It's not bad necessarily to do it that way inline in another function, because there's no way to use it elsewhere without realizing that it has the side effect

It's not bad necessarily to do it that way inline in another function, because there's no way to use it elsewhere without realizing that it has the side effect of emptying the list. However since this is recursive that's not realistically how it would be used. It wouldn't be bad if it were obvious what it did, which it isn't with the name "sum", but would be with the name 'sum_and_empty". For good measure, maybe "sum_in_linear_space_and_empty" is even better. It's also less efficient (although in the context of a recursive sum definition that's not too relevant). Essentially what you're doing here is using the length of the list as the iteration variable — the `i' in `for (i = n-1; i >= 0; i--)'. But `pop()' probably does more than decrement the length field, I imagine it also does some memory management bookkeeping at least. I also take issue with how `list' is mutated and accessed in the same expression. I don't really use python, but I think I recall reading that it in fact guarantees left-to-right evaluation of such expressions. Even still, it's confusing and makes programmers, especially those who write C/C++ where this is undefined behavior, uneasy.