Generalize list(seq) to work with iterators. This also generalizes list()

to no longer insist that len(seq) be defined.
NEEDS DOC CHANGES.
This is meant to be a model for how other functions of this ilk (max,
filter, etc) can be generalized similarly.  Feel encouraged to grab your
favorite and convert it!
Note some cute consequences:
    list(file) == file.readlines() == list(file.xreadlines())
    list(dict) == dict.keys()
    list(dict.iteritems()) = dict.items()
    list(xrange(i, j, k)) == range(i, j, k)

Lib/test/test_iter.py

@@ -243,4 +243,36 @@ class TestCase(unittest.TestCase):
             except OSError:
                 pass
 
+    # Test list()'s use of iterators.
+    def test_builtin_list(self):
+        self.assertEqual(list(SequenceClass(5)), range(5))
+        self.assertEqual(list(SequenceClass(0)), [])
+        self.assertEqual(list(()), [])
+        self.assertEqual(list(range(10, -1, -1)), range(10, -1, -1))
+
+        d = {"one": 1, "two": 2, "three": 3}
+        self.assertEqual(list(d), d.keys())
+
+        self.assertRaises(TypeError, list, list)
+        self.assertRaises(TypeError, list, 42)
+
+        f = open(TESTFN, "w")
+        try:
+            for i in range(5):
+                f.write("%d\n" % i)
+        finally:
+            f.close()
+        f = open(TESTFN, "r")
+        try:
+            self.assertEqual(list(f), ["0\n", "1\n", "2\n", "3\n", "4\n"])
+            f.seek(0, 0)
+            self.assertEqual(list(f.xreadlines()),
+                             ["0\n", "1\n", "2\n", "3\n", "4\n"])
+        finally:
+            f.close()
+            try:
+                unlink(TESTFN)
+            except OSError:
+                pass
+
 run_unittest(TestCase)

Misc/NEWS

@@ -1,3 +1,13 @@
+What's New in Python 2.2a0?
+===========================
+
+Core
+
+- The following functions were generalized to work nicely with iterator
+  arguments:
+  list()
+
+
 What's New in Python 2.1 (final)?
 =================================
 

Objects/abstract.c

@@ -1236,52 +1236,78 @@ PySequence_Tuple(PyObject *v)
 PyObject *
 PySequence_List(PyObject *v)
 {
-	PySequenceMethods *m;
+	PyObject *it;      /* iter(v) */
+	PyObject *result;  /* result list */
+	int n;		   /* guess for result list size */
+	int i;
 
 	if (v == NULL)
 		return null_error();
 
+	/* Special-case list(a_list), for speed. */
 	if (PyList_Check(v))
 		return PyList_GetSlice(v, 0, PyList_GET_SIZE(v));
 
-	m = v->ob_type->tp_as_sequence;
-	if (m && m->sq_item) {
-		int i;
-		PyObject *l;
-		int n = PySequence_Size(v);
+	/* Get iterator.  There may be some low-level efficiency to be gained
+	 * by caching the tp_iternext slot instead of using PyIter_Next()
+	 * later, but premature optimization is the root etc.
+	 */
+	it = PyObject_GetIter(v);
+	if (it == NULL)
+		return NULL;
+
+	/* Guess a result list size. */
+	n = -1;	 /* unknown */
+	if (PySequence_Check(v) &&
+	    v->ob_type->tp_as_sequence->sq_length) {
+		n = PySequence_Size(v);
 		if (n < 0)
-			return NULL;
-		l = PyList_New(n);
-		if (l == NULL)
-			return NULL;
-		for (i = 0; ; i++) {
-			PyObject *item = (*m->sq_item)(v, i);
-			if (item == NULL) {
-				if (PyErr_ExceptionMatches(PyExc_IndexError))
+			PyErr_Clear();
+	}
+	if (n < 0)
+		n = 8;	/* arbitrary */
+	result = PyList_New(n);
+	if (result == NULL) {
+		Py_DECREF(it);
+		return NULL;
+	}
+
+	/* Run iterator to exhaustion. */
+	for (i = 0; ; i++) {
+		PyObject *item = PyIter_Next(it);
+		if (item == NULL) {
+			/* We're out of here in any case, but if this is a
+			 * StopIteration exception it's expected, but if
+			 * any other kind of exception it's an error.
+			 */
+			if (PyErr_Occurred()) {
+				if (PyErr_ExceptionMatches(PyExc_StopIteration))
 					PyErr_Clear();
 				else {
-					Py_DECREF(l);
-					l = NULL;
+					Py_DECREF(result);
+					result = NULL;
 				}
-				break;
-			}
-			if (i < n)
-				PyList_SET_ITEM(l, i, item);
-			else if (PyList_Append(l, item) < 0) {
-				Py_DECREF(l);
-				l = NULL;
-				break;
 			}
+			break;
 		}
-		if (i < n && l != NULL) {
-			if (PyList_SetSlice(l, i, n, (PyObject *)NULL) != 0) {
-				Py_DECREF(l);
-				l = NULL;
-			}
+		if (i < n)
+			PyList_SET_ITEM(result, i, item);
+		else if (PyList_Append(result, item) < 0) {
+			Py_DECREF(result);
+			result = NULL;
+			break;
 		}
-		return l;
 	}
-	return type_error("list() argument must be a sequence");
+
+	/* Cut back result list if initial guess was too large. */
+	if (i < n && result != NULL) {
+		if (PyList_SetSlice(result, i, n, (PyObject *)NULL) != 0) {
+			Py_DECREF(result);
+			result = NULL;
+		}
+	}
+	Py_DECREF(it);
+	return result;
 }
 
 PyObject *