Non-blocking I/O with asyncio

Question

I'm trying to write a networked game with Pygame and asyncio, but I can't work out how to avoid hanging on reads. Here is my code for the client:

@asyncio.coroutine def handle_client():     print("Connected!")     reader, writer = yield from asyncio.open_connection('localhost', 8000)     while True:         mouse_up = False         for event in pygame.event.get():             if event.type == pygame.QUIT:                 pygame.quit()                 sys.exit()                             elif event.type == pygame.MOUSEBUTTONUP:                 mouse_up = True          if mouse_up:             print("Writing")             writer.write(b"Mouse up")         print("Waiting to read")         line = yield from reader.read(2**12)         print(line.decode())      writer.close() 

This hangs on the line line = yield from reader.read(2**12). I previously thought that the point of asyncio was that it was non-blocking, and so if there wasn't any data to read it would just continue executing. I see now that this isn't the case.

How do I integrate the asyncio networking code with the Pygame drawing and event code?

Answer 1

The point of yield from is to switch the execution to the asyncio's event loop and to block the current coroutine until the result is available. To schedule a task without blocking the current coroutine, you could use asyncio.async().

To print read-so-far data without blocking the pygame loop:

@asyncio.coroutine def read(reader, callback):     while True:         data = yield from reader.read(2**12)         if not data: # EOF             break         callback(data)  @asyncio.coroutine def echo_client():     reader, ...     chunks = []     asyncio.async(read(reader, chunks.append))     while True:         pygame.event.pump() # advance pygame event loop         ...         if chunks: # print read-so-far data             print(b''.join(chunks).decode())             del chunks[:]         yield from asyncio.sleep(0.016) # advance asyncio loop 

There should be no blocking calls inside the while loop.

read() and sleep() coroutines run concurrently in the same thread (obviously you could run other coroutines concurrently too).