Index: Python/bltinmodule.c
===================================================================
--- Python/bltinmodule.c	(revision 68609)
+++ Python/bltinmodule.c	(working copy)
@@ -268,6 +268,8 @@
 
 	/* Guess a result list size. */
 	len = _PyObject_LengthHint(seq, 8);
+	if (len == -1)
+		goto Fail_it;
 
 	/* Get a result list. */
 	if (PyList_Check(seq) && seq->ob_refcnt == 1) {
@@ -977,6 +979,8 @@
 
 		/* Update len. */
 		curlen = _PyObject_LengthHint(curseq, 8);
+		if (curlen == -1)
+			goto Fail_2;
 		if (curlen > len)
 			len = curlen;
 	}
@@ -2381,10 +2385,12 @@
 	/* Guess at result length:  the shortest of the input lengths.
 	   If some argument refuses to say, we refuse to guess too, lest
 	   an argument like xrange(sys.maxint) lead us astray.*/
-	len = -1;	/* unknown */
+	len = -2;	/* unknown */
 	for (i = 0; i < itemsize; ++i) {
 		PyObject *item = PyTuple_GET_ITEM(args, i);
-		Py_ssize_t thislen = _PyObject_LengthHint(item, -1);
+		Py_ssize_t thislen = _PyObject_LengthHint(item, -2);
+		if (thislen == -1)
+			return NULL;
 		if (thislen < 0) {
 			len = -1;
 			break;
Index: Include/abstract.h
===================================================================
--- Include/abstract.h	(revision 68609)
+++ Include/abstract.h	(working copy)
@@ -438,7 +438,7 @@
        /*
          Guess the size of object o using len(o) or o.__length_hint__().
          If neither of those return a non-negative value, then return the
-         default value.  This function never fails. All exceptions are cleared.
+         default value.  If one of the calls fails, this function returns -1.
        */
 
      PyAPI_FUNC(PyObject *) PyObject_GetItem(PyObject *o, PyObject *key);
Index: Objects/abstract.c
===================================================================
--- Objects/abstract.c	(revision 68609)
+++ Objects/abstract.c	(working copy)
@@ -85,8 +85,8 @@
 
 /* The length hint function returns a non-negative value from o.__len__()
    or o.__length_hint__().  If those methods aren't found or return a negative
-   value, then the defaultvalue is returned.  This function never fails. 
-   Accordingly, it will mask exceptions raised in either method.
+   value, then the defaultvalue is returned.  If one of the calls fails,
+   this function returns -1.
 */
 
 Py_ssize_t
@@ -100,29 +100,31 @@
 	rv = PyObject_Size(o);
 	if (rv >= 0)
 		return rv;
-	if (PyErr_Occurred())
+	if (PyErr_Occurred()) {
+		if (!PyErr_ExceptionMatches(PyExc_TypeError) &&
+			!PyErr_ExceptionMatches(PyExc_AttributeError))
+				return -1;
 		PyErr_Clear();
+	}
 
 	/* cache a hashed version of the attribute string */
 	if (hintstrobj == NULL) {
 		hintstrobj = PyString_InternFromString("__length_hint__");
 		if (hintstrobj == NULL)
-			goto defaultcase;
+			return -1;
 	}
 
 	/* try o.__length_hint__() */
 	ro = PyObject_CallMethodObjArgs(o, hintstrobj, NULL);
-	if (ro == NULL)
-		goto defaultcase;
+	if (ro == NULL && PyErr_Occurred()) {
+		if (!PyErr_ExceptionMatches(PyExc_TypeError) &&
+			!PyErr_ExceptionMatches(PyExc_AttributeError))
+				return -1;
+		PyErr_Clear();
+	}
 	rv = PyInt_AsLong(ro);
 	Py_DECREF(ro);
-	if (rv >= 0)
-		return rv;
-
-defaultcase:
-	if (PyErr_Occurred())
-		PyErr_Clear();
-	return defaultvalue;
+	return rv;
 }
 
 PyObject *
@@ -2114,7 +2116,7 @@
 {
 	PyObject *it;  /* iter(v) */
 	Py_ssize_t n;         /* guess for result tuple size */
-	PyObject *result;
+	PyObject *result = NULL;
 	Py_ssize_t j;
 
 	if (v == NULL)
@@ -2139,6 +2141,8 @@
 
 	/* Guess result size and allocate space. */
 	n = _PyObject_LengthHint(v, 10);
+	if (n == -1)
+		goto Fail;
 	result = PyTuple_New(n);
 	if (result == NULL)
 		goto Fail;
Index: Objects/listobject.c
===================================================================
--- Objects/listobject.c	(revision 68609)
+++ Objects/listobject.c	(working copy)
@@ -838,6 +838,10 @@
 
 	/* Guess a result list size. */
 	n = _PyObject_LengthHint(b, 8);
+	if (n == -1) {
+		Py_DECREF(it);
+		return NULL;
+	}
 	m = Py_SIZE(self);
 	mn = m + n;
 	if (mn >= m) {
Index: Objects/bytearrayobject.c
===================================================================
--- Objects/bytearrayobject.c	(revision 68609)
+++ Objects/bytearrayobject.c	(working copy)
@@ -2687,6 +2687,10 @@
 
     /* Try to determine the length of the argument. 32 is abitrary. */
     buf_size = _PyObject_LengthHint(arg, 32);
+	if (buf_size == -1) {
+		Py_DECREF(it);
+		return NULL;
+	}	
 
     bytes_obj = PyByteArray_FromStringAndSize(NULL, buf_size);
     if (bytes_obj == NULL)
Index: Lib/test/test_iterlen.py
===================================================================
--- Lib/test/test_iterlen.py	(revision 68609)
+++ Lib/test/test_iterlen.py	(working copy)
@@ -195,6 +195,33 @@
         d.extend(xrange(20))
         self.assertEqual(len(it), 0)
 
+## -- Check to make sure exceptions are not suppressed by __length_hint__()
+
+
+class BadLen(object):
+   def __iter__(self): return iter(range(10))
+   def __len__(self):
+       raise RuntimeError('hello')
+
+class BadLengthHint(object):
+   def __iter__(self): return iter(range(10))
+   def __length_hint__(self):
+       raise RuntimeError('hello')
+
+class TestLengthHintExceptions(unittest.TestCase):
+
+    def test_issue1242657(self):
+        self.assertRaises(RuntimeError, list, BadLen())
+        self.assertRaises(RuntimeError, list, BadLengthHint())
+        self.assertRaises(RuntimeError, [].extend, BadLen())
+        self.assertRaises(RuntimeError, [].extend, BadLengthHint())      
+        self.assertRaises(RuntimeError, zip, BadLen())
+        self.assertRaises(RuntimeError, zip, BadLengthHint())        
+        self.assertRaises(RuntimeError, filter, None, BadLen())
+        self.assertRaises(RuntimeError, filter, None, BadLengthHint())        
+        self.assertRaises(RuntimeError, map, chr, BadLen())
+        self.assertRaises(RuntimeError, map, chr, BadLengthHint())
+
 def test_main():
     unittests = [
         TestRepeat,
@@ -209,6 +236,7 @@
         TestSet,
         TestList,
         TestListReversed,
+        TestLengthHintExceptions,
     ]
     test_support.run_unittest(*unittests)