fix: slicing supports under/overflow (#576)

Author: henryiii

docs/CHANGELOG.md

@@ -1,5 +1,11 @@
 # What's new in boost-histogram
 
+## UPCOMING
+
+* Fix "picking" on a flow bin [#576][]
+
+[#576]: https://github.com/scikit-hep/boost-histogram/pull/576
+
 
 ## Version 0.13
 

src/boost_histogram/_internal/hist.py

@@ -684,16 +684,20 @@ def __getitem__(self, index):  # noqa: C901
 
         integrations = set()
         slices = []
+        pick_each = dict()
 
         # Compute needed slices and projections
         for i, ind in enumerate(indexes):
             if hasattr(ind, "__index__"):
-                ind = slice(ind.__index__(), ind.__index__() + 1, sum)
-
+                pick_each[i] = ind.__index__() + (  # type: ignore
+                    1 if self.axes[i].traits.underflow else 0
+                )
+                continue
             elif not isinstance(ind, slice):
                 raise IndexError(
                     "Must be a slice, an integer, or follow the locator protocol."
                 )
+
             # If the dictionary brackets are forgotten, it's easy to put a slice
             # into a slice - adding a nicer error message in that case
             if any(isinstance(v, slice) for v in (ind.start, ind.stop, ind.step)):
@@ -732,16 +736,25 @@ def __getitem__(self, index):  # noqa: C901
         logger.debug("Reduce with %s", slices)
         reduced = self._hist.reduce(*slices)
 
-        if not integrations:
-            return self._new_hist(reduced)
-        else:
-            projections = [i for i in range(self.ndim) if i not in integrations]
-
-            return (
-                self._new_hist(reduced.project(*projections))
-                if projections
-                else reduced.sum(flow=True)
+        if pick_each:
+            my_slice = tuple(
+                pick_each.get(i, slice(None)) for i in range(reduced.rank())
             )
+            logger.debug("Slices: %s", my_slice)
+            axes = [
+                reduced.axis(i) for i in range(reduced.rank()) if i not in pick_each
+            ]
+            logger.debug("Axes: %s", axes)
+            new_reduced = reduced.__class__(axes)
+            new_reduced.view(flow=True)[...] = reduced.view(flow=True)[my_slice]
+            reduced = new_reduced
+            integrations = {i - sum(j <= i for j in pick_each) for i in integrations}
+
+        if integrations:
+            projections = [i for i in range(reduced.rank()) if i not in integrations]
+            reduced = reduced.project(*projections)
+
+        return self._new_hist(reduced) if reduced.rank() > 0 else reduced.sum(flow=True)
 
     def __setitem__(self, index, value):
         """

tests/test_histogram_indexing.py

@@ -2,6 +2,7 @@
 import numpy as np
 import pytest
 from numpy.testing import assert_array_equal
+from pytest import approx
 
 import boost_histogram as bh
 
@@ -352,6 +353,30 @@ def test_pick_int_category():
     assert_array_equal(h[:, :, bh.loc(7)].view(), 0)
 
 
+@pytest.mark.parametrize(
+    "ax",
+    [bh.axis.Regular(3, 0, 1), bh.axis.Variable([0, 0.3, 0.6, 1])],
+    ids=["regular", "variable"],
+)
+def test_pick_flowbin(ax):
+    w = 1e-2  # e.g. a cross section for a process
+    x = [-0.1, -0.1, 0.1, 0.1, 0.1]
+    y = [-0.1, 0.1, -0.1, -0.1, 0.1]
+
+    h = bh.Histogram(
+        ax,
+        ax,
+        storage=bh.storage.Weight(),
+    )
+    h.fill(x, y, weight=w)
+
+    uf_slice = h[bh.tag.underflow, ...]
+    assert uf_slice.values(flow=True) == approx(np.array([1, 1, 0, 0, 0]) * w)
+
+    uf_slice = h[..., bh.tag.underflow]
+    assert uf_slice.values(flow=True) == approx(np.array([1, 2, 0, 0, 0]) * w)
+
+
 def test_axes_tuple():
     h = bh.Histogram(bh.axis.Regular(10, 0, 1))
     assert isinstance(h.axes[:1], bh._internal.axestuple.AxesTuple)