generic, works for lists and for strings (and probably iterables in general)

nice radix conversion from int to string

counting matches of '1' is simple, and could easily scale to higher radixes such as octal
could easily be extended to match multiple digits, such as .count('137')

range traversal is clear

accumulator function sum() over a list is also a nice aggregate function

likely to use a machine instruction underneath; prefer this over the current top solution

Thanks for your suggestion, I agree. However, it would have to be implemented in 3 languages now, and that seems too much of a hassle for a tiny added value.