Can we do a JavaScript-like .then() hack in Python?

then.py

# Can we do a JavaScript-like .then() hack in Python?

import threading
import time


class Sleeper(threading.Thread):
    def __init__(self):
        super(Sleeper, self).__init__()
        self.f = None
        self.start()

    def run(self):
        time.sleep(2)
        if callable(self.f):
            self.f()

    def then(self, f):
        self.f = f


def move_motor_up(steps):
    print 'UP', steps
    return Sleeper()

def move_motor_down(steps):
    print 'DOWN', steps
    return Sleeper()


def up():
    move_motor_up(100).then(
        lambda: move_motor_down(100).then(up)
    )

up()

When leaving this infinite loop running, would you end up accumulating a huge collection of un-garbage-collected Sleeper instances? To test that, I put print statements in Sleeper.__init__ and Sleeper.__del__. No more than two of them are ever alive, so this might be a pretty good design pattern.

This came up in the context of sending commands to a widget and getting back asynchronous responses, and there are many different flavors of command/response pairings, and several commands can be in-process at any given moment. So you want some kind of event-handler approach, and borrowing JS's .then() pattern seemed sensible.

To apply this to an embedded control scenario, Sleeper would not inherit from Thread, and the __init__ and run methods would be replaced by something like this.

    def __init__(self, *args):
        widget.sendCommand(args)
        self.f = None

    def onResponseReceived(self):
        # perform sanity checks, raise Exception if not in the expected state
        if callable(self.f):
            self.f()