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python: Fix __setitem__ with negative index preceded by None #4414

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Merged
zcbenz merged 1 commit into from
Aug 29, 2026
Merged

python: Fix __setitem__ with negative index preceded by None #4414

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2 changes: 1 addition & 1 deletion python/src/indexing.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -913,7 +913,7 @@ mlx_compute_slice_update_args(
upd_ax--;
} else if (is_index_scalar(pyidx)) {
int st = safe_to_int32(pyidx);
st = (st < 0) ? st + src.shape(i) : st;
st = (st < 0) ? st + src.shape(ax) : st;
starts[ax] = st;
stops[ax] = st + 1;
if (upd_ax >= 0) {
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7 changes: 7 additions & 0 deletions python/tests/test_array.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1348,6 +1348,13 @@ def test_setitem(self):
a[0:1] = mx.array([1])
self.assertEqual(a.tolist(), [1, 4, 4])

# Regression test: a negative integer index after a None
# (newaxis) used to be normalized against the wrong axis size,
# silently writing nothing instead of updating the last row.
b = mx.zeros((3, 4))
b[None, -1] = 9
self.assertEqual(b.tolist(), [[0, 0, 0, 0], [0, 0, 0, 0], [9, 9, 9, 9]])

with self.assertRaises(ValueError):
a[0:1] = mx.array([2, 3])

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