replace_line.py

#!/usr/bin/env python3

from argparse import ArgumentParser
from pathlib import Path


def replace_line(path, line_number, text):
    if isinstance(text, str):
        text = text.encode()
    assert isinstance(text, bytes)
    path = Path(path)
    temp_path = path.with_name(path.name + '.tmp')
    try:
        with path.open(mode='rb') as f1:
            with temp_path.open(mode='wb') as f2:
                for n, line in enumerate(f1, start=1):
                    if n == line_number:
                        f2.write(text)
                        f2.write(b'\n')
                    else:
                        f2.write(line)
        temp_path.rename(path)
    finally:
        # pokud se neco nepovedlo, smazeme temp soubor
        if temp_path.exists():
            temp_path.unlink()


def main():
    p = ArgumentParser()
    p.add_argument('file')
    p.add_argument('line', type=int)
    p.add_argument('text')
    args = p.parse_args()
    replace_line(args.file, args.line, args.text)


if __name__ == '__main__':
    main()