Change all instruments in a MIDI file.

change_inst.py

# 2022 Dobby233Liu
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"""
Change all instruments in a MIDI file.
This needs a custom version of python-midi: https://github.com/mgedmin/python-midi/tree/python3
"""

import midi
from midi import MetaEvent, ProgramChangeEvent, NoteOnEvent, NoteOffEvent
import argparse
import pathlib
import random

DRUM_CHANNEL = 10 - 1

def categorize_events(pattern):
    event_list = [[] for _ in range(16)]
    metaevent_list = []

    for track_no, track in enumerate(pattern):
        for event in track:
            if isinstance(event, MetaEvent):
                metaevent_list.append(event)
            else:
                event_list[event.channel].append((track_no, event))

    return (metaevent_list, event_list)

def change_instruments(pattern, instrument_id, change_1st_event_only=False, modify_percussion=False):

    randomize = False
    if instrument_id < 0:
        randomize = True

    for channel_no, channel in enumerate(categorize_events(pattern)[1]):
        if len(channel) <= 0 or (channel_no == DRUM_CHANNEL and not modify_percussion):
            continue

        inst_range = range(128)
        if channel_no == DRUM_CHANNEL:
            inst_range = range(35, 82)

        channel_has_prog_change_event = False
        tick0_has_change = False
        track = None

        for _track, event in channel:
            # i don't know what i'm doing
            if track is None:
                track = _track

            val = instrument_id
            if randomize:
                val = random.choice(inst_range)

            if channel_no == DRUM_CHANNEL:
                if isinstance(event, NoteOnEvent) or isinstance(event, NoteOffEvent):
                    event.pitch = val
            elif isinstance(event, ProgramChangeEvent):
                channel_has_prog_change_event = True
                event.value = val
                if event.tick == 0:
                    tick0_has_change = True
                if change_1st_event_only:
                    break

        # make sure the instrument is changed
        if not channel_no == DRUM_CHANNEL and (not channel_has_prog_change_event or not tick0_has_change):
            final_inst_id = instrument_id
            if randomize:
                final_inst_id = random.choice(inst_range)
            # add the event
            _event = ProgramChangeEvent(tick=0, channel=channel_no, value=final_inst_id)
            pattern[track].insert(0, _event)

    return pattern


def main():
    parser = argparse.ArgumentParser(description='Change all instruments in a MIDI file.')
    parser.add_argument('file', type=pathlib.Path,
                        help='source MIDI file')
    parser.add_argument('id', type=int,
                        help='what instrument to change to (if < 0, performs randomization)')
    parser.add_argument('output', type=pathlib.Path,
                        help='output file')
    parser.add_argument('-i', '--change-1st-event-only', action='store_true',
                        help='change only the first instrument change event seen')
    parser.add_argument('-p', '--percussion', action='store_true',
                        help='also change percussion')

    args = parser.parse_args()
    source_file, inst_id, output_file, change_1st_only, modify_percussion = (args.file, args.id, args.output, args.change_1st_event_only, args.percussion)

    pattern = midi.read_midifile(source_file)
    pattern = change_instruments(pattern, inst_id, change_1st_event_only=change_1st_only, modify_percussion=modify_percussion)
    midi.write_midifile(output_file, pattern)

if __name__ == "__main__":
    main()