fix: the result of group_mapping() should be checked for None

Signed-off-by: Henry Schreiner <[email protected]>

.gitmodules

@@ -1,7 +1,4 @@
-[submodule "pybind11"]
-	path = extern/pybind11
-	url = ../../pybind/pybind11.git
-[submodule "extern/boosthistogram"]
+[submodule "extern/histogram"]
 	path = extern/histogram
 	url = ../../boostorg/histogram.git
 [submodule "extern/core"]

src/boost_histogram/_internal/hist.py

@@ -862,8 +862,12 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
                 if ind.step is not None:
                     if getattr(ind.step, "factor", None) is not None:
                         merge = ind.step.factor
-                    elif getattr(ind.step, "group_mapping", None) is not None:
-                        groups = ind.step.group_mapping(self.axes[i])
+                    elif (
+                        hasattr(ind.step, "group_mapping")
+                        and (tmp_groups := ind.step.group_mapping(self.axes[i]))
+                        is not None
+                    ):
+                        groups = tmp_groups
                     elif callable(ind.step):
                         if ind.step is sum:
                             integrations.add(i)

src/boost_histogram/tag.py

@@ -11,7 +11,7 @@
 
 from ._internal.typing import AxisLike
 
-__all__ = ("Slicer", "Locator", "at", "loc", "overflow", "underflow", "sum", "rebin")
+__all__ = ("Slicer", "Locator", "at", "loc", "overflow", "underflow", "rebin", "sum")
 
 
 class Slicer: