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???)

Include/Python.h

@@ -82,6 +82,7 @@
 #include "traceback.h"
 #include "sliceobject.h"
 #include "cellobject.h"
+#include "iterobject.h"
 
 #include "codecs.h"
 #include "pyerrors.h"

Include/abstract.h

@@ -470,6 +470,11 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx*/
 
        */
 
+     DL_IMPORT(PyObject *) PyObject_GetIter(PyObject *);
+     /* Takes an object and returns an iterator for it.
+        This is typically a new iterator but if the argument
+	is an iterator, this returns itself. */
+
 /*  Number Protocol:*/
 
      DL_IMPORT(int) PyNumber_Check(PyObject *o);

Include/object.h

@@ -200,6 +200,7 @@ typedef int (*cmpfunc)(PyObject *, PyObject *);
 typedef PyObject *(*reprfunc)(PyObject *);
 typedef long (*hashfunc)(PyObject *);
 typedef PyObject *(*richcmpfunc) (PyObject *, PyObject *, int);
+typedef PyObject *(*getiterfunc) (PyObject *);
 
 typedef struct _typeobject {
 	PyObject_VAR_HEAD
@@ -249,8 +250,11 @@ typedef struct _typeobject {
 	/* weak reference enabler */
 	long tp_weaklistoffset;
 
+	/* Iterators */
+	getiterfunc tp_iter;
+
 #ifdef COUNT_ALLOCS
-	/* these must be last */
+	/* these must be last and never explicitly initialized */
 	int tp_alloc;
 	int tp_free;
 	int tp_maxalloc;
@@ -342,20 +346,22 @@ given type object has a specified feature.
 /* PyNumberMethods do their own coercion */
 #define Py_TPFLAGS_CHECKTYPES (1L<<4)
 
+/* tp_richcompare is defined */
 #define Py_TPFLAGS_HAVE_RICHCOMPARE (1L<<5)
 
 /* Objects which are weakly referencable if their tp_weaklistoffset is >0 */
-/* XXX Should this have the same value as Py_TPFLAGS_HAVE_RICHCOMPARE?
- * These both indicate a feature that appeared in the same alpha release.
- */
 #define Py_TPFLAGS_HAVE_WEAKREFS (1L<<6)
 
+/* tp_iter is defined */
+#define Py_TPFLAGS_HAVE_ITER (1L<<7)
+
 #define Py_TPFLAGS_DEFAULT  ( \
                              Py_TPFLAGS_HAVE_GETCHARBUFFER | \
                              Py_TPFLAGS_HAVE_SEQUENCE_IN | \
                              Py_TPFLAGS_HAVE_INPLACEOPS | \
                              Py_TPFLAGS_HAVE_RICHCOMPARE | \
                              Py_TPFLAGS_HAVE_WEAKREFS | \
+                             Py_TPFLAGS_HAVE_ITER | \
                             0)
 
 #define PyType_HasFeature(t,f)  (((t)->tp_flags & (f)) != 0)

Include/opcode.h

@@ -53,6 +53,7 @@ extern "C" {
 #define BINARY_XOR	65
 #define BINARY_OR	66
 #define INPLACE_POWER	67
+#define GET_ITER	68
 
 #define PRINT_EXPR	70
 #define PRINT_ITEM	71
@@ -80,6 +81,7 @@ extern "C" {
 #define STORE_NAME	90	/* Index in name list */
 #define DELETE_NAME	91	/* "" */
 #define UNPACK_SEQUENCE	92	/* Number of sequence items */
+#define FOR_ITER	93
 
 #define STORE_ATTR	95	/* Index in name list */
 #define DELETE_ATTR	96	/* "" */

Include/pyerrors.h

@@ -24,6 +24,7 @@ DL_IMPORT(void) PyErr_NormalizeException(PyObject**, PyObject**, PyObject**);
 /* Predefined exceptions */
 
 extern DL_IMPORT(PyObject *) PyExc_Exception;
+extern DL_IMPORT(PyObject *) PyExc_StopIteration;
 extern DL_IMPORT(PyObject *) PyExc_StandardError;
 extern DL_IMPORT(PyObject *) PyExc_ArithmeticError;
 extern DL_IMPORT(PyObject *) PyExc_LookupError;