Modernization¶
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
PyObject_HEAD_INIT(NULL)
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:
- Instead of
PyCObject_FromVoidPtr(obj, destr), usePyCapsule_New(obj, name, destr). If the capsule will be available as a module attribute, use"<modulename>.<attrname>"for name. Otherwise, use your best judgment, but try making the name unique. - Instead of
PyCObject_FromVoidPtrAndDesc(obj, desc, destr), usePyCapsule_New()as above; then callPyCapsule_SetContext(obj, desc). - Instead of
PyCObject_AsVoidPtr(obj), usePyCapsule_GetPointer(obj, name). You will need to provide a capsule name, which is checked at runtime as a form of type safety. - Instead of
PyCObject_GetDesc(), usePyCapsule_GetContext(). - Instead of
PyCObject_SetVoidPtr(), usePyCapsule_SetPointer(). - Change all CObject destructors to
PyCapsule destructors, which take the PyCapsule object as their only argument.