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 +3138,25 @@ 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})
 
+        # Case for non-unique 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_axis_select_reindex.py

@@ -1151,3 +1151,24 @@ def test_raise_on_drop_duplicate_index(self, actual):
         expected_no_err = actual.T.drop('c', axis=1, level=level,
                                         errors='ignore')
         assert_frame_equal(expected_no_err.T, actual)
+
+    @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/indexes/test_base.py

@@ -1565,6 +1565,79 @@ def test_drop_tuple(self, values, to_drop):
         for drop_me in to_drop[1], [to_drop[1]]:
             pytest.raises(KeyError, removed.drop, drop_me)
 
+    def test_drop_unique_and_non_unique_index(self):
+        # unique
+        s = Series([1, 2], index=['one', 'two'])
+        expected = Series([1], index=['one'])
+        result = s.drop(['two'])
+        tm.assert_series_equal(result, expected)
+        result = s.drop('two', axis='rows')
+        tm.assert_series_equal(result, expected)
+
+        # non-unique
+        # GH 5248
+        s = Series([1, 1, 2], index=['one', 'two', 'one'])
+        expected = Series([1, 2], index=['one', 'one'])
+        result = s.drop(['two'], axis=0)
+        tm.assert_series_equal(result, expected)
+        result = s.drop('two')
+        tm.assert_series_equal(result, expected)
+
+        expected = Series([1], index=['two'])
+        result = s.drop(['one'])
+        tm.assert_series_equal(result, expected)
+        result = s.drop('one')
+        tm.assert_series_equal(result, expected)
+
+        # single string/tuple-like
+        s = Series(range(3), index=list('abc'))
+        pytest.raises(KeyError, s.drop, 'bc')
+        pytest.raises(KeyError, s.drop, ('a',))
+
+        # errors='ignore'
+        s = Series(range(3), index=list('abc'))
+        result = s.drop('bc', errors='ignore')
+        tm.assert_series_equal(result, s)
+        result = s.drop(['a', 'd'], errors='ignore')
+        expected = s.iloc[1:]
+        tm.assert_series_equal(result, expected)
+
+        # bad axis
+        pytest.raises(ValueError, s.drop, 'one', axis='columns')
+
+        # GH 8522
+        s = Series([2, 3], index=[True, False])
+        assert s.index.is_object()
+        result = s.drop(True)
+        expected = Series([3], index=[False])
+        tm.assert_series_equal(result, expected)
+
+        # GH 16877
+        s = Series([2, 3], index=[0, 1])
+        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(self, 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(self, index, drop_labels):
+        # GH 21494
+        with tm.assert_raises_regex(KeyError, 'not found in axis'):
+            pd.Series(index=index).drop(drop_labels)
+
     @pytest.mark.parametrize("method,expected", [
         ('intersection', np.array([(1, 'A'), (2, 'A'), (1, 'B'), (2, 'B')],
                                   dtype=[('num', int), ('let', 'a1')])),

pandas/tests/series/indexing/test_alter_index.py

@@ -461,56 +461,3 @@ def test_rename():
     assert_series_equal(result, expected)
 
     assert result.name == expected.name
-
-
-def test_drop():
-    # unique
-    s = Series([1, 2], index=['one', 'two'])
-    expected = Series([1], index=['one'])
-    result = s.drop(['two'])
-    assert_series_equal(result, expected)
-    result = s.drop('two', axis='rows')
-    assert_series_equal(result, expected)
-
-    # non-unique
-    # GH 5248
-    s = Series([1, 1, 2], index=['one', 'two', 'one'])
-    expected = Series([1, 2], index=['one', 'one'])
-    result = s.drop(['two'], axis=0)
-    assert_series_equal(result, expected)
-    result = s.drop('two')
-    assert_series_equal(result, expected)
-
-    expected = Series([1], index=['two'])
-    result = s.drop(['one'])
-    assert_series_equal(result, expected)
-    result = s.drop('one')
-    assert_series_equal(result, expected)
-
-    # single string/tuple-like
-    s = Series(range(3), index=list('abc'))
-    pytest.raises(KeyError, s.drop, 'bc')
-    pytest.raises(KeyError, s.drop, ('a',))
-
-    # errors='ignore'
-    s = Series(range(3), index=list('abc'))
-    result = s.drop('bc', errors='ignore')
-    assert_series_equal(result, s)
-    result = s.drop(['a', 'd'], errors='ignore')
-    expected = s.iloc[1:]
-    assert_series_equal(result, expected)
-
-    # bad axis
-    pytest.raises(ValueError, s.drop, 'one', axis='columns')
-
-    # GH 8522
-    s = Series([2, 3], index=[True, False])
-    assert s.index.is_object()
-    result = s.drop(True)
-    expected = Series([3], index=[False])
-    assert_series_equal(result, expected)
-
-    # GH 16877
-    s = Series([2, 3], index=[0, 1])
-    with tm.assert_raises_regex(KeyError, 'not contained in axis'):
-        s.drop([False, True])