BUG: .iloc[:] and .loc[:] return a copy of the original object #13873 (#16443)

closes #13873

Author: margaret

doc/source/whatsnew/v0.21.0.txt

@@ -97,6 +97,7 @@ Conversion
 Indexing
 ^^^^^^^^
 
+- When called with a null slice (e.g. ``df.iloc[:]``), the``iloc`` and ``loc`` indexers return a shallow copy of the original object. Previously they returned the original object. (:issue:`13873`).
 
 
 I/O

pandas/core/indexing.py

@@ -988,6 +988,10 @@ def _getitem_lowerdim(self, tup):
                     if len(new_key) == 1:
                         new_key, = new_key
 
+                # Slices should return views, but calling iloc/loc with a null
+                # slice returns a new object.
+                if is_null_slice(new_key):
+                    return section
                 # This is an elided recursive call to iloc/loc/etc'
                 return getattr(section, self.name)[new_key]
 
@@ -1250,7 +1254,7 @@ def _get_slice_axis(self, slice_obj, axis=0):
         obj = self.obj
 
         if not need_slice(slice_obj):
-            return obj
+            return obj.copy(deep=False)
         indexer = self._convert_slice_indexer(slice_obj, axis)
 
         if isinstance(indexer, slice):
@@ -1349,7 +1353,7 @@ def _get_slice_axis(self, slice_obj, axis=0):
         """ this is pretty simple as we just have to deal with labels """
         obj = self.obj
         if not need_slice(slice_obj):
-            return obj
+            return obj.copy(deep=False)
 
         labels = obj._get_axis(axis)
         indexer = labels.slice_indexer(slice_obj.start, slice_obj.stop,
@@ -1690,7 +1694,7 @@ def _get_slice_axis(self, slice_obj, axis=0):
         obj = self.obj
 
         if not need_slice(slice_obj):
-            return obj
+            return obj.copy(deep=False)
 
         slice_obj = self._convert_slice_indexer(slice_obj, axis)
         if isinstance(slice_obj, slice):

pandas/tests/indexing/test_iloc.py

@@ -591,3 +591,21 @@ def test_iloc_empty_list_indexer_is_ok(self):
         tm.assert_frame_equal(df.iloc[[]], df.iloc[:0, :],
                               check_index_type=True,
                               check_column_type=True)
+
+    def test_identity_slice_returns_new_object(self):
+        # GH13873
+        original_df = DataFrame({'a': [1, 2, 3]})
+        sliced_df = original_df.iloc[:]
+        assert sliced_df is not original_df
+
+        # should be a shallow copy
+        original_df['a'] = [4, 4, 4]
+        assert (sliced_df['a'] == 4).all()
+
+        original_series = Series([1, 2, 3, 4, 5, 6])
+        sliced_series = original_series.iloc[:]
+        assert sliced_series is not original_series
+
+        # should also be a shallow copy
+        original_series[:3] = [7, 8, 9]
+        assert all(sliced_series[:3] == [7, 8, 9])

pandas/tests/indexing/test_loc.py

@@ -630,3 +630,28 @@ def test_loc_empty_list_indexer_is_ok(self):
         tm.assert_frame_equal(df.loc[[]], df.iloc[:0, :],
                               check_index_type=True,
                               check_column_type=True)
+
+    def test_identity_slice_returns_new_object(self):
+        # GH13873
+        original_df = DataFrame({'a': [1, 2, 3]})
+        sliced_df = original_df.loc[:]
+        assert sliced_df is not original_df
+        assert original_df[:] is not original_df
+
+        # should be a shallow copy
+        original_df['a'] = [4, 4, 4]
+        assert (sliced_df['a'] == 4).all()
+
+        # These should not return copies
+        assert original_df is original_df.loc[:, :]
+        df = DataFrame(np.random.randn(10, 4))
+        assert df[0] is df.loc[:, 0]
+
+        # Same tests for Series
+        original_series = Series([1, 2, 3, 4, 5, 6])
+        sliced_series = original_series.loc[:]
+        assert sliced_series is not original_series
+        assert original_series[:] is not original_series
+
+        original_series[:3] = [7, 8, 9]
+        assert all(sliced_series[:3] == [7, 8, 9])