PyGame Re-Initialize USB MIDI Device on Reconnect

Question

I'm using PyGame to read a USB MIDI device, very similar to what is used here , except I run it as a background service on a Raspberry Pi.

I would like to be able to disconnect and reconnect the MIDI device, and still be able to read it.

I have tried two approaches:

1. Regularly enumerate the MIDI devices using pygame.midi.get_count() and info().
2. Use pyudev to monitor USB events, similar to this example.

The problem with (1) is that it seems that pygame.midi always returns the same values (both get_count and info), regardless of whether the device is still connected.

The problem with (2) is that it would never call the asynchronous function I registered for events (though the standalone example works fine, just changing the subsytem to usb). I figured this might be a problem with threading, so I called everything to register for events from a dedicated thread, which then ran glib.MainLoop.run() to idle wait, but discovered the pygame would not be able to read the midi device if I started any thread before running my AMK class, even just a thread that printed something and returned. (I'm using glib since the version of pyudev in the Pi repo is 0.13, but I guess the newer way is the gobject equivalent).

Thus I resorted to using udevd to detect the connect event and restart my service via a /etc/udev/rules.d/ trigger, which works okay, but is kludgy, and loses the state in my script (which I would like to save).

So, before I waste many more hours debugging (2), I was hoping someone could perhaps point me in the right direction.

Answer 1

pygame uses PortMidi, which was originally designed for the Windows MIDI API and assumes that the set of MIDI ports never changes.

You have to use a separate monitor process that restarts your program whenever MIDI ports change.

Answer 2

I haven't tested this thoroughly yet but I believe that if you call quit and then again init , you can then get a properly updated list of MIDI devices. Here is an example:

import pygame, pygame.midi    
pygame.midi.init()    
print pygame.midi.get_count()    
a=raw_input('Connect or disconnect some MIDI devices')    
pygame.midi.quit()    
pygame.midi.init()
print pygame.midi.get_count()

Answer 3

This is how I monitor for existing or newly added Midi devices - wait_for_midi() will block until a MIDI device appears in the system and returns the /dev/midi* path to it.

import re
import pyudev

def is_midi_device(dev_path):
    if dev_path is None: 
        return False
    if re.match(u"^/dev/midi[0-9]+$", dev_path):
        return True
    return False

# Return path to a MIDI device when found.
def wait_for_midi():
    context = pyudev.Context()

    #  Check for existing midi devices
    for device in context.list_devices():
        dev_path = device.device_node
        if is_midi_device(dev_path) :
            print('Found {}'.format(dev_path))
            return dev_path

    # Monitor for new midi devices as added
    monitor = pyudev.Monitor.from_netlink(context)
    monitor.filter_by(subsystem='sound')
    for action, device in monitor:
        if action != "add": 
            continue
        dev_path = device.device_node
        if is_midi_device(dev_path) :
            print('Just added: {}'.format(dev_path))
            return dev_path