Argument Clinic does not increase NoneType refcount when returning NoneType from a function with no parameters

[noamcohen97]

Bug report

/*[clinic input]
function -> NoneType
[clinic start generated code]*/

will produce this function:

static PyObject *
function(PyObject *module, PyObject *Py_UNUSED(ignored))
{
    return function_impl(module);
}

while function_impl returns Py_None (as it should, if using NoneType return converter), refcount is not increased.

here is a working example of NoneType return converter:

/*[clinic input]
function -> NoneType

    a: int
    /
[clinic start generated code]*/

static PyObject *
function(PyObject *module, PyObject *arg)
{
    PyObject *return_value = NULL;
    int a;
    PyObject *_return_value;

    a = _PyLong_AsInt(arg);
    if (a == -1 && PyErr_Occurred()) {
        goto exit;
    }
    _return_value = function_impl(module, a);
    if (_return_value != Py_None) {
        goto exit;
    }
    return_value = Py_None;
    Py_INCREF(Py_None);

exit:
    return return_value;
}

Generation also fails when generating a function with a single object parameter.

It seems like the problem is with default_return_converter in clinic.py, being set to True

[serhiy-storchaka]

Good catch. This converter is not used, so the bug was not exposed. And since it has dubious semantic (requires returning a weak reference to None on success), I think that it is better to remove it.

[noamcohen97]

How about we'll get rid of the callback return value, and use PyErr_Occurred() to chose whether to return NULL or Py_None?

[arhadthedev]

How about we'll get rid of the callback return value, and use PyErr_Occurred() to chose whether to return NULL or Py_None?

When I did the same in gh-91284, I found that PyErr_Occurred takes GIL. Currently (until nogil is implemented, at least) it's cheaper to deal with PyErr_Occurred only on NULL returned from a callback.

[erlend-aasland]

Resolved with:

• gh-95007: Remove the NoneType return converter #95019
• gh-95007: Remove the NoneType return converter from Argument Clinic Doc #95529