Path: csiph.com!newsfeed.hal-mli.net!feeder3.hal-mli.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!news.tele.dk!feed118.news.tele.dk!news.tele.dk!small.news.tele.dk!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Content-Type: text/plain; charset=us-ascii
Mime-Version: 1.0 (Mac OS X Mail 7.2 \(1874\))
Subject: Re: How to create an instance of a python class from C++
From: Barry Scott <barry@barrys-emacs.org>
In-Reply-To: <0ab424e9-3a3f-4111-9f41-ff50ce73d70d@googlegroups.com>
Date: Tue, 11 Mar 2014 21:37:34 +0000
Content-Transfer-Encoding: quoted-printable
References: <0ab424e9-3a3f-4111-9f41-ff50ce73d70d@googlegroups.com>
To: Bill <galaxyblue63@gmail.com>
Cc: python-list@python.org
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.8067.1394579420.18130.python-list@python.org>
Lines: 141
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:68249

On 5 Mar 2014, at 00:14, Bill <galaxyblue63@gmail.com> wrote:

> Hello:
>=20
> I can't figure out how to create an instance
> of a python class from 'C++':
>=20

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. )
>=20
> In a .py file I create an ABC and then specialize it:
>=20
>    from MyMod import *
>    from abc import ABCMeta, abstractmethod
>=20
>    # Declare an abstract base class.
>    class Base(metaclass=3DABCMeta):
>        """Base class."""
>        @abstractmethod
>        def description(self):
>            return "=46rom the base class."
>=20
>    # Declare a class that inerits from the base.
>    class Derived(Base):
>        """Derived class."""
>        def description(self):
>            return "=46rom the Derived."
>=20
>    # Register the derived class.
>    RegisterClass(Derived)
>=20
> Then from 'C++' (my implementation of RegisterClass)
> I try to create an instance
>=20
>    static PyObject *
>    RegisterClass( PyObject *, PyObject *args ) {       // This gets =
called ok.
>=20
>        PyObject *class_decl;
>        if( ! PyArg_ParseTuple(args, "O", &class_decl) )
>            return NULL;
>        Py_INCREF(class_decl);
>=20
>        PyTypeObject *typ =3D class_decl->ob_type;
>=20
>        // Tried this.
>        // PyObject *an =3D _PyObject_New(class_decl->ob_type); =
assert(an);
>        // PyObject *desc =3D =
PyObject_CallMethod(an,"description",NULL); assert(desc);
>=20
>        // Tried this.
>        // PyObject *an =3D 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);
>=20
>        // This returns something.
>        assert(class_decl); assert(class_decl->ob_type); =
assert(class_decl->ob_type->tp_new);
>        PyObject *an_inst =3D =
class_decl->ob_type->tp_new(class_decl->ob_type,NULL, NULL); =
assert(an_inst);
>        assert(class_decl->ob_type->tp_init);
>=20
>        // This crashes.
>        int ret =3D class_decl->ob_type->tp_init(an_inst,NULL, NULL); =
assert(ret =3D=3D 0);
>        // PyObject_CallMethod(an_inst,"__init__",NULL);
>        // PyObject *an_inst =3D =
PyObject_CallMethod(class_decl,"__new__",NULL); assert(an_inst);
>=20
>        // Never get here.
>        PyObject *desc =3D =
PyObject_CallMethod(an_inst,"description",NULL); assert(desc);
>        char *cp =3D _PyUnicode_AsString(desc);
>        cerr << "Description:" << cp << endl;
>=20
>        return PyLong_FromLong(0);
>    }
>=20
>    static PyMethodDef MyModMethods[] =3D {
>        { "RegisterClass", RegisterClass, METH_VARARGS, "Register =
class." },
>        {  NULL,           NULL,          0,             NULL           =
  }
>    };
>=20
>    static struct PyModuleDef MyModMod =3D {
>       PyModuleDef_HEAD_INIT,
>       "MyMod",            // name of module
>       NULL,                // module documentation, may be NULL
>       -1,
>       MyModMethods,
>       NULL,
>       NULL,
>       NULL,
>       NULL
>    };
>=20
>    PyMODINIT_FUNC
>    PyInit_MyMod( void ) {
>        PyObject* m =3D PyModule_Create(&MyModMod);
>        if( m =3D=3D NULL )
>            return NULL;
>            return m;
>    }
>=20
>    int
>    main( int, char ** ) {
>=20
>        PyImport_AppendInittab( "MyMod", PyInit_MyMod );
>=20
>        Py_Initialize();
>=20
>        const char *file_name =3D "z.py";
>        FILE *fp =3D fopen(file_name,"r");
>        if( fp ) {
>            PyRun_SimpleFileExFlags(fp,file_name,1,0);
>        }
>=20
>        Py_Finalize();
>=20
>        return 0;
>    }
> --=20
> https://mail.python.org/mailman/listinfo/python-list
>=20