Fix exception handling in itertools.izip_longest().

Lib/test/test_itertools.py

@@ -571,6 +571,46 @@ class TestBasicOps(unittest.TestCase):
         ids = map(id, list(izip_longest('abc', 'def')))
         self.assertEqual(len(dict.fromkeys(ids)), len(ids))
 
+    def test_bug_7244(self):
+
+        class Repeater(object):
+            # this class is similar to itertools.repeat
+            def __init__(self, o, t, e):
+                self.o = o
+                self.t = int(t)
+                self.e = e
+            def __iter__(self): # its iterator is itself
+                return self
+            def next(self):
+                if self.t > 0:
+                    self.t -= 1
+                    return self.o
+                else:
+                    raise self.e
+
+        # Formerly this code in would fail in debug mode
+        # with Undetected Error and Stop Iteration
+        r1 = Repeater(1, 3, StopIteration)
+        r2 = Repeater(2, 4, StopIteration)
+        def run(r1, r2):
+            result = []
+            for i, j in izip_longest(r1, r2, fillvalue=0):
+                with test_support.captured_output('stdout'):
+                    print (i, j)
+                result.append((i, j))
+            return result
+        self.assertEqual(run(r1, r2), [(1,2), (1,2), (1,2), (0,2)])
+
+        # Formerly, the RuntimeError would be lost
+        # and StopIteration would stop as expected
+        r1 = Repeater(1, 3, RuntimeError)
+        r2 = Repeater(2, 4, StopIteration)
+        it = izip_longest(r1, r2, fillvalue=0)
+        self.assertEqual(next(it), (1, 2))
+        self.assertEqual(next(it), (1, 2))
+        self.assertEqual(next(it), (1, 2))
+        self.assertRaises(RuntimeError, next, it)
+
     def test_product(self):
         for args, result in [
             ([], [()]),                     # zero iterables

Misc/NEWS

@@ -15,6 +15,9 @@ Core and Builtins
 - Remove length limitation when constructing a complex number from a
   unicode string.
 
+- Issue #7244: itertools.izip_longest() no longer ignores exceptions
+  raised during the formation of an output tuple.
+
 - Issue #1087418: Boost performance of bitwise operations for longs.
 
 - Issue #1722344: threading._shutdown() is now called in Py_Finalize(), which

Modules/itertoolsmodule.c

@@ -3884,10 +3884,11 @@ izip_longest_next(iziplongestobject *lz)
                                 Py_INCREF(lz->fillvalue);
                                 item = lz->fillvalue;
                         } else {
-                                item = (*Py_TYPE(it)->tp_iternext)(it);
+                                item = PyIter_Next(it);
                                 if (item == NULL) {
                                         lz->numactive -= 1;
-                                        if (lz->numactive == 0) {
+                                        if (lz->numactive == 0 || PyErr_Occurred()) {
+                                                lz->numactive = 0;
                                                 Py_DECREF(result);
                                                 return NULL;
                                         } else {
@@ -3912,10 +3913,11 @@ izip_longest_next(iziplongestobject *lz)
                                 Py_INCREF(lz->fillvalue);
                                 item = lz->fillvalue;
                         } else {
-                                item = (*Py_TYPE(it)->tp_iternext)(it);
+                                item = PyIter_Next(it);
                                 if (item == NULL) {
                                         lz->numactive -= 1;
-                                        if (lz->numactive == 0) {
+                                        if (lz->numactive == 0 || PyErr_Occurred()) {
+                                                lz->numactive = 0;
                                                 Py_DECREF(result);
                                                 return NULL;
                                         } else {