Extract cover art using [`mid3v2`](https://github.com/quodlibet/mutagen/blob/master/mutagen/_tools/mid3v2.py)

extract-cover-art.py

#!/usr/bin/env python3
import os
import sys
import argparse
import subprocess
import ast

HOMEDIR = os.path.normpath(os.environ['HOME'])
MUSICDIR = os.path.join(HOMEDIR, 'Music')
COVERARTDIR = os.path.join(MUSICDIR, '.cover-arts')

OK = 0
ERRNO_MID3V2_NOT_FOUND = 1
ERRNO_MID3V2_TIMEOUT = 2
ERRNO_MID3V2_FAILED = 4
ERRNO_TAGINFO_NO_MIME = 8
ERRNO_TAGINFO_NO_DATA = 16
ERRNO_TAGINFO_NO_APIC = 32


def make_parser():
    parser = argparse.ArgumentParser(
        description=('Extract cover art, if any, from FILEs under '
                     '~/Music/ and save them to ~/Music/.cover-arts/.'))
    parser.add_argument('filenames', metavar='FILE', nargs='*')
    return parser


def extract_apic_info(filename):
    command_s = 'mid3v2 --list-raw "{}"'.format(filename)
    try:
        output = subprocess.check_output(['mid3v2', '--list-raw', filename],
                                         stderr=subprocess.STDOUT,
                                         timeout=5,
                                         cwd=MUSICDIR)
    except FileNotFoundError:
        print('`mid3v2\' is required to run this script!', file=sys.stderr)
        return ERRNO_MID3V2_NOT_FOUND
    except subprocess.TimeoutExpired:
        print('`{}\' timeout!'.format(command_s), file=sys.stderr)
        return ERRNO_MID3V2_TIMEOUT
    except subprocess.CalledProcessError as err:
        print(
            '`{}\' failed with error code {}'.format(command_s,
                                                     err.returncode),
            file=sys.stderr)
        return ERRNO_MID3V2_FAILED
    for line in output.decode('utf-8').split('\n'):
        if line.startswith('APIC('):
            try:
                mime_start_pos = line.index("mime='")
            except ValueError:
                print(
                    'failed to find MIME type in `{}\'s output'.format(
                        command_s),
                    file=sys.stderr)
                return ERRNO_TAGINFO_NO_MIME
            mime_end_pos = line.index("',", mime_start_pos)
            mime = line[mime_start_pos + len("mime='"):mime_end_pos]
            _literal_image, ext = mime.split('/', maxsplit=1)
            assert _literal_image == 'image', \
                   'unrecognized MIME {}'.format(mime)
            try:
                data_start_pos = line.index('data=', mime_end_pos)
            except ValueError:
                print(
                    'failed to find image data in `{}\'s output'.format(
                        command_s),
                    file=sys.stderr)
                return ERRNO_TAGINFO_NO_DATA
            data = line[data_start_pos + len('data='):-1]
            buffer = ast.literal_eval(data)
            fname = os.path.splitext(os.path.basename(filename))[0]
            tofile = os.path.join(COVERARTDIR, '.'.join([fname, ext]))
            with open(tofile, 'wb') as outfile:
                outfile.write(buffer)
            return OK
    print('{}: no cover art found'.format(filename), file=sys.stderr)
    return ERRNO_TAGINFO_NO_APIC


def main():
    args = make_parser().parse_args()
    for filename in args.filenames:
        returncode = extract_apic_info(filename)
        if returncode not in [OK, ERRNO_TAGINFO_NO_APIC]:
            sys.exit(returncode)


if __name__ == '__main__':
    main()