Python C api Exception class inside class - python-3.x

Question

I'm trying to throw exception which is defined inside class in extension module created with python C api.
Here is what I want in python C api, but written in python:
class Graph:
class TooManyVerticesError( Exception ): pass
def addVertex( self ):
if self.__order == 16:
raise Graph.TooManyVerticesError( "too many vertices" )
for v in range( self.__order ):
self.__matrix[v] += [False]
self.__order += 1
self.__matrix += [ [False] * self.__order ]
Here is what I have now written with python C api:
#define ERROR return NULL;
PyObject *TooManyVerticesError;
typedef struct {
PyObject_HEAD
size_t __order; /* the maximum number of elements in q_elements */
std::vector<std::vector<int>> AdjacencyList;
} ListaSasiedztwa;
static
PyObject* addVertex(ListaSasiedztwa* self, PyObject* args)
{
int u, v;
// Process arguments
PyArg_ParseTuple(args, "ii",
&u,
&v);
if (self->__order == 16)
{
PyErr_SetString(TooManyVerticesError, "too many vertices");
ERROR
}
std::vector<int> vertexList;
self->AdjacencyList.push_back(vertexList);
return NULL;
}
PyMODINIT_FUNC PyInit_simple_graphs(void)
{
if (PyType_Ready(&ListaSasiedztwaType) < 0) return NULL;
PyObject* m = PyModule_Create(&cSimpleGraphsModule);
if (m == NULL) return NULL;
TooManyVerticesError = PyErr_NewException("simple_graphs.TooManyVerticesError", NULL, NULL);
PyDict_SetItemString(ListaSasiedztwaType.tp_dict, "TooManyVerticesError", TooManyVerticesError);
Py_INCREF(&ListaSasiedztwaType);
PyModule_AddObject(m, "ListaSasiedztwa",
(PyObject *)&ListaSasiedztwaType);
return m;
}
Everything builds correctly, I can create object of type ListaSasiedztwa. But when exception should be thrown I have Exception of type SystemError with message:
class 'simple_graphs.ListaSasiedztwa' returned a result with an error set.
Should I add this Exception as structure member somehow?
I don't know if ListaSasiedztwaType is relevant in this problem so I didn't add it yet.

Answer 1

I think you're actually hitting the return NULL; at the end of the function which you reach in the "non-error" case. This NULL indicates to Python that their should be an exception set, but since there isn't one you get the general SystemError from Python..
I suspect you're trying to return the Python None object - use the macro Py_RETURN_NONE instead.

Answer 2

It occured that addVertex was called from another method which didn't pass return value which had to be NULL.