[c-api]Transmutation of an extension object into a read-only buffer adding an integer in-place.

[Giacomo Alzetta]

I'm trying to implement a c-extension which defines a new class(ModPolynomial on the python side, ModPoly on the C-side).
At the moment I'm writing the in-place addition, but I get a *really* strange behaviour.

Here's the code for the in-place addition:

#define ModPoly_Check(v) (PyObject_TypeCheck(v, &ModPolyType))
[...]
static PyObject *
ModPoly_InPlaceAdd(PyObject *self, PyObject *other)
{
    
    if (!ModPoly_Check(self)) {
        // This should never occur for in-place addition, am I correct?
        if (!ModPoly_Check(other)) {
            PyErr_SetString(PyExc_TypeError, "Neither argument is a ModPolynomial.");
            return NULL;
        }
        return ModPoly_InPlaceAdd(other, self);
    } else {
        if (!PyInt_Check(other) && !PyLong_Check(other)) {
            Py_INCREF(Py_NotImplemented);
            return Py_NotImplemented;
        }
    }
    
    ModPoly *Tself = (ModPoly *)self;
    PyObject *tmp, *tmp2;
    tmp = PyNumber_Add(Tself->ob_item[0], other);
    tmp2 = PyNumber_Remainder(tmp, Tself->n_modulus);
    
    Py_DECREF(tmp);
    tmp = Tself->ob_item[0];
    Tself->ob_item[0] = tmp2;
    Py_DECREF(tmp);
    
    printf("%d\n", (int)ModPoly_Check(self));
    return self;
    
}

And here's an example usage:

>>> from algebra import polynomials
>>> pol = polynomials.ModPolynomial(3,17)
>>> pol += 5
1
>>> pol
<read-only buffer ptr 0x22bee30, size 4 at 0x21f5af0>
>>> 

Now, how come my ModPolynomial suddenly becomes a read-only buffer, even though that last printf tells us that the object returned is of the correct type?
If I raise an exception instead of returning self, the ModPolynomial gets incremented correctly. If I use the Py_RETURN_NONE macro, the ModPolynomial is correctly replaced by None.