Mididings scene switch callback

mididingsSceneCallback.py

#!/usr/bin/env python
'''
Two alternatives for scene switch callback.

1. Using a custom made Hook() object, which responds only *after* a scene 
    change has been made. This means that if a scene switch request contains 
    a non existing scene/subscene, nothing happens.
2. Rewriting the private Engine.scene_switch_callback function, which 
    intercepts scene switch requests and can do some further actions before 
    the change is actually made.

This example uses a "Control" port and changes scene according to the octave
of a NOTEON event. The event is then discarded here, which is not required; if 
you want to keep it, remember to reroute if there are less outputs than inputs.
'''

from mididings import *
from mididings.engine import Engine, current_scene, current_subscene, switch_scene

in_ports = (
    ('Some keyboard', ), 
    ('Control', ), 
    )

config(
    backend='alsa', 
    client_name='Chatty router', 
    in_ports=in_ports, 
    silent=True, 
    )

scene_dict = {s: Pass() for s in range(1, 5)}

class Messenger(object):
    def __init__(self):
        self.scene = 0
        self.subscene = 0
        self.on_switch_scene = self.startup_switch_scene

    def on_start(self):
        print('Mididings started!')

    def startup_switch_scene(self, *args):
        #Startup callback showing "default" scene info;
        #scene and subscene are 1 index based
        self.scene = current_scene() + 1
        self.subscene = current_subscene() + 1
        print('Start scene: {}\nStart subscene: {}\n'.format(self.scene, self.subscene))
        self.on_switch_scene = self.real_switch_scene

    def real_switch_scene(self, scene, subscene):
        #Do something when a scene/subscene has changed; here we show a message
        #only when a scene/subscene actually changes
        if scene != self.scene:
            print('Scene changed from {} to {}'.format(self.scene, scene))
        elif subscene != self.subscene:
            print('Subscene changed from {} to {}'.format(self.subscene, subscene))
        self.scene = scene
        self.subscene = subscene

started = False
old_scene = 0
old_subscene = 0

def scene_switch_callback(cb, engine, scene, subscene):
    #This is called *before* trying to change the scene
    global started
    global old_scene
    global old_subscene
    #call the original Engine() class method
    cb(engine, scene, subscene)
    if not started:
        print('Mididings started!\nCurrent scene: {}\nCurrent subscene: {}'.format(old_scene + 1, old_subscene + 1))
        started = True
    else:
        #Do something before trying to change the scene
        if old_scene != scene:
            print('Scene changing from {} to {}'.format(old_scene + 1, scene + 1))
        elif old_subscene != subscene:
                print('Subscene changing from {} to {}'.format(old_subscene, current_subscene()))
        old_scene = scene
        old_subscene = subscene

#Uncomment to use the hook class
#hook(Messenger())

#Uncomment to use method renaming
#Engine.scene_switch_callback = lambda engine, scene, subscene, cb=Engine.scene_switch_callback: scene_switch_callback(cb, engine, scene, subscene)

def switch(event):
    switch_scene(event.note // 12)
    return(event)

run(scenes=scene_dict, control=PortFilter('Control')>>Filter(NOTEON)>>Process(switch)>>Discard())