python - queue.Queue 上的多路复用？

Question

如何在多个 queue.Queue 上进行“选择”同时？

Golang 有 desired feature及其 channel :

select {
case i1 = <-c1:
    print("received ", i1, " from c1\n")
case c2 <- i2:
    print("sent ", i2, " to c2\n")
case i3, ok := (<-c3):  // same as: i3, ok := <-c3
    if ok {
        print("received ", i3, " from c3\n")
    } else {
        print("c3 is closed\n")
    }
default:
    print("no communication\n")
}

其中第一个解除阻塞的 channel 执行相应的阻塞。我如何在 Python 中实现这一点？

更新0

根据 the link在 tux21b's answer 中给出，所需的队列类型具有以下属性:

• 多生产者/多消费者队列 (MPMC)
• 提供每个生产者的 FIFO/LIFO
• 当队列为空/满时，消费者/生产者会被阻塞

此外， channel 可能会被阻塞，生产者会一直阻塞，直到消费者检索到该项目。我不确定 Python 的 Queue 可以做到这一点。

Answer 1

如果您使用 queue.PriorityQueue，您可以使用 channel 对象作为优先级获得类似的行为:

import threading, logging
import random, string, time
from queue import PriorityQueue, Empty
from contextlib import contextmanager

logging.basicConfig(level=logging.NOTSET,
                    format="%(threadName)s - %(message)s")

class ChannelManager(object):
    next_priority = 0

    def __init__(self):
        self.queue = PriorityQueue()
        self.channels = []

    def put(self, channel, item, *args, **kwargs):
        self.queue.put((channel, item), *args, **kwargs)

    def get(self, *args, **kwargs):
        return self.queue.get(*args, **kwargs)

    @contextmanager
    def select(self, ordering=None, default=False):
        if default:
            try:
                channel, item = self.get(block=False)
            except Empty:
                channel = 'default'
                item = None
        else:
            channel, item = self.get()
        yield channel, item


    def new_channel(self, name):
        channel = Channel(name, self.next_priority, self)
        self.channels.append(channel)
        self.next_priority += 1
        return channel


class Channel(object):
    def __init__(self, name, priority, manager):
        self.name = name
        self.priority = priority
        self.manager = manager

    def __str__(self):
        return self.name

    def __lt__(self, other):
        return self.priority < other.priority

    def put(self, item):
        self.manager.put(self, item)


if __name__ == '__main__':
    num_channels = 3
    num_producers = 4
    num_items_per_producer = 2
    num_consumers = 3
    num_items_per_consumer = 3

    manager = ChannelManager()
    channels = [manager.new_channel('Channel#{0}'.format(i))
                for i in range(num_channels)]

    def producer_target():
        for i in range(num_items_per_producer):
            time.sleep(random.random())
            channel = random.choice(channels)
            message = random.choice(string.ascii_letters)
            logging.info('Putting {0} in {1}'.format(message, channel))
            channel.put(message)

    producers = [threading.Thread(target=producer_target,
                                  name='Producer#{0}'.format(i))
                 for i in range(num_producers)]
    for producer in producers:
        producer.start()
    for producer in producers:
        producer.join()
    logging.info('Producers finished')

    def consumer_target():
        for i in range(num_items_per_consumer):
            time.sleep(random.random())
            with manager.select(default=True) as (channel, item):
                if channel:
                    logging.info('Received {0} from {1}'.format(item, channel))
                else:
                    logging.info('No data received')

    consumers = [threading.Thread(target=consumer_target,
                                  name='Consumer#{0}'.format(i))
                 for i in range(num_consumers)]
    for consumer in consumers:
        consumer.start()
    for consumer in consumers:
        consumer.join()
    logging.info('Consumers finished')

示例输出:

Producer#0 - Putting x in Channel#2
Producer#2 - Putting l in Channel#0
Producer#2 - Putting A in Channel#2
Producer#3 - Putting c in Channel#0
Producer#3 - Putting z in Channel#1
Producer#1 - Putting I in Channel#1
Producer#1 - Putting L in Channel#1
Producer#0 - Putting g in Channel#1
MainThread - Producers finished
Consumer#1 - Received c from Channel#0
Consumer#2 - Received l from Channel#0
Consumer#0 - Received I from Channel#1
Consumer#0 - Received L from Channel#1
Consumer#2 - Received g from Channel#1
Consumer#1 - Received z from Channel#1
Consumer#0 - Received A from Channel#2
Consumer#1 - Received x from Channel#2
Consumer#2 - Received None from default
MainThread - Consumers finished

在本例中，ChannelManager 只是 queue.PriorityQueue 的一个包装器，它将 select 方法实现为 contextmanager 使其看起来类似于 Go 中的 select 语句。

需要注意的几点:

• 排序

  • 在 Go 示例中， channel 在 select 语句中的写入顺序决定了如果有多个 channel 可用的数据，将执行哪个 channel 的代码。

  • 在 python 示例中，顺序由分配给每个 channel 的优先级决定。但是，优先级可以动态分配给每个 channel (如示例中所示)，因此可以使用更复杂的 select 方法更改顺序，该方法负责根据参数分配新的优先级到方法。此外，一旦上下文管理器完成，旧的排序可能会重新建立。

• 屏蔽

  • 在 Go 示例中，如果存在 default 情况，则 select 语句将阻塞。

  • 在 python 示例中，必须将一个 bool 参数传递给 select 方法，以明确何时需要阻塞/非阻塞。在非阻塞情况下，上下文管理器返回的 channel 只是字符串 'default' 因此在里面的代码中很容易在 with 里面的代码中检测到这一点> 声明。

• 线程:queue 模块中的对象已经为多生产者、多消费者场景做好了准备，如示例中所示。