Before porting a C extension to Python 3, you’ll need to make sure that you’re not using features deprecated even in Python 2. Also, many of Python 3’s improvements have been backported to Python 2.6, and using them will make the porting process easier.

For all changes you do, be sure add tests to ensure you do not break anything.

PyObject Structure Members

To conform to C’s strict aliasing rules, PyObject_HEAD, which provides members such as ob_type and ob_refcnt, is a separate struct in Python 3. Access to these members is provided by macros, which have been ported to Python 2.6:

Instead of use
obj->ob_type Py_TYPE(obj)
obj->ob_refcnt Py_REFCNT(obj)
obj->ob_size Py_SIZE(obj)

And for initialization of type objects, the sequence

0, /* ob_size */

must be replaced with

PyVarObject_HEAD_INIT(NULL, 0)

Adding module-level constants

Often, module initialization uses code like this:

PyModule_AddObject(m, "RDWR", PyInt_FromLong(O_RDWR));
PyModule_AddObject(m, "__version__", PyString_FromString("6.28"));

Python 2.6 introduced convenience functions, which are shorter to write:

PyModule_AddIntConstant(m, "RDWR", O_RDWR)
PyModule_AddStringConstant(m, "__version__", "6.28")

These will use native int and str types in both Python versions.

New-Style Classes

The old-style classes (PyClass_* and PyInstance_*) will be removed in Python 3. Instead, use type objects, which have been available since Python 2.2.

PyCObject to PyCapsule

The PyCObject API has been removed in Python 3.3. You should instead use its replacement, PyCapsule, which is available in Python 2.7 and 3.1+. For the rationale behind Capsules, see CPython issue 5630.

If you need to support Python 2.6, you can use capsulethunk.h, which implements the PyCapsule API (with some limitations) in terms of PyCObject. For instructions, see the chapter PyCapsule API for Python 2.6.

The port to PyCapsule API should be straightforward:



In previous versions, this chapter talked about rich comparison, but those changes are better left to porting.


When your project is sufficiently modernized, and the tests still pass under Python 2, you’re ready to start the actual Porting.