PEP 227 implementation
A cell contains a reference to a single PyObject. It could be
implemented as a mutable, one-element sequence, but the separate type
has less overhead.
diff --git a/Objects/cellobject.c b/Objects/cellobject.c
new file mode 100644
index 0000000..d9ecfd7
--- /dev/null
+++ b/Objects/cellobject.c
@@ -0,0 +1,115 @@
+/* Cell object implementation */
+
+#include "Python.h"
+
+PyObject *
+PyCell_New(PyObject *obj)
+{
+ PyCellObject *op;
+
+ op = (PyCellObject *)PyObject_New(PyCellObject, &PyCell_Type);
+ op->ob_ref = obj;
+ Py_XINCREF(obj);
+
+ PyObject_GC_Init(op);
+ return (PyObject *)op;
+}
+
+PyObject *
+PyCell_Get(PyObject *op)
+{
+ if (!PyCell_Check(op)) {
+ PyErr_BadInternalCall();
+ return NULL;
+ }
+ Py_XINCREF(((PyCellObject*)op)->ob_ref);
+ return PyCell_GET(op);
+}
+
+int
+PyCell_Set(PyObject *op, PyObject *obj)
+{
+ if (!PyCell_Check(op)) {
+ PyErr_BadInternalCall();
+ return -1;
+ }
+ Py_XDECREF(((PyCellObject*)op)->ob_ref);
+ Py_XINCREF(obj);
+ PyCell_SET(op, obj);
+ return 0;
+}
+
+static void
+cell_dealloc(PyCellObject *op)
+{
+ PyObject_GC_Fini(op);
+ Py_XDECREF(op->ob_ref);
+ PyObject_Del(op);
+}
+
+static int
+cell_compare(PyCellObject *a, PyCellObject *b)
+{
+ if (a->ob_ref == NULL) {
+ if (b->ob_ref == NULL)
+ return 0;
+ return -1;
+ } else if (b->ob_ref == NULL)
+ return 1;
+ return PyObject_Compare(a->ob_ref, b->ob_ref);
+}
+
+static PyObject *
+cell_repr(PyCellObject *op)
+{
+ char buf[256];
+
+ if (op->ob_ref == NULL)
+ sprintf(buf, "<cell at %p: empty>", op);
+ else
+ sprintf(buf, "<cell at %p: %.80s object at %p>",
+ op, op->ob_ref->ob_type->tp_name, op->ob_ref);
+ return PyString_FromString(buf);
+}
+
+static int
+cell_traverse(PyCellObject *op, visitproc visit, void *arg)
+{
+ if (op->ob_ref)
+ return visit(op->ob_ref, arg);
+ return 0;
+}
+
+static int
+cell_clear(PyCellObject *op)
+{
+ op->ob_ref = NULL;
+ return 0;
+}
+
+PyTypeObject PyCell_Type = {
+ PyObject_HEAD_INIT(&PyType_Type)
+ 0,
+ "cell",
+ sizeof(PyCellObject) + PyGC_HEAD_SIZE,
+ 0,
+ (destructor)cell_dealloc, /* tp_dealloc */
+ 0, /* tp_print */
+ 0, /* tp_getattr */
+ 0, /* tp_setattr */
+ (cmpfunc)cell_compare, /* tp_compare */
+ (reprfunc)cell_repr, /* tp_repr */
+ 0, /* tp_as_number */
+ 0, /* tp_as_sequence */
+ 0, /* tp_as_mapping */
+ 0, /* tp_hash */
+ 0, /* tp_call */
+ 0, /* tp_str */
+ 0, /* tp_getattro */
+ 0, /* tp_setattro */
+ 0, /* tp_as_buffer */
+ Py_TPFLAGS_DEFAULT | Py_TPFLAGS_GC, /* tp_flags */
+ 0, /* tp_doc */
+ (traverseproc)cell_traverse, /* tp_traverse */
+ (inquiry)cell_clear, /* tp_clear */
+};