> Or you could simply Py_INCREF(args) before the Py_XDECREF...

But won't self->args point to a broken object while any callbacks triggered by Py_XDECREF() are run?

An alternative would be

    tmp = self->args;
    self->args = args;
    Py_INCREF(self->args);
    Py_XDECREF(tmp);

As far as I can see the idiom Py_?DECREF(self->...) is rarely safe outside of a deallocator unless you are sure the pointed to object has a "safe" type (or you are sure the refcount cannot fall to zero).