Well, technically a call to range_mul() is still pure and it matches the definition: it has no side effects, and it returns a new instance of the same object (t

Well, technically a call to range_mul() is still pure and it matches the definition: it has no side effects, and it returns a new instance of the same object (the generator); the generator itself is inpure of course, so I concede the point. But that's a limitation of python generators being one-shot; with proper delimited continuations you can snapshot the state at any point and replay it at leisure, not differently than a lazy stream. Regarding the rest, I was referring to this comment from int_19h re encapsulation: "In general, so long as mutation can be encapsulated in modules that only expose pure functional interfaces, I think it should still count as FP for practical purposes."