Why is PyObject_SetAttrString returning -1 (Python C-API)

Question

I am working on bridging C++ and Python.

When a new instance of my custom type gets created, I need to register certain C++ instance methods as attributes on the Python object being created.

The relevant code flow goes like this:

// during setup, we fill slots for the underlying PyTypeObject
p = new wrapperFor_PyTypeObject{ sizeof(FinalClass), 0, default_name };

p->set_tp_new(      extension_object_new );
p->set_tp_init(     extension_object_init );
p->set_tp_dealloc(  extension_object_deallocator );
:

Of interest here is set_tp_init

    static int extension_object_init( PyObject* _self, PyObject* _args, PyObject* _kwds )
    {
        try
        {
            Py::Tuple args{_args};
            Py::Dict kwds = _kwds ? Py::Dict{_kwds} : Py::Dict{};

            PythonClassInstance* self{ reinterpret_cast<PythonClassInstance*>(_self) };

            if( self->m_pycxx_object )
            {
                self->m_pycxx_object->reinit( args, kwds );
                DBG_PRINT( "reinit -" );
            }
            else
                self->m_pycxx_object = new FinalClass{ self, args, kwds };

            // here we force all c++ relevant class instance methods to register as attributes
            FinalClass* f = (FinalClass*)(self->m_pycxx_object);
            f->AddAll();
        }
        catch(...)
        :

    void AddAll()
    {
        auto& methods = FuncMapper<FinalClass>::methods();

        //auto py_method_table = new PyMethodDef[ methods.size() + 1 ]{}; // sentinel must be 0'd, which it is thx to {}

        for( auto& m : methods )
        {
            PyObject* a{ selfPtr() };  // 'this' class derives from PyObject

backing PyObject* const char* str = m.first.c_str();

            Object callable{ m.second->ConstructCFunc(this) };  // ConstructCFunc uses PyCFunction_New
            callable.increment_reference_count();
            PyObject* c{ callable.ptr() };                      // extract backing PyObject* pointer

            int ret = PyObject_SetAttrString( a, str, c );

            if( ret == -1 )
                throw AttributeError{ m.first };
        }
    }

I've declared all of the variables in a very pedantic way so as to make sure they are getting passed correctly into PyObject_SetAttrString, which it appears they are.

However, PyObject_SetAttrString is returning -1 (error).

Looking at the CPython source code, I can't make out where this error is coming from:

int
PyObject_SetAttrString(PyObject *v, const char *name, PyObject *w)
{
    PyObject *s;
    int res;

    if (Py_TYPE(v)->tp_setattr != NULL)
        return (*Py_TYPE(v)->tp_setattr)(v, (char*)name, w);
    s = PyUnicode_InternFromString(name);
    if (s == NULL)
        return -1;
    res = PyObject_SetAttr(v, s, w);
    Py_XDECREF(s);
    return res;
}

int
PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
{
    PyTypeObject *tp = Py_TYPE(v);
    int err;

    if (!PyUnicode_Check(name)) {
        PyErr_Format(PyExc_TypeError,
                     "attribute name must be string, not '%.200s'",
                     name->ob_type->tp_name);
        return -1;
    }
    Py_INCREF(name);

    PyUnicode_InternInPlace(&name);
    if (tp->tp_setattro != NULL) {
        err = (*tp->tp_setattro)(v, name, value); // <-- SHOULD HIT HERE
        Py_DECREF(name);
        return err;
    }
    if (tp->tp_setattr != NULL) {
        char *name_str = _PyUnicode_AsString(name);
        if (name_str == NULL)
            return -1;
        err = (*tp->tp_setattr)(v, name_str, value);
        Py_DECREF(name);
        return err;
    }
    Py_DECREF(name);
    assert(name->ob_refcnt >= 1);
    if (tp->tp_getattr == NULL && tp->tp_getattro == NULL)
        PyErr_Format(PyExc_TypeError,
                     "'%.100s' object has no attributes "
                     "(%s .%U)",
                     tp->tp_name,
                     value==NULL ? "del" : "assign to",
                     name);
    else
        PyErr_Format(PyExc_TypeError,
                     "'%.100s' object has only read-only attributes "
                     "(%s .%U)",
                     tp->tp_name,
                     value==NULL ? "del" : "assign to",
                     name);
    return -1;
}

Is there anything I can try short of having to add the CPython source to my project and single step through it?

By looking at the values going in, it should reach the point I've marked:

    PyUnicode_InternInPlace(&name);
    if (tp->tp_setattro != NULL) {
        err = (*tp->tp_setattro)(v, name, value); // <-- SHOULD HIT HERE
        Py_DECREF(name);
        return err;
    }

But I can't see how to debug into tp_setattro. It is a slot in a function table. Grepping through the source code reveals a huge number of accesses.