The Flask application either needs the large file to continue running, or does not release unneeded resources.
If the app needs the large file, that's it. Otherwise, the app is buggy and in need to be corrected.
In both cases, the "being open" status of the large file (that, at least on Linux, leads to the file still being present in the mass memory system) cannot be controlled  by your script.
The program is a standard flask program, and it does some cleanup as part of the initialization. In the cleanup() procedure, using os.remove("abc.txt"), I noticed that the file is removed, but not reclaimed by the OS.
I use both "python website.py" and "gunicorn website:app" to run the application and both have the same problem, in Linux environment. In MacOS, I can't reproduce it.
After the file is os.remove , it is no longer listed in "ls" command, but when I run
lsof | grep deleted
I can still see this file being listed as deleted but opened by the python application.
Because this file is already "os.remove", it is not listed in ls command, and du will not calculate this file.
But if this file is big enough, df command will show the space of this file is still being occupied, not being reclaimed. Because this file is still "being open by the flask application", as the lsof program claimed.
As soon as I stop the flask application from running, the lsof will not have this file, and the space is reclaimed.
Usually when the file is too small, or when the application stops or restarts frequently, you won't notice the space is being occupied. But this is not very reasonable for keeping the space. I would expect the website running for years.
When searching internet for "open but deleted files", most suggestions are "find the application and kill it". Is there a way to keep the flask application running without restarting it? My application doesn't actually "open" this file, but simply os.remove it.
Suggestion on how to delete file and re-claim the space immediately?
A flask website, when it delete a file (os.remove("abc.txt")), the file is removed but the space is not reclaimed