The function PyMapping_Check in Objects/abstract.c in
Python 2.4a3 is

int
PyMapping_Check(PyObject *o)
{
        if (PyInstance_Check(o))
                return PyObject_HasAttrString(o,
"__getitem__");

        return  o && o->ob_type->tp_as_mapping &&
                o->ob_type->tp_as_mapping->mp_subscript &&
                !(o->ob_type->tp_as_sequence &&
                  o->ob_type->tp_as_sequence->sq_slice);
}

where PyInstance_Check is #defined in
Include/classobject.h as

#define PyInstance_Check(op) ((op)->ob_type ==
&PyInstance_Type)

Hence, if the argument o of PyMapping_Check is NULL,
the function will crash. I first noticed this crash on
Cygwin.
The problem is that PyInstance_Check dereferences the
argument o before the check for o==NULL is made in the
return statement.

In Python 2.3.4, PyMapping_Check was

int
PyMapping_Check(PyObject *o)
{
        return o && o->ob_type->tp_as_mapping &&
                o->ob_type->tp_as_mapping->mp_subscript;
}

which checks for o==NULL correctly.

By the way, I am not sure if I am submitting this to
the correct category.

Logged In: YES 
user_id=80475

It is not clear that this is actually a bug.  Most of the
PY???_Check functions expect a non-NULL object.  The C API
docs are silent on the subject. 

However, in the spirit of defensive programming, I added the
early null checks back to PyMapping_Check and PySequence_Check.

Fixed and checked in as:
   Objects/abstract.c 2.132