aiowire - A simple event loop using asyncio
This package implements a EventLoop class
that manages concurrent coroutines.
It is based on the principles of functional reactive programming and draws inspiration from Haskell's Control.Wire library.
In particular, every co-routine started by the
event loop is a Wire.
Wire-s either return None, indicating they're done,
or another Wire.
An example helps explain the idea:
from aiowire import EventLoop
event = 0
async def show_event(ev) \
-> Optional[Callable[[EventLoop],Awaitable]]:
print("Running...")
event += 1
await asyncio.sleep(event*0.15)
print(f"Event {event}")
if event < 5:
return show_event
async with EventLoop(timeout=1) as event:
event.start(show_event)
event.start(show_event)
We start up an event loop and drop in two wires.
Each runs, then returns the show_event function.
The event loop runs those functions next... and so on.
But this isn't functional programming. The wires have access to the event loop, and can start more tasks. Easy, right?
What can I do with it?
What if you have a server that's spawning programs, working with sockets, and managing timeouts? Drop in one wire for each program, one polling on socket I/O, and another acting as a timer (as above).
The canonical task types are thus:
asyncio.create_subprocess_exec # run a process asyncio.sleep # awake the loop after a given time lapse zmq.asyncio.Poller.poll # awake the loop after I/O on socket/file # Note: see aiowire.Poller for a Wire-y interface.
Now your sockets can launch programs, and your program results can start/stop sockets, and everyone can start background tasks.
Poller?
The Poller class lets you schedule callbacks in response
to socket or file-descriptor activity. Of course, the callbacks
are wires, and run concurrently.
Poller is also a Wire, created as, Poller(dictionary mapping sockets / fd-s to callback wires).
You add it to your event loop as usual:
# ... create sock from zmq.asyncio.Context
async def echo(ev):
await sock.send( await sock.recv() )
todo = { 0: Call(print, "received input on sys.stdin"),
sock: echo
}
async with EventLoop() as ev:
ev.start( Poller(todo) )
Tell me more
Yes, you could just send async functions taking one
argument to EventLoop.start, but where's the fun in
writing closures everywhere?
To take it to the next level, aiowire comes with a
Wire convenience class that lets you write Wire-s expressively.
The following class extensions help you make Wire-s out of common
programming idioms:
- Wire(w): acts like an identity over "async func(ev):" functions
- Repeat(w, n): repeat wire
wn times in a row - Forever(w): repeat forever -- like Repeat(w) * infinity
- Call(fn, *args): call fn (normal or async), ignore the return, and exit
Consider, for example, printing 4 alarms separated by some time interval:
from aiowire import EventLoop, Call
prog = ( Call(asyncio.sleep, 0.1) >> Call(print, 'beep\a') ) * 4
async with EventLoop() as ev:
ev.start(prog)