fix: avoid in-place modification of groups in rebinning (#1047)

* Fix: avoid in-place modification of groups in rebinning (use [1, *groups])

* Test: ensure rebinning does not mutate input groups list in-place

* style: pre-commit fixes

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: henryiii

src/boost_histogram/histogram.py

@@ -1067,7 +1067,7 @@ def __getitem__(self, index: IndexingExpr) -> Self | float | Accumulator:
                     new_j_base = 0
 
                     if old_axis.traits_underflow and axes[i].traits_underflow:
-                        groups.insert(0, 1)
+                        groups = [1, *groups]
                     elif axes[i].traits_underflow:
                         new_j_base = 1
 

tests/test_histogram_indexing.py

@@ -554,3 +554,19 @@ def test_setting_histogram_mismatch():
         h[:len, 0] = bh.Histogram(
             bh.axis.Regular(10, 0, 10, underflow=False, overflow=False)
         )
+
+
+def test_rebin_groups_no_inplace_modification():
+    """
+    Test that rebinning with a groups list does not mutate the input list in-place.
+    This ensures consecutive rebin operations with the same list do not fail.
+    """
+    h1 = bh.Histogram(bh.axis.Regular(60, 0, 600))
+    h2 = bh.Histogram(bh.axis.Regular(60, 0, 600))
+    rebinner = [5] * 12
+    h3 = h1[bh.rebin(groups=rebinner)]
+    # The original list should remain unchanged
+    assert rebinner == [5] * 12, "rebinner list was mutated in-place"
+    # Second rebin should not raise
+    h4 = h2[bh.rebin(groups=rebinner)]
+    assert h3 == h4