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gh-154570: Fix itertools.accumulate() silently corrupting its total on reentrancy #154571

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gh-154570: Fix itertools.accumulate() silently corrupting its total on reentrancy #154571

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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 @@ -157,6 +157,25 @@ def test_accumulate(self):
with self.assertRaises(TypeError):
list(accumulate([10, 20], 100))

def test_accumulate_reenter(self):
# A source iterable (or binop) that calls back into next() on the
# same accumulate object must not be allowed to silently corrupt
# the running total.
class I:
count = 0
def __iter__(self):
return self
def __next__(self):
self.count += 1
if self.count == 2:
return next(it)
return self.count

it = accumulate(I())
self.assertEqual(next(it), 1)
with self.assertRaisesRegex(RuntimeError, "accumulate"):
next(it)

def test_batched(self):
self.assertEqual(list(batched('ABCDEFG', 3)),
[('A', 'B', 'C'), ('D', 'E', 'F'), ('G',)])
Expand Down
6 changes: 6 additions & 0 deletions ...WS.d/next/Library/2026-07-23-22-00-00.gh-issue-154570.accumulate-reentrancy.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
Fix :func:`itertools.accumulate` silently corrupting its running total
when the source iterable (or the ``func`` callable) re-enters the same
:func:`!accumulate` iterator via a nested call to :func:`!next`. It now
raises :exc:`RuntimeError` on re-entry instead of producing wrong,
silently-dropped values, matching the existing behavior of
:func:`itertools.tee`.
14 changes: 13 additions & 1 deletion Modules/itertoolsmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3079,6 +3079,7 @@ typedef struct {
PyObject *binop;
PyObject *initial;
itertools_state *state;
int running;
} accumulateobject;

#define accumulateobject_CAST(op) ((accumulateobject *)(op))
Expand Down Expand Up @@ -3120,6 +3121,7 @@ itertools_accumulate_impl(PyTypeObject *type, PyObject *iterable,
lz->it = it;
lz->initial = Py_XNewRef(initial);
lz->state = find_state_by_type(type);
lz->running = 0;
return (PyObject *)lz;
}

Expand Down Expand Up @@ -3160,12 +3162,21 @@ accumulate_next_lock_held(PyObject *op)
lz->initial = Py_NewRef(Py_None);
return Py_NewRef(lz->total);
}
if (lz->running) {
PyErr_SetString(PyExc_RuntimeError,
"cannot re-enter the accumulate iterator");
return NULL;
}
lz->running = 1;
val = (*Py_TYPE(lz->it)->tp_iternext)(lz->it);
if (val == NULL)
if (val == NULL) {
lz->running = 0;
return NULL;
}

if (lz->total == NULL) {
lz->total = Py_NewRef(val);
lz->running = 0;
return lz->total;
}

Expand All @@ -3174,6 +3185,7 @@ accumulate_next_lock_held(PyObject *op)
else
newtotal = PyObject_CallFunctionObjArgs(lz->binop, lz->total, val, NULL);
Py_DECREF(val);
lz->running = 0;
if (newtotal == NULL)
return NULL;

Expand Down
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