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gh-134119: Fix crash from calling next() on exhausted template iterator #134120

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Merged
merged 2 commits into from
May 17, 2025
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gh-134119: Fix crash from calling next() on exhausted template iterator #134120

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gh-134119: Fix crash from calling next() on exhausted template iterator
JelleZijlstra May 17, 2025
c6d78ee
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JelleZijlstra May 17, 2025
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7 changes: 7 additions & 0 deletions Lib/test/test_string/test_templatelib.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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()
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-20-59-12.gh-issue-134119.w8expI.rst
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@@ -0,0 +1,2 @@
Fix crash when calling :func:`next` on an exhausted template string iterator.
Patch by Jelle Zijlstra.
3 changes: 3 additions & 0 deletions Objects/templateobject.c
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Expand Up @@ -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) {
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Maybe it is worth to add condition here like

if (!item || PyUnicode_GET_LENGTH(item) == 0) {
  Py_XSETREF(item, PyItem_Next(self->interpolationsiter));
  self->from_strings = 1;
}

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I think that would potentially make an API call with an exception set, which is not allowed, right?

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Yeah, good point. Thanks!

Py_SETREF(item, PyIter_Next(self->interpolationsiter));
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Should the result of PyIter_Next(self->interpolationsiter) be checked for NULL too?

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@davepeck davepeck May 17, 2025 •
edited
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That case just falls through to returning NULL safely, so it's probably fine as-is.

(The fall-through is intentional: since there is always one more string than interpolation, the first NULL returned by the iterator will always be here.)

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Py_SETREF is safe if it assigns to NULL, the behavior should be correct in that case.

self->from_strings = 1;
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