Porting Extension Modules to Python 3
*************************************

author:
   Benjamin Peterson


Résumé
^^^^^^

Although changing the C-API was not one of Python 3’s objectives, the
many Python-level changes made leaving Python 2’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 Python 3 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 merged together some types with similar functions while
cleanly separating others.


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

Python 3’s "str()" type is equivalent to Python 2’s "unicode()"; the C
functions are called "PyUnicode_*" for both.  The old 8-bit string
type has become "bytes()", with C functions called "PyBytes_*".
Python 2.6 and later provide a compatibility header, "bytesobject.h",
mapping "PyBytes" names to "PyString" ones.  For best compatibility
with Python 3, "PyUnicode" should be used for textual data and
"PyBytes" for binary data.  It’s also important to remember that
"PyBytes" and "PyUnicode" in Python 3 are not interchangeable like
"PyString" and "PyUnicode" are in Python 2.  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
--------------------

Python 3 has only one integer type, "int()".  But it actually
corresponds to Python 2’s "long()" type—the "int()" type used in
Python 2 was removed.  In the C-API, "PyInt_*" functions are replaced
by their "PyLong_*" equivalents.


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

Python 3 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 Python 2 and Python 3 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

   PyMODINIT_FUNC
   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
   }


CObject replaced with Capsule
=============================

The "Capsule" object was introduced in Python 3.1 and 2.7 to replace
"CObject".  CObjects were useful, but the "CObject" API was
problematic: it didn’t permit distinguishing between valid CObjects,
which allowed mismatched CObjects to crash the interpreter, and some
of its APIs relied on undefined behavior in C. (For further reading on
the rationale behind Capsules, please see bpo-5630.)

If you’re currently using CObjects, and you want to migrate to 3.1 or
newer, you’ll need to switch to Capsules. "CObject" was deprecated in
3.1 and 2.7 and completely removed in Python 3.2.  If you only support
2.7, or 3.1 and above, you can simply switch to "Capsule".  If you
need to support Python 3.0, or versions of Python earlier than 2.7,
you’ll have to support both CObjects and Capsules. (Note that Python
3.0 is no longer supported, and it is not recommended for production
use.)

The following example header file "capsulethunk.h" may solve the
problem for you.  Simply write your code against the "Capsule" API and
include this header file after "Python.h".  Your code will
automatically use Capsules in versions of Python with Capsules, and
switch to CObjects when Capsules are unavailable.

"capsulethunk.h" simulates Capsules using CObjects.  However,
"CObject" provides no place to store the capsule’s « name ».  As a
result the simulated "Capsule" objects created by "capsulethunk.h"
behave slightly differently from real Capsules.  Specifically:

   * The name parameter passed in to "PyCapsule_New()" is ignored.

   * The name parameter passed in to "PyCapsule_IsValid()" and
     "PyCapsule_GetPointer()" is ignored, and no error checking of the
     name is performed.

   * "PyCapsule_GetName()" always returns NULL.

   * "PyCapsule_SetName()" always raises an exception and returns
     failure.  (Since there’s no way to store a name in a CObject,
     noisy failure of "PyCapsule_SetName()" was deemed preferable to
     silent failure here.  If this is inconvenient, feel free to
     modify your local copy as you see fit.)

You can find "capsulethunk.h" in the Python source distribution as
Doc/includes/capsulethunk.h.  We also include it here for your
convenience:

   #ifndef __CAPSULETHUNK_H
   #define __CAPSULETHUNK_H

   #if (    (PY_VERSION_HEX <  0x02070000) \
        || ((PY_VERSION_HEX >= 0x03000000) \
         && (PY_VERSION_HEX <  0x03010000)) )

   #define __PyCapsule_GetField(capsule, field, default_value) \
       ( PyCapsule_CheckExact(capsule) \
           ? (((PyCObject *)capsule)->field) \
           : (default_value) \
       ) \

   #define __PyCapsule_SetField(capsule, field, value) \
       ( PyCapsule_CheckExact(capsule) \
           ? (((PyCObject *)capsule)->field = value), 1 \
           : 0 \
       ) \


   #define PyCapsule_Type PyCObject_Type

   #define PyCapsule_CheckExact(capsule) (PyCObject_Check(capsule))
   #define PyCapsule_IsValid(capsule, name) (PyCObject_Check(capsule))


   #define PyCapsule_New(pointer, name, destructor) \
       (PyCObject_FromVoidPtr(pointer, destructor))


   #define PyCapsule_GetPointer(capsule, name) \
       (PyCObject_AsVoidPtr(capsule))

   /* Don't call PyCObject_SetPointer here, it fails if there's a destructor */
   #define PyCapsule_SetPointer(capsule, pointer) \
       __PyCapsule_SetField(capsule, cobject, pointer)


   #define PyCapsule_GetDestructor(capsule) \
       __PyCapsule_GetField(capsule, destructor)

   #define PyCapsule_SetDestructor(capsule, dtor) \
       __PyCapsule_SetField(capsule, destructor, dtor)


   /*
    * Sorry, there's simply no place
    * to store a Capsule "name" in a CObject.
    */
   #define PyCapsule_GetName(capsule) NULL

   static int
   PyCapsule_SetName(PyObject *capsule, const char *unused)
   {
       unused = unused;
       PyErr_SetString(PyExc_NotImplementedError,
           "can't use PyCapsule_SetName with CObjects");
       return 1;
   }



   #define PyCapsule_GetContext(capsule) \
       __PyCapsule_GetField(capsule, descr)

   #define PyCapsule_SetContext(capsule, context) \
       __PyCapsule_SetField(capsule, descr, context)


   static void *
   PyCapsule_Import(const char *name, int no_block)
   {
       PyObject *object = NULL;
       void *return_value = NULL;
       char *trace;
       size_t name_length = (strlen(name) + 1) * sizeof(char);
       char *name_dup = (char *)PyMem_MALLOC(name_length);

       if (!name_dup) {
           return NULL;
       }

       memcpy(name_dup, name, name_length);

       trace = name_dup;
       while (trace) {
           char *dot = strchr(trace, '.');
           if (dot) {
               *dot++ = '\0';
           }

           if (object == NULL) {
               if (no_block) {
                   object = PyImport_ImportModuleNoBlock(trace);
               } else {
                   object = PyImport_ImportModule(trace);
                   if (!object) {
                       PyErr_Format(PyExc_ImportError,
                           "PyCapsule_Import could not "
                           "import module \"%s\"", trace);
                   }
               }
           } else {
               PyObject *object2 = PyObject_GetAttrString(object, trace);
               Py_DECREF(object);
               object = object2;
           }
           if (!object) {
               goto EXIT;
           }

           trace = dot;
       }

       if (PyCObject_Check(object)) {
           PyCObject *cobject = (PyCObject *)object;
           return_value = cobject->cobject;
       } else {
           PyErr_Format(PyExc_AttributeError,
               "PyCapsule_Import \"%s\" is not valid",
               name);
       }

   EXIT:
       Py_XDECREF(object);
       if (name_dup) {
           PyMem_FREE(name_dup);
       }
       return return_value;
   }

   #endif /* #if PY_VERSION_HEX < 0x02070000 */

   #endif /* __CAPSULETHUNK_H */


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 and Python 2.
