Calling coroutines in asyncio.Protocol.data_received

user_884770 0
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I am having a problem doing asynchronous stuff in the asyncio.Protocol.data_received callback of the new Python asyncio module.

Consider the following server:

class MathServer(asyncio.Protocol):

   @asyncio.coroutine
   def slow_sqrt(self, x):
      yield from asyncio.sleep(1)
      return math.sqrt(x)

   def fast_sqrt(self, x):
      return math.sqrt(x)

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

   #@asyncio.coroutine
   def data_received(self, data):
      print('data received: {}'.format(data.decode()))
      x = json.loads(data.decode())
      #res = self.fast_sqrt(x)
      res = yield from self.slow_sqrt(x)
      self.transport.write(json.dumps(res).encode('utf8'))
      self.transport.close()

used with the following client:

class MathClient(asyncio.Protocol):

   def connection_made(self, transport):
      transport.write(json.dumps(2.).encode('utf8'))

   def data_received(self, data):
      print('data received: {}'.format(data.decode()))

   def connection_lost(self, exc):
      asyncio.get_event_loop().stop()

With self.fast_sqrt being called, everything works as expected.

With self.slow_sqrt, it does not work.

It also does not work with self.fast_sqrt and the @asyncio.coroutine decorator on data_received.

I feel I am missing something fundamental here.

Complete code is here:

Tested with:

  • Python 3.4.0b1 (Windows)
  • Python 3.3.3 + asyncio-0.2.1 (FreeBSD)

The issue is the same on both: with slow_sqrt, the client/server will just hang doing nothing.

python-asyncio yield coroutine network-programming python
2 Answers

I had a similar problem where I wanted to run a coroutine when my MyProtocol.connection_made was called. My solution was quite similar, except that my Protocol had access to the loop. For those using a more recent version of python, the following worked for me (I'm using python 3.6.8):

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

    async def do_async_thing(self):
        await asyncio.sleep(1)

    def connection_made(self, transport):
        self.transport = transport
        self.loop.create_task(self.do_async_thing())

    # Other member functions left out for brevity.

It makes sense that this works - the loop needs to schedule a task which needs to have independent context, i.e. can be run independent of any other call stack. This is why you give the loop a coroutine that it can run, do_async_thing() along with a class instance in this case, which it will call when it can. When it is called is has nothing to do with the connection_made member function anymore.

It's worth noting that this can also be achieved by using asyncio.ensure_future(coro, loop=None) instead of self.loop.create_task(coro), but the latter would presumably use the default loop. In fact, it does - I just checked the source code.

user_6525850
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