fix: handle exhausted iterators

Author: KowalskiThomas

Lib/test/test_itertools.py

@@ -1223,6 +1223,14 @@ def __index__(self):
         self.assertEqual(islice(range(10), 3, None, 2).__length_hint__(), 4)
         self.assertEqual(islice(range(10), 2, 2, 1).__length_hint__(), 0)
 
+        it = islice(iter(range(5)), None)
+        list(it)  # exhaust
+        self.assertEqual(it.__length_hint__(), 0)
+
+        it = islice(iter(range(5)), 3)
+        list(it)  # exhaust
+        self.assertEqual(it.__length_hint__(), 0)
+
     def test_takewhile(self):
         data = [1, 3, 5, 20, 2, 4, 6, 8]
         self.assertEqual(list(takewhile(underten, data)), [1, 3, 5])

Modules/itertoolsmodule.c

@@ -1665,7 +1665,7 @@ islice_length_hint(PyObject *op, PyObject *Py_UNUSED(dummy))
 {
     isliceobject *lz = isliceobject_CAST(op);
 
-    if (lz->stop >= 0 && lz->stop <= lz->next) {
+    if (lz->it == NULL || (lz->stop >= 0 && lz->stop <= lz->next)) {
         return PyLong_FromSsize_t(0);
     }