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.

Include/Python.h

@@ -81,6 +81,7 @@
 #include "cobject.h"
 #include "traceback.h"
 #include "sliceobject.h"
+#include "cellobject.h"
 
 #include "codecs.h"
 #include "pyerrors.h"

Include/cellobject.h

+/* Cell object interface */
+
+#ifndef Py_CELLOBJECT_H
+#define Py_CELLOBJECT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct {
+	PyObject_VAR_HEAD
+	PyObject *ob_ref;
+} PyCellObject;
+
+extern DL_IMPORT(PyTypeObject) PyCell_Type;
+
+#define PyCell_Check(op) ((op)->ob_type == &PyCell_Type)
+
+extern DL_IMPORT(PyObject *) PyCell_New(PyObject *);
+extern DL_IMPORT(PyObject *) PyCell_Get(PyObject *);
+extern DL_IMPORT(int) PyCell_Set(PyObject *, PyObject *);
+
+#define PyCell_GET(op) (((PyCellObject *)(op))->ob_ref)
+#define PyCell_SET(op, v) (((PyCellObject *)(op))->ob_ref = v)
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_TUPLEOBJECT_H */

Makefile.pre.in

@@ -236,6 +236,7 @@ PYTHON_OBJS=	\
 OBJECT_OBJS=	\
 		Objects/abstract.o \
 		Objects/bufferobject.o \
+		Objects/cellobject.o \
 		Objects/classobject.o \
 		Objects/cobject.o \
 		Objects/complexobject.o \

Objects/cellobject.c

+/* 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 */
+};