aiowire 3.0.1

A simple event loop using asyncio

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[Wire]:
    print("Running...")
    event += 1
    await asyncio.sleep(event*0.15)
    print(f"Event {event}")
    if event < 5:
        return Wire(show_event)
    return None

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 since 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).

Some canonical task types that do these include:

asyncio.create_subprocess_exec # run a program

asyncio.sleep # awake the loop after a given time lapse

zmq.asyncio.Poller.poll # awake the loop after I/O on socket/file

aiowire.Poller # Wire-y interface to zmq.asyncio.Poller

Think about each wire as a finite state machine. For example,

flowchart LR
    R[Ready] --> N{New Task?};
    N -- Yes --> W[Working];
    W --> C{Complete?};
    C -- Yes --> R;

can be implemented like so:

async def ready(ev : EventLoop, info : X) -> Optional[Wire]:
    if info.new_task():
        do_working_action()
        return Wire(working, info) # move to working state

    # Return a sequence of 2 wires:
    return Call(asyncio.sleep, 1.0) >> Wire(ready, info)

async def working(ev : EventLoop, info : X) -> Wire:
    if info.complete():
        do_complete_action()
        return Wire(ready, info)
    await asyncio.sleep(0.5) # directly sleep a bit
    return Wire(working, info)

Note how 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: Wire(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 w n times in a row

• Forever(w): repeat forever – like Repeat(w) * infinity

• Call(fn, *args, **kargs): 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)

References

• https://pyzmq.readthedocs.io/en/latest/api/zmq.html#poller

• https://pythontic.com/modules/select/poll

• https://blog.tomecek.net/post/non-blocking-stdin-in-python/