Argument Clinic no longer works single optional argument. For example see _tkinter.tkapp.wantobjects in the patch in bpo-20168.

/*[clinic input]
_tkinter.tkapp.wantobjects

[
value: int
]
/

[clinic start generated code]*/

It is converted to the methoddef of type METH_O.

#define _TKINTER_TKAPP_WANTOBJECTS_METHODDEF    \
    {"wantobjects", (PyCFunction)_tkinter_tkapp_wantobjects, METH_O, _tkinter_tkapp_wantobjects__doc__},

static PyObject *
_tkinter_tkapp_wantobjects(PyObject *self, PyObject *arg)
{
    PyObject *return_value = NULL;
    int group_right_1 = 0;
    int value = 0;

    if (!PyArg_Parse(arg, "i:wantobjects", &value))
        goto exit;
    return_value = _tkinter_tkapp_wantobjects_impl(self, group_right_1, value);

exit:
    return return_value;
}

As result wantobjects() can't be called without an argument.

Optional groups were designed to support two specific legacy use cases:

• functions that have groups of parameters that are
• functions with optional parameters to the left of required parameters

They are not intended to be used in cases like this. I really, really wish you would stop using them except for these specific legacy cases.

Furthermore, the argument parsing for tkinter.tkapp.wantobjects has a very clear translation into Argument Clinic:

value: int = -1
/

Optional groups should not be used for single-argument functions. I have no interest in fixing this bug.

Sorry, I forgot to finish the first bullet point. It should have read:

• functions that have groups of parameters that are optional only as a group (e.g. curses.window.getch)

Then it would be better to not convert wantobjects() to Argument Clinic. Using -1 as a value for not passed argument is a hack (added in 1ab9978123be) and I don't want to propagate it to the signature.

But I afraid that there are other functions that suffer from this bug (may be in the curses module).

If the argument currently uses a default value of -1, then I see no problem with converting it to Argument Clinic using a default value of -1. If you claim it's a "hack" then you should discuss that with the author of 1ab9978123be.

Closing as wontfix. This is not a supported use of optional groups.