clarify-error-message-slice-bound-non-monotonic-index-without-specifying-the-label

Author: natmokval

pandas/core/indexes/base.py

@@ -6868,8 +6868,11 @@ 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 non-specific label: {original_label!r}. "
+                    "Please specify the label."
+                ) from err
 
         if isinstance(slc, np.ndarray):
             # get_loc may return a boolean array, which

pandas/tests/series/indexing/test_indexing.py

@@ -404,6 +404,16 @@ def test_setitem_empty_indexer(indexer, val):
     tm.assert_frame_equal(df, expected)
 
 
+def test_loc_non_monotonic_index_with_non_specific_label():
+    msg = (
+        "Cannot get left slice bound for non-monotonic "
+        "index with a non-specific 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):