static PyObject*
foo(PyObject *, PyObject *arg)
{
    void *buf;
    Py_ssize_t size;
    if (PyObject_AsReadBuffer(arg, &buf, &size))
        size = -1;
    return PyInt_FromLong(size);
}

>>> import tst, re
>>> re.search("a", "a")
<_sre.SRE_Match object at 0xb76d0950>
>>> tst.foo("abc")
3
>>> re.search("a", "a")
<_sre.SRE_Match object at 0xb76d0950>
>>> tst.foo(None)
-1
>>> re.search("a", "a")
TypeError: expected a readable buffer object
>>>