gh-134119: Fix crash from calling next() on exhausted template iterat…

…or (GH-134120)

(cherry picked from commit fc7f4c3)

Co-authored-by: Jelle Zijlstra <[email protected]>
Co-authored-by: Bénédikt Tran <[email protected]>

Lib/test/test_string/test_templatelib.py

@@ -148,6 +148,13 @@ def test_iter(self):
         self.assertEqual(res[1].format_spec, '')
         self.assertEqual(res[2], ' yz')
 
+    def test_exhausted(self):
+        # See https://github.com/python/cpython/issues/134119.
+        template_iter = iter(t"{1}")
+        self.assertIsInstance(next(template_iter), Interpolation)
+        self.assertRaises(StopIteration, next, template_iter)
+        self.assertRaises(StopIteration, next, template_iter)
+
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-20-59-12.gh-issue-134119.w8expI.rst

@@ -0,0 +1,2 @@
+Fix crash when calling :func:`next` on an exhausted template string iterator.
+Patch by Jelle Zijlstra.

Objects/templateobject.c

@@ -23,6 +23,9 @@ templateiter_next(PyObject *op)
     if (self->from_strings) {
         item = PyIter_Next(self->stringsiter);
         self->from_strings = 0;
+        if (item == NULL) {
+            return NULL;
+        }
         if (PyUnicode_GET_LENGTH(item) == 0) {
             Py_SETREF(item, PyIter_Next(self->interpolationsiter));
             self->from_strings = 1;