Guido van Rossum writes:
I don't actually understand why Stephen made this claim about arithmetic operations,
Stephen is often mistaken about computers (among many topics). That's why he mostly posts on -ideas, and mostly throws drafts away rather than post them. :-)
I would not claim that evaluating a literal such as 1e1000 is not an arithmetic operation, I just "forgot" that these examples exist (mostly in a context of trying to get NaN with math.asin(2) and got ValueError instead ;-/ ).
I still stand by the argument that since some common ways of producing true infinities (rather than overflows) and NaNs such as 1.0/0.0 and 0.0/0.0 end up as Exceptions rather than float values, inf and nan have rather small usefulness in Python with only builtins, especially nan. "from math import inf" has to be the least annoying thing about working with infinities and overflows in Python (which, to be fair, is quite annoying in any context, on computers or on paper).