asyncio queue consumer coroutine

Question

I have a asyncio.Protocol subclass receiving data from a server. I am storing this data (each line, because the data is text) in a asyncio.Queue.

import asyncio

q = asyncio.Queue()

class StreamProtocol(asyncio.Protocol):
    def __init__(self, loop):
        self.loop = loop
        self.transport = None

    def connection_made(self, transport):
        self.transport = transport

    def data_received(self, data):
        for message in data.decode().splitlines():
            yield q.put(message.rstrip())

    def connection_lost(self, exc):
        self.loop.stop()

loop = asyncio.get_event_loop()
coro = loop.create_connection(lambda: StreamProtocol(loop),
                              '127.0.0.1', '42')
loop.run_until_complete(coro)
loop.run_forever()
loop.close()

I want to have another coroutine responsible for consuming the data in the queue and processing it.

• Should this be a asyncio.Task?
• What if the queue becomes empty because for a few seconds no data is received? How can I make sure my consumer doesn't stop (run_until_complete)?
• Is there a cleaner way than using a global variable for my queue?

Answer 1

Should this be a asyncio.Task?

Yes, create it using asyncio.ensure_future or loop.create_task.

What if the queue becomes empty because for a few seconds no data is received?

Simply use queue.get to wait until an item is available:

async def consume(queue):
    while True:
        item = await queue.get()
        print(item)

Is there a cleaner way than using a global variable for my queue?

Yes, simply pass it as argument to the consumer coroutine and stream protocol:

class StreamProtocol(asyncio.Protocol):
    def __init__(self, loop, queue):
        self.loop = loop
        self.queue = queue

    def data_received(self, data):
        for message in data.decode().splitlines():
            self.queue.put_nowait(message.rstrip())

    def connection_lost(self, exc):
        self.loop.stop()

How can I make sure my consumer doesn't stop (run_until_complete)?

Once the connection is closed, use queue.join to wait until the queue is empty.

---

Full example:

loop = asyncio.get_event_loop()
queue = asyncio.Queue()
# Connection coroutine
factory = lambda: StreamProtocol(loop, queue)
connection = loop.create_connection(factory, '127.0.0.1', '42')
# Consumer task
consumer = asyncio.ensure_future(consume(queue))
# Set up connection
loop.run_until_complete(connection)
# Wait until the connection is closed
loop.run_forever()
# Wait until the queue is empty
loop.run_until_complete(queue.join())
# Cancel the consumer
consumer.cancel()
# Let the consumer terminate
loop.run_until_complete(consumer)
# Close the loop
loop.close()

Alternatively, you can also use streams:

async def tcp_client(host, port, loop=None):
    reader, writer = await asyncio.open_connection(host, port, loop=loop)
    async for line in reader:
        print(line.rstrip())
    writer.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(tcp_client('127.0.0.1', 42, loop))
loop.close()