
Porting Extension Modules to 3.0
********************************

author:
   Benjamin Peterson


Abstract
^^^^^^^^

Although changing the C-API was not one of Python 3.0's objectives,
the many Python level changes made leaving 2.x's API intact
impossible.  In fact, some changes such as ``int()`` and ``long()``
unification are more obvious on the C level.  This document endeavors
to document incompatibilities and how they can be worked around.


Conditional compilation
=======================

The easiest way to compile only some code for 3.0 is to check if
``PY_MAJOR_VERSION`` is greater than or equal to 3.

   #if PY_MAJOR_VERSION >= 3
   #define IS_PY3K
   #endif

API functions that are not present can be aliased to their equivalents
within conditional blocks.


Changes to Object APIs
======================

Python 3.0 merged together some types with similar functions while
cleanly separating others.


str/unicode Unification
-----------------------

Python 3.0's ``str()`` (``PyString_*`` functions in C) type is
equivalent to 2.x's ``unicode()`` (``PyUnicode_*``).  The old 8-bit
string type has become ``bytes()``.  Python 2.6 and later provide a
compatibility header, ``bytesobject.h``, mapping ``PyBytes`` names to
``PyString`` ones.  For best compatibility with 3.0, ``PyUnicode``
should be used for textual data and ``PyBytes`` for binary data.  It's
also important to remember that ``PyBytes`` and ``PyUnicode`` in 3.0
are not interchangeable like ``PyString`` and ``PyString`` are in 2.x.
The following example shows best practices with regards to
``PyUnicode``, ``PyString``, and ``PyBytes``.

   #include "stdlib.h"
   #include "Python.h"
   #include "bytesobject.h"

   /* text example */
   static PyObject *
   say_hello(PyObject *self, PyObject *args) {
       PyObject *name, *result;

       if (!PyArg_ParseTuple(args, "U:say_hello", &name))
           return NULL;

       result = PyUnicode_FromFormat("Hello, %S!", name);
       return result;
   }

   /* just a forward */
   static char * do_encode(PyObject *);

   /* bytes example */
   static PyObject *
   encode_object(PyObject *self, PyObject *args) {
       char *encoded;
       PyObject *result, *myobj;

       if (!PyArg_ParseTuple(args, "O:encode_object", &myobj))
           return NULL;

       encoded = do_encode(myobj);
       if (encoded == NULL)
           return NULL;
       result = PyBytes_FromString(encoded);
       free(encoded);
       return result;
   }


long/int Unification
--------------------

In Python 3.0, there is only one integer type.  It is called ``int()``
on the Python level, but actually corresponds to 2.x's ``long()``
type.  In the C-API, ``PyInt_*`` functions are replaced by their
``PyLong_*`` neighbors.  The best course of action here is using the
``PyInt_*`` functions aliased to ``PyLong_*`` found in
``intobject.h``.  The abstract ``PyNumber_*`` APIs can also be used in
some cases.

   #include "Python.h"
   #include "intobject.h"

   static PyObject *
   add_ints(PyObject *self, PyObject *args) {
       int one, two;
       PyObject *result;

       if (!PyArg_ParseTuple(args, "ii:add_ints", &one, &two))
           return NULL;

       return PyInt_FromLong(one + two);
   }


Module initialization and state
===============================

Python 3.0 has a revamped extension module initialization system.
(See **PEP 3121**.)  Instead of storing module state in globals, they
should be stored in an interpreter specific structure.  Creating
modules that act correctly in both 2.x and 3.0 is tricky.  The
following simple example demonstrates how.

   #include "Python.h"

   struct module_state {
       PyObject *error;
   };

   #if PY_MAJOR_VERSION >= 3
   #define GETSTATE(m) ((struct module_state*)PyModule_GetState(m))
   #else
   #define GETSTATE(m) (&_state)
   static struct module_state _state;
   #endif

   static PyObject *
   error_out(PyObject *m) {
       struct module_state *st = GETSTATE(m);
       PyErr_SetString(st->error, "something bad happened");
       return NULL;
   }

   static PyMethodDef myextension_methods[] = {
       {"error_out", (PyCFunction)error_out, METH_NOARGS, NULL},
       {NULL, NULL}
   };

   #if PY_MAJOR_VERSION >= 3

   static int myextension_traverse(PyObject *m, visitproc visit, void *arg) {
       Py_VISIT(GETSTATE(m)->error);
       return 0;
   }

   static int myextension_clear(PyObject *m) {
       Py_CLEAR(GETSTATE(m)->error);
       return 0;
   }


   static struct PyModuleDef moduledef = {
           PyModuleDef_HEAD_INIT,
           "myextension",
           NULL,
           sizeof(struct module_state),
           myextension_methods,
           NULL,
           myextension_traverse,
           myextension_clear,
           NULL
   };

   #define INITERROR return NULL

   PyObject *
   PyInit_myextension(void)

   #else
   #define INITERROR return

   void
   initmyextension(void)
   #endif
   {
   #if PY_MAJOR_VERSION >= 3
       PyObject *module = PyModule_Create(&moduledef);
   #else
       PyObject *module = Py_InitModule("myextension", myextension_methods);
   #endif

       if (module == NULL)
           INITERROR;
       struct module_state *st = GETSTATE(module);

       st->error = PyErr_NewException("myextension.Error", NULL, NULL);
       if (st->error == NULL) {
           Py_DECREF(module);
           INITERROR;
       }

   #if PY_MAJOR_VERSION >= 3
       return module;
   #endif
   }


Other options
=============

If you are writing a new extension module, you might consider Cython.
It translates a Python-like language to C.  The extension modules it
creates are compatible with Python 3.x and 2.x.
