How can the built-in range function take a single argument or three?

Question

I would like to know, if anyone can tell me, how the range function can take either: a single argument, range(stop), or range(start, stop), or

Answer 1

The beauty of open-source software is that you can just look it up in the source:

(TL;DR: yes, it uses varargs)

if (PyTuple_Size(args) <= 1) {
    if (!PyArg_UnpackTuple(args, "range", 1, 1, &stop))
        return NULL;
    stop = PyNumber_Index(stop);
    if (!stop)
        return NULL;
    start = PyLong_FromLong(0);
    if (!start) {
        Py_DECREF(stop);
        return NULL;
    }
    step = PyLong_FromLong(1);
    if (!step) {
        Py_DECREF(stop);
        Py_DECREF(start);
        return NULL;
    }
}
else {
    if (!PyArg_UnpackTuple(args, "range", 2, 3,
                           &start, &stop, &step))
        return NULL;

    /* Convert borrowed refs to owned refs */
    start = PyNumber_Index(start);
    if (!start)
        return NULL;
    stop = PyNumber_Index(stop);
    if (!stop) {
        Py_DECREF(start);
        return NULL;
    }
    step = validate_step(step);    /* Caution, this can clear exceptions */
    if (!step) {
        Py_DECREF(start);
        Py_DECREF(stop);
        return NULL;
    }
}