> You're using recursion.

No, see GP.

> `runreq()` calls `sum_()` which calls `sum()` in `return l[0] + f(f, l[1:])`, where `f` is `sum()`

Also no, see GP.

I am too stupid to understand this. This:

    def sum_(f, l):
      if not l: return 0
      return l[0] + f(f, l[1:])

    def runreq(f, *args):
      return f(f, *args)

    print(995,runreq(sum_, range(1,995)))
    print(1000,runreq(sum_, range(1,1000)))

when run with python3.11 gives me this output:

    995 494515
    Traceback (most recent call last):
      File "/tmp/sum.py", line 9, in <module>
        print(1000,runreq(sum_, range(1,1000)))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/tmp/sum.py", line 6, in runreq
        return f(f, *args)
               ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      [Previous line repeated 995 more times]
    RecursionError: maximum recursion depth exceeded in comparison

A RecursionError seems to indicate there must have been recursion, no?