C extension type gives type error in power operator

[Paul Moore]

I'm trying to implement an extension type with a power operator. The
operator is unusual in that I want to allow my objects to be raised to
an integer power:

   p = Pattern()
   p3 = p ** 3

I've implemented the code for a nb_power slot, it converts the "other"
argument to a C long using PyInt_AsLong().

static PyObject *
Pattern_pow (PyObject *self, PyObject *other, PyObject *modulo)
{
    long n = PyInt_AsLong(other);
    ...
    /* Ignore modulo argument - not meaningful */

    if (n == -1 && PyErr_Occurred())
        return NULL;
...
}

However, when I try to use the operator, I get the following error:
TypeError: unsupported operand type(s) for ** or pow():
'_ppeg.Pattern' and 'int'

I'm not sure where this error is coming from, as I don't have any type
checks in my code. Is there something else I should add to allow mixed-
type operations? (This is Python 2.5, if it matters).

Oh, and is there a good reference for writing C extensions for Python
anywhere? The manuals aren't bad, but I keep hitting empty sections
(e.g., 10.5 Number Object Structures).

Thanks
Paul.