[tsbischof]

One of the big changes was to make many basic tools into lazy iterators instead of greedy lists (map, filter, range, etc.). I can remember having to teach my fellow scientist/engineer colleagues not to just loop over an index variable when working with certain datasets, but this was a major bit of friction because they were so used to Matlab and other languages where direct indexing is the primary method. While lazy execution is great for many things, it is not something that is necessarily common knowledge among people who use coding as a means to an end.

Additionally, because so many of the core libraries were 32-bit only or Python 2 only, you ended up having to either write your own version of them or just go back to 32-bit Python 2. Numpy in particular (and therefore transitively anything halfway useful for science and engineering) took several years to stabilize and I have many memories of having to dig into things like https://www.lfd.uci.edu/~gohlke/pythonlibs/ to get unofficial but viable builds going for the Windows machines we used. It was enough of a pain to deal with dependencies that I actually ended up rolling my own ndarray class that was horrendous but just good enough to get the job done.