The problem I've noticed is that if I append the file with each of the 100k runs (one at a time), it can happen that two threads try to save to the file at the same time and some row(s) end up empty.

The “w+” tells Python that we are writing to a new file. If the file already exists, then that file is overwritten. If the file doesn’t already exist, then it will be created.

How to write four million lines of Python: Lessons from Dropbox on using the programming language at scale Your email has been sent While Python is a hugely popular programming language it has ...