I had a similar problem. I was attempting to have a file written every time a user visits a website.
The problem ended up being twofold.
1: the permissions were not set correctly
2: I attempted to use
f = open(r"newfile.txt","w+")
(Wrong)
After changing the file to 777 (all users can read/write)
chmod 777 /var/www/path/to/file
and changing the path to an absolute path, my problem was solved
f = open(r"/var/www/path/to/file/newfile.txt","w+")
(Right)