Create a Python type from C that implements a __dict__?

Question

• How is a type created to have a __dict__ as per a \"normal\" class would have were it defined in Python?
• Are there any examples of non-dynamic types

Answer 1

The following code will generate a class that implements a __dict__ in Python 2.x:

typedef struct {
  PyObject_HEAD
  PyObject* dict;
} BarObject;

static PyTypeObject BarObject_Type = {
  PyObject_HEAD_INIT(NULL)
};

PyMODINIT_FUNC
initFoo(void)
{
  PyObject *m;

  m = Py_InitModule("Foo", NULL);
  if (m == NULL)
    return;

  BarObject_Type.tp_new = PyType_GenericNew;
  BarObject_Type.tp_name = "Foo.Bar";
  BarObject_Type.tp_basicsize = sizeof(BarObject);
  BarObject_Type.tp_getattro = PyObject_GenericGetAttr;
  BarObject_Type.tp_setattro = PyObject_GenericSetAttr;
  BarObject_Type.tp_flags = Py_TPFLAGS_DEFAULT;
  BarObject_Type.tp_dictoffset = offsetof(BarObject,dict);
  BarObject_Type.tp_doc = "Doc string for class Bar in module Foo.";
  if (PyType_Ready(&BarObject_Type) < 0)
    return;

  Py_INCREF(&BarObject_Type);
  PyModule_AddObject(m, "Bar", (PyObject*)&BarObject_Type);
}

The important bit is the tp_dictoffset member of the PyTypeObject struct (http://docs.python.org/c-api/typeobj.html):

If the instances of this type have a dictionary containing instance variables, this field is non-zero and contains the offset in the instances of the type of the instance variable dictionary; this offset is used by PyObject_GenericGetAttr().

Do not confuse this field with tp_dict; that is the dictionary for attributes of the type object itself.