"""In Python 3.9, the following code can be run after commenting sleep, otherwise the file will always be empty"""
import time

file=open("data.txt","a+")
while True:
    time.sleep(0.00001)
    file.write("test\n")
    print("add over")

I unlinked the preliminary versions of the code minimized in the 3 post.

The posted code is buggy: it opens a file and then start an infinite loop.  It has to be interrupted someway, and the file may not be properly closed and flushed to disk.  When I make the loop finite and explicitly close the file, or even better, use a 'with' statement, the file is written as expected.  At least on Windows, it runs noticeably slower with the sleep because the minimum sleep is 1/16 second.

Minaki, if you find a problem with unbuggy code, you can post and reopen.  If you are a beginner who does not know about things like closing file, flushing to disk, and the with statement, you should post to a question and answer forum such as python-list and ask 'Is Python or my code buggy?'