I've used pandas off and on for the better of a seven or eight years and, unlike other python libraries, I always feel like I'm starting from scratch every time I begin a new project. following a tutorial/official API on a second screen just to remember how to do fairly basic stuff.

The reason is that once I'm done building whatever model I've needed it works so well I don't have to touch it again for a few years and I forget everything I learned (or the API changes again).