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???)

Python/bltinmodule.c

@@ -1311,6 +1311,32 @@ static char float_doc[] =
 Convert a string or number to a floating point number, if possible.";
 
 
+static PyObject *
+builtin_iter(PyObject *self, PyObject *args)
+{
+	PyObject *v, *w = NULL;
+
+	if (!PyArg_ParseTuple(args, "O|O:iter", &v, &w))
+		return NULL;
+	if (w == NULL)
+		return PyObject_GetIter(v);
+	if (!PyCallable_Check(v)) {
+		PyErr_SetString(PyExc_TypeError,
+				"iter(v, w): v must be callable");
+		return NULL;
+	}
+	return PyCallIter_New(v, w);
+}
+
+static char iter_doc[] =
+"iter(collection) -> iterator\n\
+iter(callable, sentinel) -> iterator\n\
+\n\
+Get an iterator from an object.  In the first form, the argument must\n\
+supply its own iterator, or be a sequence.\n\
+In the second form, the callable is called until it returns the sentinel.";
+
+
 static PyObject *
 builtin_len(PyObject *self, PyObject *args)
 {
@@ -2148,6 +2174,7 @@ static PyMethodDef builtin_methods[] = {
 	{"int",		builtin_int, 1, int_doc},
 	{"isinstance",  builtin_isinstance, 1, isinstance_doc},
 	{"issubclass",  builtin_issubclass, 1, issubclass_doc},
+	{"iter",	builtin_iter, 1, iter_doc},
 	{"len",		builtin_len, 1, len_doc},
 	{"list",	builtin_list, 1, list_doc},
 	{"locals",	builtin_locals, 1, locals_doc},

Python/ceval.c

@@ -381,6 +381,7 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 	/* Make it easier to find out where we are with a debugger */
 	char *filename = PyString_AsString(co->co_filename);
 #endif
+	static PyObject *nextstr;
 
 /* Code access macros */
 
@@ -416,6 +417,11 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 				     GETLOCAL(i) = value; } while (0)
 
 /* Start of code */
+	if (nextstr == NULL) {
+		nextstr = PyString_InternFromString("next");
+		if (nextstr == NULL)
+			return NULL;
+	}
 
 #ifdef USE_STACKCHECK
 	if (tstate->recursion_depth%10 == 0 && PyOS_CheckStack()) {
@@ -1875,6 +1881,41 @@ eval_code2(PyCodeObject *co, PyObject *globals, PyObject *locals,
 			JUMPTO(oparg);
 			continue;
 
+		case GET_ITER:
+			/* before: [obj]; after [getiter(obj)] */
+			v = POP();
+			x = PyObject_GetIter(v);
+			Py_DECREF(v);
+			if (x != NULL) {
+				w = x;
+				x = PyObject_GetAttr(w, nextstr);
+				Py_DECREF(w);
+				if (x != NULL) {
+					PUSH(x);
+					continue;
+				}
+			}
+			break;
+
+		case FOR_ITER:
+			/* before: [iter]; after: [iter, iter()] *or* [] */
+			v = TOP();
+			x = PyObject_CallObject(v, NULL);
+			if (x == NULL) {
+				if (PyErr_ExceptionMatches(
+					PyExc_StopIteration))
+				{
+					PyErr_Clear();
+					x = v = POP();
+					Py_DECREF(v);
+					JUMPBY(oparg);
+					continue;
+				}
+				break;
+			}
+			PUSH(x);
+			continue;
+
 		case FOR_LOOP:
 			/* for v in s: ...
 			   On entry: stack contains s, i.

Python/compile.c

@@ -1232,21 +1232,15 @@ parsestrplus(node *n)
 static void
 com_list_for(struct compiling *c, node *n, node *e, char *t)
 {
-	PyObject *v;
 	int anchor = 0;
 	int save_begin = c->c_begin;
 
 	/* list_iter: for v in expr [list_iter] */
 	com_node(c, CHILD(n, 3)); /* expr */
-	v = PyInt_FromLong(0L);
-	if (v == NULL)
-		c->c_errors++;
-	com_addoparg(c, LOAD_CONST, com_addconst(c, v));
-	com_push(c, 1);
-	Py_XDECREF(v);
+	com_addbyte(c, GET_ITER);
 	c->c_begin = c->c_nexti;
 	com_addoparg(c, SET_LINENO, n->n_lineno);
-	com_addfwref(c, FOR_LOOP, &anchor);
+	com_addfwref(c, FOR_ITER, &anchor);
 	com_push(c, 1);
 	com_assign(c, CHILD(n, 1), OP_ASSIGN, NULL);
 	c->c_loops++;
@@ -1255,7 +1249,7 @@ com_list_for(struct compiling *c, node *n, node *e, char *t)
 	com_addoparg(c, JUMP_ABSOLUTE, c->c_begin);
 	c->c_begin = save_begin;
 	com_backpatch(c, anchor);
-	com_pop(c, 2); /* FOR_LOOP has popped these */
+	com_pop(c, 1); /* FOR_ITER has popped this */
 }  
 
 static void
@@ -2873,7 +2867,6 @@ com_while_stmt(struct compiling *c, node *n)
 static void
 com_for_stmt(struct compiling *c, node *n)
 {
-	PyObject *v;
 	int break_anchor = 0;
 	int anchor = 0;
 	int save_begin = c->c_begin;
@@ -2882,15 +2875,10 @@ com_for_stmt(struct compiling *c, node *n)
 	com_addfwref(c, SETUP_LOOP, &break_anchor);
 	block_push(c, SETUP_LOOP);
 	com_node(c, CHILD(n, 3));
-	v = PyInt_FromLong(0L);
-	if (v == NULL)
-		c->c_errors++;
-	com_addoparg(c, LOAD_CONST, com_addconst(c, v));
-	com_push(c, 1);
-	Py_XDECREF(v);
+	com_addbyte(c, GET_ITER);
 	c->c_begin = c->c_nexti;
 	com_addoparg(c, SET_LINENO, n->n_lineno);
-	com_addfwref(c, FOR_LOOP, &anchor);
+	com_addfwref(c, FOR_ITER, &anchor);
 	com_push(c, 1);
 	com_assign(c, CHILD(n, 1), OP_ASSIGN, NULL);
 	c->c_loops++;
@@ -2899,7 +2887,7 @@ com_for_stmt(struct compiling *c, node *n)
 	com_addoparg(c, JUMP_ABSOLUTE, c->c_begin);
 	c->c_begin = save_begin;
 	com_backpatch(c, anchor);
-	com_pop(c, 2); /* FOR_LOOP has popped these */
+	com_pop(c, 1); /* FOR_ITER has popped this */
 	com_addbyte(c, POP_BLOCK);
 	block_pop(c, SETUP_LOOP);
 	if (NCH(n) > 8)

Python/exceptions.c

@@ -52,6 +52,7 @@ recommended that user defined class based exceptions be derived from the\n\
 Exception\n\
  |\n\
  +-- SystemExit\n\
+ +-- StopIteration\n\
  +-- StandardError\n\
  |    |\n\
  |    +-- KeyboardInterrupt\n\
@@ -369,6 +370,9 @@ StandardError__doc__[] = "Base class for all standard Python exceptions.";
 static char
 TypeError__doc__[] = "Inappropriate argument type.";
 
+static char
+StopIteration__doc__[] = "Signal the end from iterator.next().";
+
 
 
 static char
@@ -924,6 +928,7 @@ static PyMethodDef functions[] = {
 /* Global C API defined exceptions */
 
 PyObject *PyExc_Exception;
+PyObject *PyExc_StopIteration;
 PyObject *PyExc_StandardError;
 PyObject *PyExc_ArithmeticError;
 PyObject *PyExc_LookupError;
@@ -985,6 +990,8 @@ static struct {
   * The first three classes MUST appear in exactly this order
   */
  {"Exception", &PyExc_Exception},
+ {"StopIteration", &PyExc_StopIteration, &PyExc_Exception,
+  StopIteration__doc__},
  {"StandardError", &PyExc_StandardError, &PyExc_Exception,
   StandardError__doc__},
  {"TypeError", &PyExc_TypeError, 0, TypeError__doc__},

Python/import.c

@@ -43,7 +43,7 @@ extern time_t PyOS_GetLastModificationTime(char *, FILE *);
 /* XXX Perhaps the magic number should be frozen and a version field
    added to the .pyc file header? */
 /* New way to come up with the magic number: (YEAR-1995), MONTH, DAY */
-#define MAGIC (60202 | ((long)'\r'<<16) | ((long)'\n'<<24))
+#define MAGIC (60420 | ((long)'\r'<<16) | ((long)'\n'<<24))
 
 /* Magic word as global; note that _PyImport_Init() can change the
    value of this global to accommodate for alterations of how the