9.5. Async Programming

promises
futures
coroutines
awaitable
queue

9.5.1. Rationale

  • asyncio in stdlib

  • async keyword

  • await keyword

  • CPU-bound Concurrency:

    • Using Queues and Multiprocessing

    • Using Futures and Multiprocessing

  • I/O-bound Concurrency:

    • Using Queues and Threading

    • Using Futures and Threading

9.5.2. Async Programming

../_images/sync-execution-sequence.png
../_images/sync-execution-timeline.png
../_images/async-execution-sequence.png
../_images/async-execution-timeline.png
../_images/eventloop-sync.png
../_images/eventloop-async.png
../_images/async-python.png
../_images/async-threads.png
../_images/async-gil.png
../_images/async-anatomy.png
../_images/uvloop-doc.png
../_images/uvloop-using.png

9.5.3. Awaitable

  • Coroutine

  • __await__

9.5.4. Iterator

  • __aiter__

  • __anext__

class Reader:
    async def readline(self):
        ...

    def __aiter__(self):
        return self

    async def __anext__(self):
        val = await self.readline()
        if val == b'':
            raise StopAsyncIteration
        return val

9.5.5. Context Manager

  • __aenter__

  • __aexit__

class AsyncContextManager:
    async def __aenter__(self):
        await print('entering context')

    async def __aexit__(self, exc_type, exc, tb):
        await print('exiting context')

9.5.6. 3rd Party Library - Trio

$ pip install trio

import trio

async def child1():
    print("  child1: started! sleeping now...")
    await trio.sleep(1)
    print("  child1: exiting!")

async def child2():
    print("  child2: started! sleeping now...")
    await trio.sleep(1)
    print("  child2: exiting!")

async def parent():
    print("parent: started!")
    async with trio.open_nursery() as nursery:
        print("parent: spawning child1...")
        nursery.start_soon(child1)

        print("parent: spawning child2...")
        nursery.start_soon(child2)

        print("parent: waiting for children to finish...")
        # -- we exit the nursery block here --
    print("parent: all done!")

trio.run(parent)

Client:

import sys
import trio

# arbitrary, but:
# - must be in between 1024 and 65535
# - can't be in use by some other program on your computer
# - must match what we set in our echo server
PORT = 12345
# How much memory to spend (at most) on each call to recv. Pretty arbitrary,
# but shouldn't be too big or too small.
BUFSIZE = 16384

async def sender(client_stream):
    print("sender: started!")
    while True:
        data = b"async can sometimes be confusing, but I believe in you!"
        print(f"sender: sending {data!r}")
        await client_stream.send_all(data)
        await trio.sleep(1)

async def receiver(client_stream):
    print("receiver: started!")
    while True:
        data = await client_stream.receive_some(BUFSIZE)
        print(f"receiver: got data {data!r}")
        if not data:
            print("receiver: connection closed")
            sys.exit()

async def parent():
    print(f"parent: connecting to 127.0.0.1:{PORT}")
    client_stream = await trio.open_tcp_stream("127.0.0.1", PORT)
    async with client_stream:
        async with trio.open_nursery() as nursery:
            print("parent: spawning sender...")
            nursery.start_soon(sender, client_stream)

            print("parent: spawning receiver...")
            nursery.start_soon(receiver, client_stream)

trio.run(parent)

Server:

import trio
from itertools import count

# Port is arbitrary, but:
# - must be in between 1024 and 65535
# - can't be in use by some other program on your computer
# - must match what we set in our echo client
PORT = 12345
# How much memory to spend (at most) on each call to recv. Pretty arbitrary,
# but shouldn't be too big or too small.
BUFSIZE = 16384

CONNECTION_COUNTER = count()

async def echo_server(server_stream):
    # Assign each connection a unique number to make our debug prints easier
    # to understand when there are multiple simultaneous connections.
    ident = next(CONNECTION_COUNTER)
    print("echo_server {}: started".format(ident))
    try:
        while True:
            data = await server_stream.receive_some(BUFSIZE)
            print(f"echo_server {ident}: received data {data!r}")
            if not data:
                print(f"echo_server {ident}: connection closed")
                return
            print(f"echo_server {ident}: sending data {data!r}")
            await server_stream.send_all(data)
    # FIXME: add discussion of MultiErrors to the tutorial, and use
    # MultiError.catch here. (Not important in this case, but important if the
    # server code uses nurseries internally.)
    except Exception as exc:
        # Unhandled exceptions will propagate into our parent and take
        # down the whole program. If the exception is KeyboardInterrupt,
        # that's what we want, but otherwise maybe not...
        print(f"echo_server {ident}: crashed: {exc!r}")

async def main():
    await trio.serve_tcp(echo_server, PORT)

# We could also just write 'trio.run(serve_tcp, echo_server, PORT)', but real
# programs almost always end up doing other stuff too and then we'd have to go
# back and factor it out into a separate function anyway. So it's simplest to
# just make it a standalone function from the beginning.
trio.run(main)

9.5.7. 3rd Party Library - Unsync

  • Library decides which to run, thread, asyncio or sync

$ pip install unsync

@unsync
def my_function():
    pass