This is a list of suggestions about the migration of Python C extension modules to Python 3.0. Feel free to expand the list!

• First of all, see Porting extension modules to 3.0 in the official Python 3 documentation.

• Use conditional compilation to make your code able to compile under both Python 2.x and 3.0 C-API.

• Now Python 3 uses Unicode as internal string representation, all the class name, method name, etc are Unicode strings. So check all the occurrences of PyString_*, most of them should be replaced with PyUnicode_*. When you suffer a segmentation fault, sometimes it's because you provided a PyString where the C-API is expecting a PyUnicode. Also, for the return value of your C extension function, you may use PyUnicode instead of PyString, otherwise you may break your client's code.

• PyInt_* removed, but you can explicitly #include <intobject.h> to define aliases that redirect most PyInt_* symbols to PyLong_*.

• PEP 3123: Making PyObject_HEAD conform to standard C (http://www.python.org/dev/peps/pep-3123/). When initializing type objects, replace

   1 PyObject_HEAD_INIT(NULL)
   2 0, /* ob_size */

• with

   1 PyVarObject_HEAD_INIT(NULL, 0)

• And you'd better use pre-defined macros Py_TYPE(o), Py_REFCNT(o), Py_SIZE(o) to access o->ob_type, o->ob_refcnt and o->ob_size separately. For more details please read the PEP.

• Changes in PyNumberMethods: nb_divide, nb_coerce, nb_oct, nb_hex and nb_inplace_divide removed.

• Read PEP 3121 Extension Module Initialization and Finalization (http://www.python.org/dev/peps/pep-3121/) and change your code to the new API. It solved many problems for module initialization and finalization.

• Now PyObject_Compare() raises exception when comparing two objects of different type. (In case of Python 2.x, it compares their pointer.)

• If your object has slicing interface (eg. obj[2:5]), you should notice that getitem, setitem and delitem now receive PySliceObject instead of a pair of integer value.

• Unbound method removed. In Python 3 you can use PyInstanceMethod_New() C-API to generate an unbound method for your C function.

• PyClass_Check() removed. Consider this as a workaround:

   1 #define PyClass_Check(obj) PyObject_IsInstance(obj, (PyObject *)&PyType_Type)
   2 

• PyInstance_NewRaw() removed, try to use PyBaseObject_Type.tp_new() for replacement.

• In py3k dict.items() method returns a dict_items object, you can use PySequence_Fast() to convert it to a sequence.

• struct member flags: The RO shorthand for READONLY is gone.