Program hang with asyncio

Question

Here is my code:

import asyncio, socket sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.bind(('', 1234)) sock.setblocking(False) queue = asyncio.Queue() def sock_reader(): print(sock.recv(1024)) # x = yield from queue def test_sock_reader(): sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.sendto(b'HELLO', ('127.0.0.1', 1234)) loop = asyncio.get_event_loop() loop.add_reader(sock, sock_reader) loop.call_later(0.5, test_sock_reader) loop.run_forever() loop.close()

This is the output:

b'HELLO'

When the line # x = yield from queue is uncommented the program is not printing b'Hello' anymore.

Why is the yield from affecting a command that should already be executed?

bagrat · Accepted Answer · 2016-09-13 12:02:35Z

The problem is a combination of syntax and API definition.

First of, refer to the documentation of add_reader, which states that it expects a callback. It is not obvious from the word itself, but by saying callback it means a regular function.

Now, when you uncomment the # x = yield from queue line, your sock_reader function actually becomes a generator/coroutine due to yield from, in which case when called like a regular function (i.e. sock_reader(...)), it returns a generator object, and does not get executed.

i want sock_reader to wait until the queue has an item. how to do it?
@DombiSzabolcs Have a look at the UDP examples from the asyncio documentation, and the loop.create_datagram_endpoint method

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