Fix passing empty label to df drop

doc/source/whatsnew/v0.23.2.txt

@@ -55,6 +55,7 @@ Bug Fixes
 
 - Bug in :meth:`Index.get_indexer_non_unique` with categorical key (:issue:`21448`)
 - Bug in comparison operations for :class:`MultiIndex` where error was raised on equality / inequality comparison involving a MultiIndex with ``nlevels == 1`` (:issue:`21149`)
+- Bug in :meth:`DataFrame.drop` behaviour is not consistent for unique and non-unique indexes (:issue:`21494`)
 -
 
 **I/O**

pandas/core/generic.py

@@ -3129,7 +3129,7 @@ def _drop_axis(self, labels, axis, level=None, errors='raise'):
         """
         axis = self._get_axis_number(axis)
         axis_name = self._get_axis_name(axis)
-        axis, axis_ = self._get_axis(axis), axis
+        axis = self._get_axis(axis)
 
         if axis.is_unique:
             if level is not None:
@@ -3138,24 +3138,23 @@ def _drop_axis(self, labels, axis, level=None, errors='raise'):
                 new_axis = axis.drop(labels, level=level, errors=errors)
             else:
                 new_axis = axis.drop(labels, errors=errors)
-            dropped = self.reindex(**{axis_name: new_axis})
-            try:
-                dropped.axes[axis_].set_names(axis.names, inplace=True)
-            except AttributeError:
-                pass
-            result = dropped
+            result = self.reindex(**{axis_name: new_axis})
 
         else:
             labels = _ensure_object(com._index_labels_to_array(labels))
             if level is not None:
                 if not isinstance(axis, MultiIndex):
                     raise AssertionError('axis must be a MultiIndex')
                 indexer = ~axis.get_level_values(level).isin(labels)
+                # GH 18561 MultiIndex.drop should raise if label is absent
+                if errors == 'raise' and indexer.all():
+                    raise KeyError('{} not found in axis'.format(labels))
             else:
                 indexer = ~axis.isin(labels)
-
-            if errors == 'raise' and indexer.all():
-                raise KeyError('{} not found in axis'.format(labels))
+                # Check if label doesn't exist along axis
+                labels_missing = (axis.get_indexer_for(labels) == -1).any()
+                if errors == 'raise' and labels_missing:
+                    raise KeyError('{} not found in axis'.format(labels))
 
             slicer = [slice(None)] * self.ndim
             slicer[self._get_axis_number(axis_name)] = indexer

pandas/core/indexes/base.py

@@ -4335,7 +4335,7 @@ def drop(self, labels, errors='raise'):
         Raises
         ------
         KeyError
-            If none of the labels are found in the selected axis
+            If not all of the labels are found in the selected axis
         """
         arr_dtype = 'object' if self.dtype == 'object' else None
         labels = com._index_labels_to_array(labels, dtype=arr_dtype)
@@ -4344,7 +4344,7 @@ def drop(self, labels, errors='raise'):
         if mask.any():
             if errors != 'ignore':
                 raise KeyError(
-                    'labels %s not contained in axis' % labels[mask])
+                    '{} not found in axis'.format(labels[mask]))
             indexer = indexer[~mask]
         return self.delete(indexer)
 

pandas/core/indexes/multi.py

@@ -1719,7 +1719,6 @@ def drop(self, labels, level=None, errors='raise'):
                 if errors != 'ignore':
                     raise ValueError('labels %s not contained in axis' %
                                      labels[mask])
-                indexer = indexer[~mask]
         except Exception:
             pass
 

pandas/tests/frame/test_indexing.py

@@ -3515,3 +3515,24 @@ def test_functions_no_warnings(self):
         with tm.assert_produces_warning(False):
             df['group'] = pd.cut(df.value, range(0, 105, 10), right=False,
                                  labels=labels)
+
+    @pytest.mark.parametrize('index, drop_labels', [
+        ([1, 2, 3], []),
+        ([1, 1, 2], []),
+        ([1, 2, 3], [2]),
+        ([1, 1, 3], [1]),
+    ])
+    def test_drop_empty_list(self, index, drop_labels):
+        # GH 21494
+        expected_index = [i for i in index if i not in drop_labels]
+        frame = pd.DataFrame(index=index).drop(drop_labels)
+        tm.assert_frame_equal(frame, pd.DataFrame(index=expected_index))
+
+    @pytest.mark.parametrize('index, drop_labels', [
+        ([1, 2, 3], [1, 4]),
+        ([1, 2, 2], [1, 4]),
+    ])
+    def test_drop_non_empty_list(self, index, drop_labels):
+        # GH 21494
+        with tm.assert_raises_regex(KeyError, 'not found in axis'):
+            pd.DataFrame(index=index).drop(drop_labels)

pandas/tests/series/indexing/test_alter_index.py

@@ -512,5 +512,28 @@ def test_drop():
 
     # GH 16877
     s = Series([2, 3], index=[0, 1])
-    with tm.assert_raises_regex(KeyError, 'not contained in axis'):
+    with tm.assert_raises_regex(KeyError, 'not found in axis'):
         s.drop([False, True])
+
+
+@pytest.mark.parametrize('index, drop_labels', [
+    ([1, 2, 3], []),
+    ([1, 1, 2], []),
+    ([1, 2, 3], [2]),
+    ([1, 1, 3], [1]),
+])
+def test_drop_empty_list(index, drop_labels):
+    # GH 21494
+    expected_index = [i for i in index if i not in drop_labels]
+    series = pd.Series(index=index).drop(drop_labels)
+    tm.assert_series_equal(series, pd.Series(index=expected_index))
+
+
+@pytest.mark.parametrize('index, drop_labels', [
+    ([1, 2, 3], [1, 4]),
+    ([1, 2, 2], [1, 4]),
+])
+def test_drop_non_empty_list(index, drop_labels):
+    # GH 21494
+    with tm.assert_raises_regex(KeyError, 'not found in axis'):
+        pd.Series(index=index).drop(drop_labels)