Pass a callback function from python to c using cython

Question

First off I want to say that I do not have the option to modify or even view the c source code so anything that involves modifying the c file will not be helpful.

In VP.h:

typedef enum VPEvent { 
  ...
  EVENT_OBJECT_CLICK,
  ...
}
...
typedef void *VPInstance;
typedef void(*VPEventHandler)(VPInstance);
...
VPSDK_API VPInstance vp_create(void);
...
VPSDK_API int vp_event_set(VPInstance instance, VPEvent eventname, VPEventHandler event);
...

In VP.pyx:

cdef extern from "VP.h":
  ...
  cdef enum VPEvent:
    ...
    VP_EVENT_OBJECT_CLICK,
    ...

  ...
  ctypedef void *VPInstance
  ctypedef void(*VPEventHandler)(VPInstance)
  ...
  VPInstance vp_create()
  ...
  int vp_event_set(VPInstance instance, VPEvent eventname, VPEventHandler event)
  ...

...
EVENT_OBJECT_CLICK = VP_EVENT_OBJECT_CLICK
...

cdef class create:
  cdef VPInstance instance

  def __init__(self):
    self.instance = vp_create()
  ...
  def event_set(self, eventname, event):
    return vp_event_set(self.instance, eventname, event)

What I want to have In Python:

import VP
...
def click(bot):
  bot.say("Someone clicked something!")
...
bot = VP.create()
bot.event_set(VP.EVENT_OBJECT_CLICK, click)

This is how you would do it in c:

#include <VP.h>

void click(VPInstance instance) {
  vp_say(instance, "Someone clicked something!");
}

int main(int argc, char ** argv) {
  ...
  VPInstance instance;
  instance = vp_create();
  ...
  vp_event_set(instance, VP_EVENT_OBJECT_CLICK, click)
}

However the problem is that when compiling VP.pyx I get

Cannot convert Python object to 'VPEventHandler'

As well, by default the callback is given a VPInstance pointer but I want to abstract this value into a class.

Answer 1

As you probably figured out, the problem in the call

bot.event_set(VP.EVENT_OBJECT_CLICK, click)

Indeed, the third argument click is a Python function object which you are passing in event_set to vp_event_set. Alas vp_event_set is expecting a VPEventHandler that is a C function pointer of type void(*VPEventHandler)(VPInstance);

I think I would build a dictionary associating to a VPInstance (void * pointer casted as integer) an instance of some PyEvent class which should contains itself the function click. Using that you can ensure that you need on one C function as callback.

In foo.pxd:

cdef class PyEvent(object):
     cdef VPInstance instance
     cdef object py_callback

In foo.pyx:

events = dict()

cdef void EventCallBack(VPInstance instance):
     PyEvent ev = <PyEvent> dict[events[<size_t> self.instance]
     ev.py_callback(ev)

cdef class PyEvent(object):
     def __init__(self, click):
         self.instance = vp_create()
         self.py_callback = click

     def event_set(self, eventname):
         global events
         events[<size_t> self.instance] = self
         return vp_event_set(self.instance, eventname, EventCallBack)

I don't have a chance to test this, so I hope it more or less works. Also I would recommend asking on [email protected] as they are usually really helpful and more expert than me.

Answer 2

I eventually solved this by taking a similar approach to what hivert suggested, however I decided to switch to swig as it is specifically built for language bindings.

This is a trimmed down version of what I setup

int swig_event_set(VPInstance instance, VPEvent eventname, PyObject * event);

%{
static PyObject * py_events[VP_HIGHEST_EVENT];
static void PythonEvent(VPInstance instance, int eventname) {
  PyObject * func, * arglist;
  PyObject * result;

  func = py_events[eventname];
  arglist = Py_BuildValue("(O)", SWIG_NewPointerObj(SWIG_as_voidptr(instance), SWIGTYPE_p_void, 0));
  result = PyEval_CallObject(func, arglist);
  Py_DECREF(arglist);
  Py_XDECREF(result);
}

static void vp_event_object_click(VPInstance instance) {
  PythonEvent(instance, VP_EVENT_OBJECT_CLICK);
}

int swig_event_set(VPInstance instance, VPEvent eventname, PyObject * event) {
  Py_XDECREF(py_events[eventname]);  /* Dispose of previous event callback */
  Py_XINCREF(event);                 /* Add a reference to new event callback */
  py_events[eventname] = event;      /* Remember new event callback */

  switch(eventname) {
    case VP_EVENT_OBJECT_CLICK:
      return vp_event_set(instance, eventname, vp_event_object_click);
      break;
    case VP_HIGHEST_EVENT:
      break;
  }

  return 1;
}
%}