modern rewrite of BitTorrent .torrent file parser from here https://web.archive.org/web/20200919043954/https://effbot.org/zone/bencode.htm

torrent.py

#!/usr/bin/env python3
from collections.abc import Buffer, Generator
from typing import TypeAlias
import re

Token: TypeAlias = int | bytes | str
Result: TypeAlias = int | bytes | list['Result'] | dict[bytes, 'Result']

regexp = re.compile(rb'([del])|i(-?\d+)e|(\d+):')

def tokenize(data: Buffer) -> Generator[Token, None, None]:
    pos = 0
    while pos < len(data):
        m = regexp.match(data, pos)
        if m is None:
            raise ValueError
        pos = m.end()
        s = m[m.lastindex]
        match m.lastindex:
            case 1:
                yield s.decode()
            case 2:
                yield int(s)
            case 3:
                pos += int(s)
                if pos > len(data):
                    raise ValueError
                yield data[m.end():pos]

def decode(data: Buffer) -> Result:
    gen = tokenize(data)
    def decode_with_token(token: Token) -> Result:
        match token:
            case 'd' | 'l':
                elems = []
                while (_token := next(gen)) != 'e':
                    elems.append(decode_with_token(_token))
                if token == 'd':
                    return dict(zip(elems[::2], elems[1::2], strict=True))
                else:
                    return elems
            case int() | bytes():
                return token
            case _:
                raise ValueError
    result = decode_with_token(next(gen))
    for token in gen:
        raise ValueError
    return result

'''
'd' k0 v0 k1 v1 ... 'e' => dict
'l' e0 e1 ... 'e'       => list
'i' int 'e'             => int
len ':' bytes[len]      => bytes
'''

def main() -> None:
    import argparse
    import pprint
    parser = argparse.ArgumentParser()
    parser.add_argument('path', help='path to .torrent file')
    args = parser.parse_args()
    pprint.pprint(decode(open(args.path, 'rb').read()))

if __name__ == '__main__':
    main()