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277 lines
8.2 KiB
ReStructuredText
277 lines
8.2 KiB
ReStructuredText
.. highlightlang:: c
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.. _cporting-howto:
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*************************************
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Porting Extension Modules to Python 3
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*************************************
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:author: Benjamin Peterson
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.. topic:: Abstract
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Although changing the C-API was not one of Python 3's objectives,
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the many Python-level changes made leaving Python 2's API intact
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impossible. In fact, some changes such as :func:`int` and
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:func:`long` unification are more obvious on the C level. This
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document endeavors to document incompatibilities and how they can
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be worked around.
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Conditional compilation
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=======================
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The easiest way to compile only some code for Python 3 is to check
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if :c:macro:`PY_MAJOR_VERSION` is greater than or equal to 3. ::
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#if PY_MAJOR_VERSION >= 3
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#define IS_PY3K
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#endif
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API functions that are not present can be aliased to their equivalents within
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conditional blocks.
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Changes to Object APIs
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======================
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Python 3 merged together some types with similar functions while cleanly
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separating others.
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str/unicode Unification
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-----------------------
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Python 3's :func:`str` (``PyString_*`` functions in C) type is equivalent to
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Python 2's :func:`unicode` (``PyUnicode_*``). The old 8-bit string type has
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become :func:`bytes`. Python 2.6 and later provide a compatibility header,
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:file:`bytesobject.h`, mapping ``PyBytes`` names to ``PyString`` ones. For best
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compatibility with Python 3, :c:type:`PyUnicode` should be used for textual data and
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:c:type:`PyBytes` for binary data. It's also important to remember that
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:c:type:`PyBytes` and :c:type:`PyUnicode` in Python 3 are not interchangeable like
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:c:type:`PyString` and :c:type:`PyUnicode` are in Python 2. The following example
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shows best practices with regards to :c:type:`PyUnicode`, :c:type:`PyString`,
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and :c:type:`PyBytes`. ::
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#include "stdlib.h"
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#include "Python.h"
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#include "bytesobject.h"
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/* text example */
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static PyObject *
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say_hello(PyObject *self, PyObject *args) {
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PyObject *name, *result;
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if (!PyArg_ParseTuple(args, "U:say_hello", &name))
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return NULL;
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result = PyUnicode_FromFormat("Hello, %S!", name);
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return result;
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}
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/* just a forward */
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static char * do_encode(PyObject *);
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/* bytes example */
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static PyObject *
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encode_object(PyObject *self, PyObject *args) {
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char *encoded;
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PyObject *result, *myobj;
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if (!PyArg_ParseTuple(args, "O:encode_object", &myobj))
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return NULL;
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encoded = do_encode(myobj);
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if (encoded == NULL)
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return NULL;
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result = PyBytes_FromString(encoded);
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free(encoded);
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return result;
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}
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long/int Unification
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--------------------
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Python 3 has only one integer type, :func:`int`. But it actually
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corresponds to Python 2's :func:`long` type--the :func:`int` type
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used in Python 2 was removed. In the C-API, ``PyInt_*`` functions
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are replaced by their ``PyLong_*`` equivalents.
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The best course of action here is using the ``PyInt_*`` functions aliased to
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``PyLong_*`` found in :file:`intobject.h`. The abstract ``PyNumber_*`` APIs
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can also be used in some cases. ::
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#include "Python.h"
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#include "intobject.h"
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static PyObject *
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add_ints(PyObject *self, PyObject *args) {
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int one, two;
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PyObject *result;
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if (!PyArg_ParseTuple(args, "ii:add_ints", &one, &two))
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return NULL;
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return PyInt_FromLong(one + two);
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}
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Module initialization and state
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===============================
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Python 3 has a revamped extension module initialization system. (See
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:pep:`3121`.) Instead of storing module state in globals, they should
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be stored in an interpreter specific structure. Creating modules that
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act correctly in both Python 2 and Python 3 is tricky. The following
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simple example demonstrates how. ::
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#include "Python.h"
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struct module_state {
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PyObject *error;
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};
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#if PY_MAJOR_VERSION >= 3
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#define GETSTATE(m) ((struct module_state*)PyModule_GetState(m))
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#else
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#define GETSTATE(m) (&_state)
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static struct module_state _state;
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#endif
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static PyObject *
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error_out(PyObject *m) {
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struct module_state *st = GETSTATE(m);
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PyErr_SetString(st->error, "something bad happened");
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return NULL;
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}
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static PyMethodDef myextension_methods[] = {
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{"error_out", (PyCFunction)error_out, METH_NOARGS, NULL},
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{NULL, NULL}
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};
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#if PY_MAJOR_VERSION >= 3
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static int myextension_traverse(PyObject *m, visitproc visit, void *arg) {
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Py_VISIT(GETSTATE(m)->error);
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return 0;
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}
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static int myextension_clear(PyObject *m) {
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Py_CLEAR(GETSTATE(m)->error);
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return 0;
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}
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static struct PyModuleDef moduledef = {
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PyModuleDef_HEAD_INIT,
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"myextension",
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NULL,
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sizeof(struct module_state),
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myextension_methods,
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NULL,
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myextension_traverse,
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myextension_clear,
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NULL
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};
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#define INITERROR return NULL
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PyObject *
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PyInit_myextension(void)
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#else
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#define INITERROR return
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void
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initmyextension(void)
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#endif
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{
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#if PY_MAJOR_VERSION >= 3
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PyObject *module = PyModule_Create(&moduledef);
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#else
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PyObject *module = Py_InitModule("myextension", myextension_methods);
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#endif
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if (module == NULL)
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INITERROR;
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struct module_state *st = GETSTATE(module);
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st->error = PyErr_NewException("myextension.Error", NULL, NULL);
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if (st->error == NULL) {
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Py_DECREF(module);
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INITERROR;
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}
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#if PY_MAJOR_VERSION >= 3
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return module;
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#endif
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}
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CObject replaced with Capsule
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=============================
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The :c:type:`Capsule` object was introduced in Python 3.1 and 2.7 to replace
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:c:type:`CObject`. CObjects were useful,
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but the :c:type:`CObject` API was problematic: it didn't permit distinguishing
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between valid CObjects, which allowed mismatched CObjects to crash the
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interpreter, and some of its APIs relied on undefined behavior in C.
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(For further reading on the rationale behind Capsules, please see :issue:`5630`.)
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If you're currently using CObjects, and you want to migrate to 3.1 or newer,
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you'll need to switch to Capsules.
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:c:type:`CObject` was deprecated in 3.1 and 2.7 and completely removed in
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Python 3.2. If you only support 2.7, or 3.1 and above, you
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can simply switch to :c:type:`Capsule`. If you need to support Python 3.0,
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or versions of Python earlier than 2.7,
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you'll have to support both CObjects and Capsules.
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(Note that Python 3.0 is no longer supported, and it is not recommended
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for production use.)
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The following example header file :file:`capsulethunk.h` may
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solve the problem for you. Simply write your code against the
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:c:type:`Capsule` API and include this header file after
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:file:`Python.h`. Your code will automatically use Capsules
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in versions of Python with Capsules, and switch to CObjects
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when Capsules are unavailable.
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:file:`capsulethunk.h` simulates Capsules using CObjects. However,
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:c:type:`CObject` provides no place to store the capsule's "name". As a
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result the simulated :c:type:`Capsule` objects created by :file:`capsulethunk.h`
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behave slightly differently from real Capsules. Specifically:
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* The name parameter passed in to :c:func:`PyCapsule_New` is ignored.
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* The name parameter passed in to :c:func:`PyCapsule_IsValid` and
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:c:func:`PyCapsule_GetPointer` is ignored, and no error checking
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of the name is performed.
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* :c:func:`PyCapsule_GetName` always returns NULL.
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* :c:func:`PyCapsule_SetName` always throws an exception and
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returns failure. (Since there's no way to store a name
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in a CObject, noisy failure of :c:func:`PyCapsule_SetName`
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was deemed preferable to silent failure here. If this is
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inconvenient, feel free to modify your local
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copy as you see fit.)
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You can find :file:`capsulethunk.h` in the Python source distribution
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as :source:`Doc/includes/capsulethunk.h`. We also include it here for
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your convenience:
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.. literalinclude:: ../includes/capsulethunk.h
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Other options
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=============
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If you are writing a new extension module, you might consider `Cython
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<http://www.cython.org>`_. It translates a Python-like language to C. The
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extension modules it creates are compatible with Python 3 and Python 2.
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