[C++-sig] How to expose to a python user defined type with to/from_python convertors?

[X Wang]

> How do I expose my C++ type to a user
> defined python type? I think I could use boost python
> class_<> wrapper to wrap around, but if I want to use
> to_python/from_python convertors?
> 
> Say I want to expose my C++ class(CA) to a python
> type(PA):
> 
> C++:
> class CA
> {
>   public:
>     //ctors, accessors...
>   private:
>     std::string name_;
>     int value_;
> };
> 
> Python:
> class PA(object):
>   def __init__(self, name, value):
>     self.name=name
>     self.value=value
> 
> If I use,
> 
> void initializeConverters()
> {
>    using namespace boost::python;
>    to_python_converter<CA,
> CA_to_pA>();
> }
> 
> with
> 
> struct CA_to_PA
> {
>       static PyObject* convert(CA const&
> s)
>       {
>          printf("In C++ ->
> python converter?\n");
> 
>          std::string
> name=s.name();
>          int
> value=s.value();
> 
> #if 0
>          static
> boost::python::object obj;
>          PyObject*
> obj_ptr=obj.ptr();
> 
>      
>    PyObject_SetAttrString(obj_ptr, "name", 
>                
> PyString_FromString(name.c_str()));
>      
>    PyObject_SetAttrString(obj_ptr, "value", 
>                
> PyInt_FromLong(value));
> 
>          return
> boost::python::incref(obj_ptr);
> #else
>          return
> boost::python::incref(
>            
> //PyInt_FromLong(1999)
>            
> Py_BuildValue("s", "native string works")
>            
> //Py_BuildValue("{s:i,s:i}", "abc", 123, "def", 456)
>             );
> #endif
>       }
> };
> 
> How do I create a boost::python::object (or PyObject*) with
> "name" and "value" attributes? I could get creation and
> return of python native types to work, but can not get user
> defined types returned to the python side -- always get
> NoneType returned.
> 
> boost::python::object C2Py()
> {
>    printf("Pre assignment\n");
> 
> #if 0
>    boost::python::object obj(new CA("From
> C++ to python", 987654321));
>    PyObject* o=obj.ptr();
> #else
>    CA a("From C++ to python", 987654321);
>    PyObject *o=CA_to_PA::convert(a);
> #endif
> 
>    printf("Post assignment\n");
> 
>    PyObject *tmp=NULL;
> 
>    std::string name="No Name";
>    int value=-1;
> 
>    tmp=PyObject_GetAttrString(o, "name");
>    if (tmp!=NULL)
> name=PyString_AsString(tmp);
> 
>    tmp=PyObject_GetAttrString(o, "value");
>    if (tmp!=NULL) value=PyInt_AsLong(tmp);
> 
>    printf("(%s, %d)\n", name.c_str(),
> value);
> 
>    return obj;
> }
> 
> $ python
> Python 2.5.1 (r251:54863, Oct  5 2007, 11:16:46)
> [GCC 4.1.1 20070105 (Red Hat 4.1.1-53)] on linux2
> Type "help", "copyright", "credits" or "license" for more
> information.
> >>> import mytypes
> >>> pa=mytypes.C2Py()
> ("No Name", -1)
> >>>type(pa)
> NoneType
> 
> Any help is appreciated, thanks!
> 
Any comments? Am I asking the right question? Basically I have a simple C++ type and want to extend it to python. However, I haven't figured out a way to do the C++ to python conversion with boost python's to_python/from_python converter scheme. Am I missing something?