end):
    for i in range(10, 100):
        queue.put(i)
    else:
        queue.put(_end)

def consumer(queue, _end):
    iterator = iter(queue.get, _end)
    # Consumes the queue in batches
    while batch := list(islice(iterator, 10)):
        print(batch)

def main():

    queue = Queue()
    _end = object()

    t1 = threading.Thread(target=consumer, args=(queue, _end))
    t2 = threading.Thread(target=producer, args=(queue, _end))

    t1.start()
    t2.start()

    t2.join()
    t1.join()

if __name__ == '__main__':
    main()

In [2]: main()                                                                  
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[20, 21, 22, 23, 24, 25, 26, 27, 28, 29]
[30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
[40, 41, 42, 43, 44, 45, 46, 47, 48, 49]
[50, 51, 52, 53, 54, 55, 56, 57, 58, 59]
[60, 61, 62, 63, 64, 65, 66, 67, 68, 69]
[70, 71, 72, 73, 74, 75, 76, 77, 78, 79]
[80, 81, 82, 83, 84, 85, 86, 87, 88, 89]
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

如何从队列中获取元素块？

How to get chunks of elements from a queue?

python

sentinel