While your example of `sum` is a nice, pure function, it'll unfortunately blow up in python on even moderately sized inputs (we're talking thousands of elements, not millions) due to lack of tail calls in Python (currently) and the restrictions on recursion depth. The CPython interpreter as of 3.14 [0] is now capable of using tail calls in the interpreter itself, but it's not yet in Python, proper.
[0]: https://docs.python.org/3/whatsnew/3.14.html#a-new-type-of-i...