code: def get_arg(context, index):
      
      if index < len(context.argstreams):
          arg = ""
          chunk = yield context.argstreams[index].read()
          while chunk:
              arg += chunk
              chunk = yield context.argstreams[index].read()
  
          raise tornado.gen.Return(arg)
      else:
          raise TChannelError()
docs: get value from arg stream in async way