C module calls python script: how to set exception in C

[Alex]

> However, is it safe to call PyObject_Repr here, or can it itself err
> again?

Yes, you can get an error in the __repr__ method of a class, for
instance.  You may want something like

callable_repr = PyObject_Repr(callable_repr);
if (!callable_repr) { return NULL; }
PyErr_Format(PyExc_TypeError, "%s blah", callable_repr);

If it were me, I would probably just say that the callable passed to the
function returned the wrong type, and let the user figure out what that
was.

HTH
Alex.