Custom PyObject by inheritance in C++

Question

Long term python programmer, first time C++ extension writer guy. Anyways, for fun, I'm trying to create a linked list module for python in C++. Here's my code

#include <python2.7/Python.h>
#include <iostream>

using namespace std;

template <typename T>
class LinkedList : public PyObject {
private:
  struct ListNode {
    ListNode(T value, ListNode* next)
      : value(value), next(next) {}
    T value;
    ListNode* next;
  };
  ListNode* head;

public:
  LinkedList(T value)
    : head(new ListNode(value, 0)) {
    cout << "class constructed" << endl;
    Py_INCREF(this);
  }
  void get_value() {
    cout << "test" << endl;
  }
  ~LinkedList() {
    delete head;
    Py_DECREF(this);
    cout << "class destructed" << endl;
  }
};

static PyObject* linkedlist_new(PyObject* self, PyObject* args) {
  LinkedList<char*> ll("hello");
  return Py_BuildValue("O", &ll);
}

static PyMethodDef LinkedListMethods[] = {
    {"new", linkedlist_new, METH_VARARGS,
     "Create a new linked list."},
    {NULL, NULL, 0, NULL}
};

extern "C" PyMODINIT_FUNC initLinkedList(void) {
  (void) Py_InitModule("LinkedList", LinkedListMethods);
}

Can I do this? Most of the docs are for C, but can I inherit from PyObject and return it like this? What works right now is this:

import LinkedList

print "start"
l = LinkedList.new()
print "done"

but as soon as I call l.get_value() in python, I get a segfault. I know that what I'm doing is probably wrong, so would anybody be so kind as to point me into the right direction?

And for clarification, I know the LinkedList<char*> named “ll” is destroyed after the linkedlist_new function is done, which is part of the problem I'm having. Let's just assume I'm very, very lost...

Answer 1

First off: you may need to manually set up your object header – in other words, change

template <typename T>
class LinkedList : public PyObject { /* … */ }

to something like

template <typename T>
class LinkedList {
    public:
        PyObject_HEAD
        /// …
}

… in my own experience the latter works, provided the rest of the Python object’s API is filled out properly. Which is the second point: you don’t define a PyTypeObject, which is a little more involved (q.v. https://docs.python.org/2/c-api/typeobj.html) than what you have here.

Specifically, you’ll need a PyTypeObject that corresponds to each PyObject-derived structure you intend to expose to the user – and so while a templated PyObject-derived LinkedList class may sound great at first, keep the PyTypeObject structure in mind (as your user-facing module representation will inevitably have to concretely define one or more of them) as well as which typename parameters upon which your LinkList ends up specialized.