BUG: Bug Multiindex assignment with an aligned rhs (but at different levels) (GH7475)

doc/source/v0.14.1.txt

@@ -156,8 +156,7 @@ Bug Fixes
 ~~~~~~~~~
 
 
-
-
+- Bug Multiindex assignment with an aligned rhs (but at different levels) (:issue:`7475`)
 
 
 

pandas/core/frame.py

@@ -1980,6 +1980,19 @@ def _set_item(self, key, value):
 
         is_existing = key in self.columns
         self._ensure_valid_index(value)
+
+        # have a new column, a multi-index, but no specification for the index,
+        # have to assume first level; this in effect broadcasts across the index
+        # GH 7475
+        if not is_existing and isinstance(self.columns, MultiIndex) and not isinstance(key, tuple):
+            value_columns = self.columns.set_levels([[key]] + self.columns.levels[1:])
+            value = self._sanitize_column(key, value)
+            if len(value_columns) != value.ndim:
+                raise ValueError("cannot assign to a multi-index with invalid dimensions")
+            for i, v in zip(value_columns,value):
+                NDFrame._set_item(self, i, v)
+            return self
+
         value = self._sanitize_column(key, value)
         NDFrame._set_item(self, key, value)
 

pandas/tests/test_indexing.py

@@ -1828,6 +1828,18 @@ def f():
             df.loc['bar'] *= 2
         self.assertRaises(TypeError, f)
 
+    def test_multiindex_setitem(self):
+
+        # GH 7475
+        # multi-index assignment
+
+        df_orig = DataFrame(np.arange(10,dtype='int64').reshape(-1,2), columns=pd.MultiIndex.from_tuples([('Val','A'),('Val','B')]))
+        expected = concat([df_orig['Val'],df_orig['Val']/2],axis=1,keys=['Val','Half'])
+
+        df = df_orig.copy()
+        df['Half'] = df['Val']/2
+        assert_frame_equal(df,expected)
+
     def test_getitem_multiindex(self):
 
         # GH 5725