I feel the author's pain. I had a short consulting gig recently where I was asked to speed up a simulation job written in Python by a mechanical engineer. Smart

I feel the author's pain. I had a short consulting gig recently where I was asked to speed up a simulation job written in Python by a mechanical engineer. Smart guy but not a programmer, so the script was full of global variables, side effects and imperative loops. I parallelised the script, which is pretty easy in Python but involved refactoring into a more pure functional style. I abstracted out the algorithms and added in some persistent memoization as a bonus. It was clean and did what they needed, but... unfortunately was now completely unintelligible to the engineer who wrote it. When I was asked back 6 months later to help again, they had reverted to using and modifying the old script. This time I kept the changes to the absolute minimum.