How to create an instance of a python class from C++

[Barry Scott]

On 5 Mar 2014, at 00:14, Bill <galaxyblue63 at gmail.com> wrote:

> Hello:
> 
> I can't figure out how to create an instance
> of a python class from 'C++':
> 

Why not use pycxx from http://sourceforge.net/projects/cxx/?

This lib does all the heavy lifting for you for both python2 and python3.
Has docs and examples.

Barry
PyCXX maintainer.



> ( I am relatively new to Python so excuse some of
>  the following. )
> 
> In a .py file I create an ABC and then specialize it:
> 
>    from MyMod import *
>    from abc import ABCMeta, abstractmethod
> 
>    # Declare an abstract base class.
>    class Base(metaclass=ABCMeta):
>        """Base class."""
>        @abstractmethod
>        def description(self):
>            return "From the base class."
> 
>    # Declare a class that inerits from the base.
>    class Derived(Base):
>        """Derived class."""
>        def description(self):
>            return "From the Derived."
> 
>    # Register the derived class.
>    RegisterClass(Derived)
> 
> Then from 'C++' (my implementation of RegisterClass)
> I try to create an instance
> 
>    static PyObject *
>    RegisterClass( PyObject *, PyObject *args ) {       // This gets called ok.
> 
>        PyObject *class_decl;
>        if( ! PyArg_ParseTuple(args, "O", &class_decl) )
>            return NULL;
>        Py_INCREF(class_decl);
> 
>        PyTypeObject *typ = class_decl->ob_type;
> 
>        // Tried this.
>        // PyObject *an = _PyObject_New(class_decl->ob_type); assert(an);
>        // PyObject *desc = PyObject_CallMethod(an,"description",NULL); assert(desc);
> 
>        // Tried this.
>        // PyObject *an = PyType_GenericNew((PyTypeObject *)class_decl->ob_type, NULL, NULL); assert(an);
>        // assert(class_decl); assert(class_decl->ob_type); assert(class_decl->ob_type->tp_new);
> 
>        // This returns something.
>        assert(class_decl); assert(class_decl->ob_type); assert(class_decl->ob_type->tp_new);
>        PyObject *an_inst = class_decl->ob_type->tp_new(class_decl->ob_type,NULL, NULL); assert(an_inst);
>        assert(class_decl->ob_type->tp_init);
> 
>        // This crashes.
>        int ret = class_decl->ob_type->tp_init(an_inst,NULL, NULL); assert(ret == 0);
>        // PyObject_CallMethod(an_inst,"__init__",NULL);
>        // PyObject *an_inst = PyObject_CallMethod(class_decl,"__new__",NULL); assert(an_inst);
> 
>        // Never get here.
>        PyObject *desc = PyObject_CallMethod(an_inst,"description",NULL); assert(desc);
>        char *cp = _PyUnicode_AsString(desc);
>        cerr << "Description:" << cp << endl;
> 
>        return PyLong_FromLong(0);
>    }
> 
>    static PyMethodDef MyModMethods[] = {
>        { "RegisterClass", RegisterClass, METH_VARARGS, "Register class." },
>        {  NULL,           NULL,          0,             NULL             }
>    };
> 
>    static struct PyModuleDef MyModMod = {
>       PyModuleDef_HEAD_INIT,
>       "MyMod",            // name of module
>       NULL,                // module documentation, may be NULL
>       -1,
>       MyModMethods,
>       NULL,
>       NULL,
>       NULL,
>       NULL
>    };
> 
>    PyMODINIT_FUNC
>    PyInit_MyMod( void ) {
>        PyObject* m = PyModule_Create(&MyModMod);
>        if( m == NULL )
>            return NULL;
>            return m;
>    }
> 
>    int
>    main( int, char ** ) {
> 
>        PyImport_AppendInittab( "MyMod", PyInit_MyMod );
> 
>        Py_Initialize();
> 
>        const char *file_name = "z.py";
>        FILE *fp = fopen(file_name,"r");
>        if( fp ) {
>            PyRun_SimpleFileExFlags(fp,file_name,1,0);
>        }
> 
>        Py_Finalize();
> 
>        return 0;
>    }
> -- 
> https://mail.python.org/mailman/listinfo/python-list
>