Well, py2 -> py3 is obviously one. Before that, well it's been a while. But with and yield were added as a keywords (breaking any code using them as a variable

Well, py2 -> py3 is obviously one. Before that, well it's been a while. But with and yield were added as a keywords (breaking any code using them as a variables). The xreadlines module disappeared at some point (I'm sure a few other modules have died along the way). At some point you couldn't raise anything you wanted as exceptions any more. Oh, and the source encoding declaration. That's what I remember off the top of my head. Minor things that are handled in things that are maintained, sure. But many things are also built once and then expected to run for a long time - especially in the role shell scripts usually fill. Note, that I don't argue that any of these changes were bad. They were all good changes that made the language better. But, they resulted in old code breaking. One extra headache to deal with for the poor schmuck that was responsible for upgrading this server or that to the latest OS release.