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

This is consistent with the behavior of [:] on a list
https://docs.python.org/2/tutorial/introduction.html#lists
https://docs.python.org/3/tutorial/introduction.html#lists

Author: margaret

pandas/core/indexing.py

@@ -1250,7 +1250,7 @@ def _get_slice_axis(self, slice_obj, axis=0):
         obj = self.obj
 
         if not need_slice(slice_obj):
-            return obj
+            return obj.copy()
         indexer = self._convert_slice_indexer(slice_obj, axis)
 
         if isinstance(indexer, slice):
@@ -1349,7 +1349,7 @@ 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()
 
         labels = obj._get_axis(axis)
         indexer = labels.slice_indexer(slice_obj.start, slice_obj.stop,
@@ -1690,7 +1690,7 @@ def _get_slice_axis(self, slice_obj, axis=0):
         obj = self.obj
 
         if not need_slice(slice_obj):
-            return obj
+            return obj.copy()
 
         slice_obj = self._convert_slice_indexer(slice_obj, axis)
         if isinstance(slice_obj, slice):

pandas/tests/indexing/test_iloc.py

@@ -591,3 +591,8 @@ 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_loc_identity_slice_returns_new_object(self):
+        # GH13873
+        df = DataFrame({'a': [1, 3, 5], 'b': [2, 4, 6]})
+        assert not df.iloc[:] is df

pandas/tests/indexing/test_loc.py

@@ -630,3 +630,8 @@ 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_loc_identity_slice_returns_new_object(self):
+        # GH13873
+        df = DataFrame({'a': [1, 3, 5], 'b': [2, 4, 6]})
+        assert not df.loc[:] is df