> Just to be more pedantic: it did use tail calls, but the recursive calls weren't the tail calls.

I was talking about this code:

    def fib(n):
      if n =< 0:
        return (0, 1)
      else:
        a, b = f(n-1)
        return (b, a + b)

Not sure what you mean by tail calls here. I don't see Python-level tail calls. The interpreter will call C code for tuple packing, but those aren't in tail position with respect to the Python code either.

The last call (to a function or operator) is in the tail position. In this case, it is tuple packing.

Why wouldn't the tuple packing be in tail position in the Python code?

Because after tuple packing returns (a C return) you still need to perform a Python return. The C tuple packing function cannot return directly from the Python function.

Looking at https://github.com/python/cpython/blob/master/Python/ceval.c, here is the code that needs to be executed after the tuple is constructed:

        case TARGET(RETURN_VALUE): {
            retval = POP();
            assert(f->f_iblock == 0);
            assert(EMPTY());
            f->f_state = FRAME_RETURNED;
            f->f_stackdepth = 0;
            goto exiting;
        }

    // ...

    exiting:
    if (tstate->use_tracing) {
        if (tstate->c_tracefunc) {
            if (call_trace_protected(tstate->c_tracefunc, tstate->c_traceobj,
                                     tstate, f, PyTrace_RETURN, retval)) {
                Py_CLEAR(retval);
            }
        }
        if (tstate->c_profilefunc) {
            if (call_trace_protected(tstate->c_profilefunc, tstate->c_profileobj,
                                     tstate, f, PyTrace_RETURN, retval)) {
                Py_CLEAR(retval);
            }
        }
    }

    /* pop frame */
    exit_eval_frame:
    if (PyDTrace_FUNCTION_RETURN_ENABLED())
        dtrace_function_return(f);
    _Py_LeaveRecursiveCall(tstate);
    tstate->frame = f->f_back;

    return _Py_CheckFunctionResult(tstate, NULL, retval, __func__);

I guess you could duplicate all this code in a special "pack tuple and return from Python function" C function which you could then really tail call.

Oh, yes, I was only saying that as far as the Python language is concerned the tuple packing is in the tail position.

Any specific implementation, and in this case cpython, could do arbitrary weird things after.

Thanks for looking up the code!