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gh-142732: Prevent reentrancy in itertools.zip_longest #150675

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gh-142732: Prevent reentrancy in itertools.zip_longest #150675

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gh-142732: Prevent reentrancy in `functools.zip_longest`
brijkapadia May 31, 2026
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Merge branch 'main' into itertools-reentrancy
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19 changes: 19 additions & 0 deletions Lib/test/test_itertools.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2488,6 +2488,25 @@ def __eq__(self, other):
for j in range(2):
next(g, None) # shouldn't crash

def test_zip_longest_reentrancy(self):
class Reenter:
def __iter__(self):
return self

def __next__(self):
z = self.zip_longest
if z is None:
raise StopIteration
self.zip_longest = None
next(z, None)
raise StopIteration

driver = Reenter()
driver.zip_longest = itertools.zip_longest(itertools.chain(driver), itertools.repeat(None))

with self.assertRaises(ValueError):
next(driver.zip_longest)


class SubclassWithKwargsTest(unittest.TestCase):
def test_keywords_in_subclass(self):
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2026-05-31-16-58-14.gh-issue-142732.6Q80QA.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Now :func:`itertools.zip_longest` raises a :exc:`ValueError` when a reentrancy is detected.
32 changes: 25 additions & 7 deletions Modules/itertoolsmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3847,6 +3847,7 @@ struct ziplongestobject {
PyObject *ittuple; /* tuple of iterators */
PyObject *result;
PyObject *fillvalue;
uint8_t running;
};

#define ziplongestobject_CAST(op) ((ziplongestobject *)(op))
Expand Down Expand Up @@ -3916,6 +3917,7 @@ zip_longest_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
lz->numactive = tuplesize;
lz->result = result;
lz->fillvalue = Py_NewRef(fillvalue);
lz->running = 0;
return (PyObject *)lz;
}

Expand Down Expand Up @@ -3960,10 +3962,21 @@ zip_longest_next_impl(ziplongestobject *self)
PyObject *item;
PyObject *olditem;

if (tuplesize == 0)
return NULL;
if (lz->numactive == 0)
if (lz->running == 1) {
PyErr_SetString(PyExc_ValueError,

@eendebakpt eendebakpt May 31, 2026

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itertools.tee raises a RuntimeError, but generators raise a ValueError on re-entrant calls. In #150589 I picked RuntimeError for pairwise re-entrant calls.

I have no strong opinion on the type of error, but lets make it an informed decision and then be consistent. @rhettinger

@brijkapadia brijkapadia Jul 18, 2026

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@eendebakpt @rhettinger Has there been any update yet on what error is best to raise?

"zip_longest() iterator already executing");
return NULL;
}
lz->running = 1;

if (tuplesize == 0) {
result = NULL;
goto done;
}
if (lz->numactive == 0) {
result = NULL;
goto done;
}
if (_PyObject_IsUniquelyReferenced(result)) {
Py_INCREF(result);
for (i=0 ; i < tuplesize ; i++) {
Expand All @@ -3977,7 +3990,8 @@ zip_longest_next_impl(ziplongestobject *self)
if (lz->numactive == 0 || PyErr_Occurred()) {
lz->numactive = 0;
Py_DECREF(result);
return NULL;
result = NULL;
goto done;
} else {
item = Py_NewRef(lz->fillvalue);
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Expand All @@ -3994,8 +4008,9 @@ zip_longest_next_impl(ziplongestobject *self)
_PyTuple_Recycle(result);
} else {
result = PyTuple_New(tuplesize);
if (result == NULL)
return NULL;
if (result == NULL) {
goto done;
}
for (i=0 ; i < tuplesize ; i++) {
it = PyTuple_GET_ITEM(lz->ittuple, i);
if (it == NULL) {
Expand All @@ -4007,7 +4022,8 @@ zip_longest_next_impl(ziplongestobject *self)
if (lz->numactive == 0 || PyErr_Occurred()) {
lz->numactive = 0;
Py_DECREF(result);
return NULL;
result = NULL;
goto done;
} else {
item = Py_NewRef(lz->fillvalue);
PyTuple_SET_ITEM(lz->ittuple, i, NULL);
Expand All @@ -4018,6 +4034,8 @@ zip_longest_next_impl(ziplongestobject *self)
PyTuple_SET_ITEM(result, i, item);
}
}
done:
lz->running = 0;
return result;
}

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