Pad numeral part in file name to correctly order files

pad_file_name.py

#!/usr/bin/python3

import os
import re
import sys
import argparse

def match_files(regex):
    return [name for name in os.listdir() if regex.match(name)]

# regex should group on the numeral part of file name
def pad_length(regex, files):
    max_num = max([int(regex.search(filename).group(1)) for filename in files])
    return len(str(max_num))

def new_file_names(pad, files):
    matches = [(regex.search(filename).group(1), filename) for filename in files]
    return [name.replace(m, m.zfill(pad)) for (m, name) in matches]

def rename_files(names, new_names):
    for (name, new_name) in zip(names, new_names):
        os.rename(name, new_name)

if __name__ == '__main__':
    parser = argparse.ArgumentParser(description='Pad numbers in file name')
    parser.add_argument("regex", help='regex pattern for file which groups numerical pattern. for e.g. playlist_(\d*)_.*_album.mp3', type=str, action='store')
    parser.add_argument('--debug', '-d', help='perform a dry run without actually changing file names', action='store_true')

    args = parser.parse_args()
    regex = re.compile(args.regex)
    debug = args.debug

    names = match_files(regex)
    pad = pad_length(regex, names)
    new_names = new_file_names(pad, names)

    if debug:
        print("{} files found".format(len(names)))
        print('\n'.join(names))
        print()
        print("New names")
        print('\n'.join(new_names))
    else:
        rename_files(names, new_names)

Usage

1. Save the file as 'pad_file_name.py'
2. chmod +x pad_file_name.py
3. mv pad_file_name.py /usr/bin/
4. cd directory/where/files/names/are/not/padded
5. run script with a regex for the files

Example

pad_file_name.py --debug "playlist_(\d+)_.*_album.mp3"
pad_file_name.py "playlist_(\d+)_.*_album.mp3"