ENH: clarify error message when getting slice bounds for a non-monotonic index without specifying the label

pandas/core/indexes/base.py

@@ -6869,8 +6869,11 @@ def get_slice_bound(self, label, side: Literal["left", "right"]) -> int:
  try:
  return self._searchsorted_monotonic(label, side)
  except ValueError:
- # raise the original KeyError
- raise err from None
+ raise KeyError(
+ f"Cannot get {side} slice bound for non-monotonic index "
+ f"with a missing label {original_label!r}. "
+ "Either sort the index or specify an existing label."
+ ) from None
 
  if isinstance(slc, np.ndarray):
  # get_loc may return a boolean array, which

pandas/tests/frame/indexing/test_indexing.py

@@ -564,9 +564,10 @@ def test_getitem_setitem_integer_slice_keyerrors(self):
 
  # non-monotonic, raise KeyError
  df2 = df.iloc[list(range(5)) + list(range(5, 10))[::-1]]
- with pytest.raises(KeyError, match=r"^3$"):
+ msg = "non-monotonic index with a missing label 3"
+ with pytest.raises(KeyError, match=msg):
  df2.loc[3:11]
- with pytest.raises(KeyError, match=r"^3$"):
+ with pytest.raises(KeyError, match=msg):
  df2.loc[3:11] = 0
 
  def test_fancy_getitem_slice_mixed(self, float_frame, float_string_frame):

pandas/tests/indexes/numeric/test_indexing.py

@@ -600,10 +600,11 @@ def test_slice_locs_na(self):
 
  def test_slice_locs_na_raises(self):
  index = Index([np.nan, 1, 2])
- with pytest.raises(KeyError, match="1.5"):
+ msg = "non-monotonic index with a missing label 1.5"
+ with pytest.raises(KeyError, match=msg):
  index.slice_locs(start=1.5)
 
- with pytest.raises(KeyError, match="1.5"):
+ with pytest.raises(KeyError, match=msg):
  index.slice_locs(end=1.5)
 
 

pandas/tests/indexing/test_loc.py

@@ -3162,9 +3162,10 @@ def test_loc_getitem_setitem_integer_slice_keyerrors(self):
 
  # non-monotonic, raise KeyError
  s2 = ser.iloc[list(range(5)) + list(range(9, 4, -1))]
- with pytest.raises(KeyError, match=r"^3$"):
+ msg = "non-monotonic index with a missing label 3"
+ with pytest.raises(KeyError, match=msg):
  s2.loc[3:11]
- with pytest.raises(KeyError, match=r"^3$"):
+ with pytest.raises(KeyError, match=msg):
  s2.loc[3:11] = 0
 
  def test_loc_getitem_iterator(self, string_series):

pandas/tests/series/indexing/test_indexing.py

@@ -404,6 +404,13 @@ def test_setitem_empty_indexer(indexer, val):
  tm.assert_frame_equal(df, expected)
 
 
+def test_loc_non_monotonic_index_with_a_missing_label():
+ msg = "Cannot get left slice bound for non-monotonic index with a missing label 4"
+ ser = Series([3, 6, 7, 6], index=[3, 8, 7, 6])
+ with pytest.raises(KeyError, match=msg):
+ ser.loc[4:7]
+
+
 class TestDeprecatedIndexers:
  @pytest.mark.parametrize("key", [{1}, {1: 1}])
  def test_getitem_dict_and_set_deprecated(self, key):