A simple Python script that sets the track number of all (supposedly) audio files found in folder "tracks" based on their respective names

set_all_track_numbers_from_name.py

#!/usr/bin env python3

# This is a very simple gist based on
# https://stackoverflow.com/questions/8948/accessing-mp3-meta-data-with-python/34970600#34970600
# to add track number metadata to music when the tracks are formatted as "{track_number}_Title.mp3"
# but the track number is missing in metadata.

# This script doesn't check that the file is an mp3, only that the file starts with a number.
# If you need another separator than '_', replace it directly in the script or add a parameter "separator"
# that you would pass to the script.

# This script changes metadata for any file matching this pattern, use with caution.

import os

from mutagen.easyid3 import EasyID3

def set_track_to_prefix_number():
    """
    Walk over all files in "tracks" folder and set their audio tag "tracknumber" to
    "{track_number}/{max_track_number}" where:
    - track_number is extracted from the beginning
        of the filename (e.g. 23 for a file named 23_My_Dearest.mp3)
    - max_track_number is the total number of files in the current subfolder
        (not in the whole "tracks" folder)

    """
    for path, children_dirs, files in os.walk("tracks"):
        max_track_number = len(files)
        for file in files:
            track_number = file.split("_")[0]
            if not str.isdigit(track_number):
                print(f"{file} does not start with '[integer]_', skip")
                continue

            audio = EasyID3(os.path.join(path, file))
            audio['tracknumber'] = f"{track_number}/{max_track_number}"
            audio.save()
            print(f"Set tracknumber for {audio['tracknumber']}")


if __name__ == '__main__':
    set_track_to_prefix_number()