self-modifying-test.py

from random import randrange
from re import sub
from time import sleep


def update(lineno: int, pattern: str, repl: str) -> None:
    """Update part of this file by given pattern and replacement."""
    with open(__file__, "r") as f:
        lines = f.readlines()

    lines[lineno - 1] = sub(pattern, repl, lines[lineno - 1])

    with open(__file__, "w") as f:
        f.writelines(lines)


def main():
    """Regularly update the integer of line no.20 in this file."""
    while True:
        print(0)  # integer here will be updated
        sleep(1)
        update(20, r"\d+", str(randrange(10)))


if __name__ == "__main__":
    main()