[Python-Dev] Change to PyIter_Next()?

[Tim Peters]

In spare moments, I've been plugging away at making various functions work
nice with iterators (map, min, max, etc).

Over and over this requires writing code of the form:

	op2 = PyIter_Next(it);
	if (op2 == NULL) {
		/* StopIteration is *implied* by a NULL return from
		 * PyIter_Next() if PyErr_Occurred() is false.
		 */
		if (PyErr_Occurred()) {
			if (PyErr_ExceptionMatches(PyExc_StopIteration))
				PyErr_Clear();
			else
				goto Fail;
		}
		break;
	}

This is wordy, obscure, and in my experience is needed every time I call
PyIter_Next().

So I'd like to hide this in PyIter_Next instead, like so:

/* Return next item.
 * If an error occurs, return NULL and set *error=1.
 * If the iteration terminated normally, return NULL and set *error=0.
 * Else return the next object and set *error=0.
 */
PyObject *
PyIter_Next(PyObject *iter, int *error)
{
	PyObject *result;
	if (!PyIter_Check(iter)) {
		PyErr_Format(PyExc_TypeError,
			     "'%.100s' object is not an iterator",
			     iter->ob_type->tp_name);
		*error = 1;
		return NULL;
	}
	result = (*iter->ob_type->tp_iternext)(iter);
	*error = 0;
	if (result)
		return result;
	if (PyErr_Occurred()) {
		if (PyErr_ExceptionMatches(PyExc_StopIteration))
			PyErr_Clear();
		else
			*error = 1;
	}
	/* Else StopIteration is implicit, and there is no error. */
	return NULL;
}

Then *calls* could be the simpler:

	op2 = PyIter_Next(it, &error);
	if (op2 == NULL) {
		if {error)
			goto Fail;
		break;
	}

Objections?  So far I'm almost the only user of PyIter_Next(); the only other
use is in ceval's FOR_ITER, which goes thru a similar dance.

However, I'm not clear on why FOR_ITER doesn't clear the exception if
PyErr_Occurred() and PyErr_ExceptionMatches(PyExc_StopIteration) are both
true -- that sure smells like a bug (but, if so, the change above would
squash it by magic).

Note that I'm not proposing to change the signature of the tp_iternext slot
similarly.  PyIter_Next() is a (IMO appropriately) higher-level function.