API: spurious setting_with_copy warning (GH8730)

Author: jreback

doc/source/whatsnew/v0.16.0.txt

@@ -454,6 +454,15 @@ Other API Changes
 
   To reproduce the old behavior, simply add more precision to the label (e.g., use ``2000-02-01`` instead of ``2000-02``).
 
+- A Spurious ``SettingWithCopy`` Warning was generated when setting a new item in a frame in some cases (:issue:`8730`)
+
+  The following would previously report a ``SettingWithCopy`` Warning.
+
+  .. ipython:: python
+
+     df1 = DataFrame({'x': Series(['a','b','c']), 'y': Series(['d','e','f'])})
+     df2 = df1[['x']]
+     df2['y'] = ['g', 'h', 'i']
 
 .. _whatsnew_0160.deprecations:
 

pandas/core/generic.py

@@ -1265,6 +1265,14 @@ def _check_setitem_copy(self, stacklevel=4, t='setting', force=False):
             except:
                 pass
 
+            # we might be a false positive
+            try:
+                if self.is_copy().shape == self.shape:
+                    self.is_copy = None
+                    return
+            except:
+                pass
+
             # a custom message
             if isinstance(self.is_copy, string_types):
                 t = self.is_copy
@@ -1344,8 +1352,9 @@ def take(self, indices, axis=0, convert=True, is_copy=True):
         result = self._constructor(new_data).__finalize__(self)
 
         # maybe set copy if we didn't actually change the index
-        if is_copy and not result._get_axis(axis).equals(self._get_axis(axis)):
-            result._set_is_copy(self)
+        if is_copy:
+            if not result._get_axis(axis).equals(self._get_axis(axis)):
+                result._set_is_copy(self)
 
         return result
 

pandas/tests/test_indexing.py

@@ -3751,14 +3751,6 @@ def f():
         assert_series_equal(s,df.iloc[:,0].order())
         assert_series_equal(s,df[0].order())
 
-        # operating on a copy
-        df = pd.DataFrame({'a': list(range(4)), 'b': list('ab..'), 'c': ['a', 'b', np.nan, 'd']})
-        mask = pd.isnull(df.c)
-
-        def f():
-            df[['c']][mask] = df[['b']][mask]
-        self.assertRaises(com.SettingWithCopyError, f)
-
         # false positives GH6025
         df = DataFrame ({'column1':['a', 'a', 'a'], 'column2': [4,8,9] })
         str(df)
@@ -3790,6 +3782,24 @@ def f():
             df['C'][2] = 'foo'
         self.assertRaises(com.SettingWithCopyError, f)
 
+    def test_setting_with_copy_bug(self):
+
+        # operating on a copy
+        df = pd.DataFrame({'a': list(range(4)), 'b': list('ab..'), 'c': ['a', 'b', np.nan, 'd']})
+        mask = pd.isnull(df.c)
+
+        def f():
+            df[['c']][mask] = df[['b']][mask]
+        self.assertRaises(com.SettingWithCopyError, f)
+
+        # invalid warning as we are returning a new object
+        # GH 8730
+        df1 = DataFrame({'x': Series(['a','b','c']), 'y': Series(['d','e','f'])})
+        df2 = df1[['x']]
+
+        # this should not raise
+        df2['y'] = ['g', 'h', 'i']
+
     def test_detect_chained_assignment_warnings(self):
 
         # warnings