gh-148829: Implement PEP 661

[JelleZijlstra]

Replaces JelleZijlstra#4. See python/steering-council#258.

• Issue: Implement PEP 661 Sentinel Values #148829

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📚 Documentation preview 📚: https://cpython-previews--148831.org.readthedocs.build/

[ZeroIntensity]

I don't think PyFunction_GetModule can fail here, because f_funcobj shouldn't ever be anything other than a function object. We can use _PyFrame_GetFunction to assert this:

Suggested change

	PyObject *r = PyFunction_GetModule(PyStackRef_AsPyObjectBorrow(f->f_funcobj));
	if (!r) {
	PyErr_Clear();
	Py_RETURN_NONE;
	}
	return Py_NewRef(r);
	PyObject *r = PyFunction_GetModule(_PyFrame_GetFunction(f));
	if (!r) {
	Py_RETURN_NONE;
	}
	return Py_NewRef(r);

[JelleZijlstra]

hm, do we even need the null check for funcobj on the line above? Not sure it can ever be NULL.

[JelleZijlstra]

I'll keep that for paranoia. Also keeping an assertion that it really is a PyFunctionObject.