make it work for nd hists

src/boost_histogram/_internal/hist.py

@@ -826,6 +826,7 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
         slices: list[_core.algorithm.reduce_command] = []
         pick_each: dict[int, int] = {}
         pick_set: dict[int, list[int]] = {}
+        reduced: CppHistogram | None = None
 
         # Compute needed slices and projections
         for i, ind in enumerate(indexes):
@@ -890,7 +891,8 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
                     )
                 # rebinning with groups
                 elif len(groups) != 0:
-                    reduced = self._hist
+                    if not reduced:
+                        reduced = self._hist
                     axes = [reduced.axis(x) for x in range(reduced.rank())]
                     reduced_view = reduced.view(flow=True)
                     new_axes_indices = [axes[i].edges[0]]
@@ -913,7 +915,7 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
                     j = 1
                     for new_j, group in enumerate(groups):
                         for _ in range(group):
-                            pos = [slice] * (i)
+                            pos = [slice(None)] * (i)
                             new_view[(*pos, new_j + 1, ...)] += reduced_view[  # type: ignore[arg-type]
                                 (*pos, j, ...)  # type: ignore[arg-type]
                             ]
@@ -922,10 +924,9 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
                     reduced = new_reduced
 
         # Will be updated below
-        if slices or pick_set or pick_each or integrations:
+        if (slices or pick_set or pick_each or integrations) and not reduced:
             reduced = self._hist
-        elif len(groups) == 0:
-            logger.debug("Reduce actions are all empty, just making a copy")
+        elif not reduced:
             reduced = copy.copy(self._hist)
 
         if pick_each:

tests/test_histogram.py

@@ -636,7 +636,6 @@ def test_rebin_1d():
 
     hs = h[{0: slice(None, None, bh.tag.Rebinner(4))}]
     assert_array_equal(hs.view(), [1, 1, 1, 0, 1])
-    print("Here")
 
     hs = h[{0: bh.tag.Rebinner(4)}]
     assert_array_equal(hs.view(), [1, 1, 1, 0, 1])
@@ -664,8 +663,59 @@ def test_rebin_nd():
     assert h[{1: s[:: bh.rebin(2)]}].axes.size == (20, 15, 40)
     assert h[{2: s[:: bh.rebin(2)]}].axes.size == (20, 30, 20)
 
+    assert h[{0: s[:: bh.rebin(groups=[1, 2, 3])]}].axes.size == (3, 30, 40)
+    assert h[{1: s[:: bh.rebin(groups=[1, 2, 3])]}].axes.size == (20, 3, 40)
+    assert h[{2: s[:: bh.rebin(groups=[1, 2, 3])]}].axes.size == (20, 30, 3)
+    assert np.all(
+        np.isclose(
+            h[{0: s[:: bh.rebin(groups=[1, 2, 3])]}].axes[0].edges, [1.0, 1.1, 1.3, 1.6]
+        )
+    )
+    assert np.all(
+        np.isclose(
+            h[{1: s[:: bh.rebin(groups=[1, 2, 3])]}].axes[1].edges,
+            [1.0, 1.06666667, 1.2, 1.4],
+        )
+    )
+    assert np.all(
+        np.isclose(
+            h[{2: s[:: bh.rebin(groups=[1, 2, 3])]}].axes[2].edges,
+            [1.0, 1.05, 1.15, 1.3],
+        )
+    )
+
     assert h[{0: s[:: bh.rebin(2)], 2: s[:: bh.rebin(2)]}].axes.size == (10, 30, 20)
 
+    assert h[
+        {0: s[:: bh.rebin(groups=[1, 2, 3])], 2: s[:: bh.rebin(groups=[1, 2, 3])]}
+    ].axes.size == (3, 30, 3)
+    assert np.all(
+        np.isclose(
+            h[
+                {
+                    0: s[:: bh.rebin(groups=[1, 2, 3])],
+                    2: s[:: bh.rebin(groups=[1, 2, 3])],
+                }
+            ]
+            .axes[0]
+            .edges,
+            [1.0, 1.1, 1.3, 1.6],
+        )
+    )
+    assert np.all(
+        np.isclose(
+            h[
+                {
+                    0: s[:: bh.rebin(groups=[1, 2, 3])],
+                    2: s[:: bh.rebin(groups=[1, 2, 3])],
+                }
+            ]
+            .axes[2]
+            .edges,
+            [1.0, 1.05, 1.15, 1.3],
+        )
+    )
+
     assert h[{1: s[:: bh.sum]}].axes.size == (20, 40)
     assert h[{1: bh.sum}].axes.size == (20, 40)