Iterators phase 1. This comprises:

new slot tp_iter in type object, plus new flag Py_TPFLAGS_HAVE_ITER
new C API PyObject_GetIter(), calls tp_iter
new builtin iter(), with two forms: iter(obj), and iter(function, sentinel)
new internal object types iterobject and calliterobject
new exception StopIteration
new opcodes for "for" loops, GET_ITER and FOR_ITER (also supported by dis.py)
new magic number for .pyc files
new special method for instances: __iter__() returns an iterator
iteration over dictionaries: "for x in dict" iterates over the keys
iteration over files: "for x in file" iterates over lines

TODO:

documentation
test suite
decide whether to use a different way to spell iter(function, sentinal)
decide whether "for key in dict" is a good idea
use iterators in map/filter/reduce, min/max, and elsewhere (in/not in?)
speed tuning (make next() a slot tp_next???)

Objects/abstract.c

@@ -1738,3 +1738,20 @@ PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 
 	return retval;
 }
+
+PyObject *
+PyObject_GetIter(PyObject *o)
+{
+	PyTypeObject *t = o->ob_type;
+	getiterfunc f = NULL;
+	if (PyType_HasFeature(t, Py_TPFLAGS_HAVE_ITER))
+		f = t->tp_iter;
+	if (f == NULL) {
+		if (PySequence_Check(o))
+			return PyIter_New(o);
+		PyErr_SetString(PyExc_TypeError, "iter() of non-sequence");
+		return NULL;
+	}
+	else
+		return (*f)(o);
+}

Objects/classobject.c

@@ -848,7 +848,7 @@ instance_traverse(PyInstanceObject *o, visitproc visit, void *arg)
 	return 0;
 }
 
-static PyObject *getitemstr, *setitemstr, *delitemstr, *lenstr;
+static PyObject *getitemstr, *setitemstr, *delitemstr, *lenstr, *iterstr;
 
 static int
 instance_length(PyInstanceObject *inst)
@@ -1712,6 +1712,32 @@ instance_richcompare(PyObject *v, PyObject *w, int op)
 }
 
 
+/* Get the iterator */
+static PyObject *
+instance_getiter(PyInstanceObject *self)
+{
+	PyObject *func;
+
+	if (iterstr == NULL)
+		iterstr = PyString_InternFromString("__iter__");
+	if (getitemstr == NULL)
+		getitemstr = PyString_InternFromString("__getitem__");
+
+	if ((func = instance_getattr(self, iterstr)) != NULL) {
+		PyObject *res = PyEval_CallObject(func, (PyObject *)NULL);
+		Py_DECREF(func);
+		return res;
+	}
+	PyErr_Clear();
+	if ((func = instance_getattr(self, getitemstr)) == NULL) {
+		PyErr_SetString(PyExc_TypeError, "iter() of non-sequence");
+		return NULL;
+	}
+	Py_DECREF(func);
+	return PyIter_New((PyObject *)self);
+}
+
+
 static PyNumberMethods instance_as_number = {
 	(binaryfunc)instance_add,		/* nb_add */
 	(binaryfunc)instance_sub,		/* nb_subtract */
@@ -1775,7 +1801,8 @@ PyTypeObject PyInstance_Type = {
 	(traverseproc)instance_traverse,	/* tp_traverse */
 	0,					/* tp_clear */
 	instance_richcompare,			/* tp_richcompare */
- 	offsetof(PyInstanceObject, in_weakreflist) /* tp_weaklistoffset */
+ 	offsetof(PyInstanceObject, in_weakreflist), /* tp_weaklistoffset */
+	(getiterfunc)instance_getiter,		/* tp_iter */
 };
 
 

Objects/dictobject.c

@@ -1324,6 +1324,8 @@ static PySequenceMethods dict_as_sequence = {
 	0,					/* sq_inplace_repeat */
 };
 
+staticforward PyObject *dictiter_new(dictobject *);
+
 PyTypeObject PyDict_Type = {
 	PyObject_HEAD_INIT(&PyType_Type)
 	0,
@@ -1350,6 +1352,8 @@ PyTypeObject PyDict_Type = {
 	(traverseproc)dict_traverse,		/* tp_traverse */
 	(inquiry)dict_tp_clear,			/* tp_clear */
 	0,					/* tp_richcompare */
+	0,					/* tp_weaklistoffset */
+	(getiterfunc)dictiter_new,		/* tp_iter */
 };
 
 /* For backward compatibility with old dictionary interface */
@@ -1392,3 +1396,102 @@ PyDict_DelItemString(PyObject *v, char *key)
 	Py_DECREF(kv);
 	return err;
 }
+
+/* Dictionary iterator type */
+
+extern PyTypeObject PyDictIter_Type; /* Forward */
+
+typedef struct {
+	PyObject_HEAD
+	dictobject *di_dict;
+	int di_size;
+	int di_pos;
+} dictiterobject;
+
+static PyObject *
+dictiter_new(dictobject *dict)
+{
+	dictiterobject *di;
+	di = PyObject_NEW(dictiterobject, &PyDictIter_Type);
+	if (di == NULL)
+		return NULL;
+	Py_INCREF(dict);
+	di->di_dict = dict;
+	di->di_size = dict->ma_size;
+	di->di_pos = 0;
+	return (PyObject *)di;
+}
+
+static void
+dictiter_dealloc(dictiterobject *di)
+{
+	Py_DECREF(di->di_dict);
+	PyObject_DEL(di);
+}
+
+static PyObject *
+dictiter_next(dictiterobject *di, PyObject *args)
+{
+	PyObject *key;
+	if (di->di_size != di->di_dict->ma_size) {
+		PyErr_SetString(PyExc_RuntimeError,
+				"dictionary changed size during iteration");
+		return NULL;
+	}
+	if (PyDict_Next((PyObject *)(di->di_dict), &di->di_pos, &key, NULL)) {
+		Py_INCREF(key);
+		return key;
+	}
+	PyErr_SetObject(PyExc_StopIteration, Py_None);
+	return NULL;
+}
+
+static PyObject *
+dictiter_getiter(PyObject *it)
+{
+	Py_INCREF(it);
+	return it;
+}
+
+static PyMethodDef dictiter_methods[] = {
+	{"next",	(PyCFunction)dictiter_next,	METH_VARARGS,
+	 "it.next() -- get the next value, or raise StopIteration"},
+	{NULL,		NULL}		/* sentinel */
+};
+
+static PyObject *
+dictiter_getattr(dictiterobject *it, char *name)
+{
+	return Py_FindMethod(dictiter_methods, (PyObject *)it, name);
+}
+
+PyTypeObject PyDictIter_Type = {
+	PyObject_HEAD_INIT(&PyType_Type)
+	0,					/* ob_size */
+	"dictionary-iterator",			/* tp_name */
+	sizeof(dictiterobject),			/* tp_basicsize */
+	0,					/* tp_itemsize */
+	/* methods */
+	(destructor)dictiter_dealloc, 		/* tp_dealloc */
+	0,					/* tp_print */
+	(getattrfunc)dictiter_getattr,		/* tp_getattr */
+	0,					/* tp_setattr */
+	0,					/* tp_compare */
+	0,					/* 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,			/* tp_flags */
+ 	0,					/* tp_doc */
+ 	0,					/* tp_traverse */
+ 	0,					/* tp_clear */
+	0,					/* tp_richcompare */
+	0,					/* tp_weaklistoffset */
+	(getiterfunc)dictiter_getiter,		/* tp_iter */
+};

Objects/stringobject.c

@@ -3232,6 +3232,8 @@ PyString_Fini(void)
 void _Py_ReleaseInternedStrings(void)
 {
 	if (interned) {
+		fprintf(stderr, "releasing interned strings\n");
+		PyDict_Clear(interned);
 		Py_DECREF(interned);
 		interned = NULL;
 	}