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.

The newly approved Python Enhancement Proposal 751 gives Python a standard lock file format for specifying the dependencies of projects. Here’s the what, why, and when.

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.