diff --git a/Include/abstract.h b/Include/abstract.h
index b3e7030fa19..f4e31c77c0c 100644
--- a/Include/abstract.h
+++ b/Include/abstract.h
@@ -392,7 +392,7 @@ xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx*/
        /*
          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);
diff --git a/Lib/test/test_iterlen.py b/Lib/test/test_iterlen.py
index 72e92a58013..2123501bcb5 100644
--- a/Lib/test/test_iterlen.py
+++ b/Lib/test/test_iterlen.py
@@ -195,6 +195,29 @@ class TestListReversed(TestInvariantWithoutMutations):
         d.extend(range(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())
+        b = bytearray(range(10))
+        self.assertRaises(RuntimeError, b.extend, BadLen())
+        self.assertRaises(RuntimeError, b.extend, BadLengthHint())
 
 def test_main():
     unittests = [
@@ -210,6 +233,7 @@ def test_main():
         TestSet,
         TestList,
         TestListReversed,
+        TestLengthHintExceptions,
     ]
     support.run_unittest(*unittests)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 71f9ca19dbf..54260466306 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -17,6 +17,9 @@ Core and Builtins
   the type definition cmpfunc.  The tp_compare slot has been renamed
   to tp_reserved, and is reserved for future usage.
 
+- Issue 1242657: the __len__() and __length_hint__() calls in several tools
+  were suppressing all exceptions.  These include list() and bytearray().
+
 - Issue #4707: round(x, n) now returns an integer if x is an integer.
   Previously it returned a float.
 
diff --git a/Objects/abstract.c b/Objects/abstract.c
index e42008a1488..86e2c394804 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -67,8 +67,8 @@ PyObject_Length(PyObject *o)
 
 /* 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
@@ -82,29 +82,32 @@ _PyObject_LengthHint(PyObject *o, Py_ssize_t defaultvalue)
 	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 = PyUnicode_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) {
+		if (!PyErr_ExceptionMatches(PyExc_TypeError) &&
+			!PyErr_ExceptionMatches(PyExc_AttributeError))
+				return -1;
+		PyErr_Clear();
+		return defaultvalue;
+	}
 	rv = PyLong_AsSsize_t(ro);
 	Py_DECREF(ro);
-	if (rv >= 0)
-		return rv;
-
-defaultcase:
-	if (PyErr_Occurred())
-		PyErr_Clear();
-	return defaultvalue;
+	return rv;
 }
 
 PyObject *
@@ -1742,7 +1745,7 @@ PySequence_Tuple(PyObject *v)
 {
 	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)
@@ -1767,6 +1770,8 @@ PySequence_Tuple(PyObject *v)
 
 	/* 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;
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 9e93ddaf37c..868013a8905 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -2616,6 +2616,10 @@ bytes_extend(PyByteArrayObject *self, PyObject *arg)
 
     /* 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)
diff --git a/Objects/listobject.c b/Objects/listobject.c
index b8343631f44..2bab4ef26b3 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -802,6 +802,10 @@ listextend(PyListObject *self, PyObject *b)
 
 	/* 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) {