Remove redundant copying

Author: Nick Eubank

pandas/core/generic.py

@@ -1217,7 +1217,7 @@ def _slice(self, slobj, axis=0, kind=None):
     def _set_item(self, key, value):
 
         # If children are views, reset to copies before setting.
-        self._convert_views_to_copies()
+        self._execute_copy_on_write()
 
         self._data.set(key, value)
         self._clear_item_cache()
@@ -1231,23 +1231,13 @@ def _set_is_copy(self, ref=None, copy=True):
             else:
                 self.is_copy = None
 
-    def _convert_views_to_copies(self):
+    def _execute_copy_on_write(self):
 
         # Don't set on views.         
-        if self._is_view and not self._is_column_view:
+        if (self._is_view and not self._is_column_view) or len(self._children) is not 0:
             self._data = self._data.copy()
-    
-        
-        # Before setting values, make sure children converted to copies. 
-        for child in self._children.valuerefs():
-                
-                # Make sure children of children converted. 
-                child()._convert_views_to_copies()
-                
-                if child()._is_view and not child()._is_column_view:
-                    child()._data = child()._data.copy()
+            self._children = weakref.WeakValueDictionary()
 
-        self._children = weakref.WeakValueDictionary()
 
     def _add_to_children(self, view_to_append):
         self._children[id(view_to_append)] = view_to_append

pandas/core/indexing.py

@@ -115,7 +115,7 @@ def _get_setitem_indexer(self, key):
 
     def __setitem__(self, key, value):
         # Make sure changes don't propagate to children
-        self.obj._convert_views_to_copies()
+        self.obj._execute_copy_on_write()
 
         indexer = self._get_setitem_indexer(key)
         self._setitem_with_indexer(indexer, value)

pandas/tests/test_generic.py

@@ -1745,7 +1745,6 @@ def test_copy_on_write(self):
 
         for v in views.keys():
             tm.assert_frame_equal(views[v], copies[v])
-            self.assertFalse(views[v]._is_view)
 
         # Test different forms of value setting 
         # all trigger conversions
@@ -1768,7 +1767,6 @@ def test_copy_on_write(self):
 
         for v in views.keys():
             tm.assert_frame_equal(views[v], copies[v])
-            self.assertFalse(views[v]._is_view)
 
         ########
         # No Backward Propogation