ENH: enable unstack to create Series (#477)

pandas/core/reshape.py

@@ -283,7 +283,10 @@ def _slow_pivot(index, columns, values):
 
 def unstack(obj, level):
     if isinstance(obj, DataFrame):
-        return _unstack_frame(obj, level)
+        if isinstance(obj.index, MultiIndex):
+            return _unstack_frame(obj, level)
+        else:
+            return obj.transpose().stack(dropna=False)
     else:
         unstacker = _Unstacker(obj.values, obj.index, level=level)
         return unstacker.get_result()

pandas/tests/test_frame.py

@@ -3889,6 +3889,30 @@ def test_stack_unstack(self):
         assert_frame_equal(unstacked_cols.T, self.frame)
         assert_frame_equal(unstacked_cols_df['bar'].T, self.frame)
 
+    def test_unstack_to_series(self):
+        # check reversibility
+        data = self.frame.unstack()
+        self.assertTrue(isinstance(data, Series))
+        undo = data.unstack().transpose()
+        assert_frame_equal(undo, self.frame)
+
+        # check NA handling
+        data = DataFrame({'x': [1, 2, np.NaN], 'y': [3.0, 4, np.NaN]})
+        data.index = Index(['a','b','c'])
+        result = data.unstack()
+
+        midx = MultiIndex(levels=[['x','y'],['a','b','c']],
+                          labels=[[0,0,0,1,1,1],[0,1,2,0,1,2]])
+        expected = Series([1,2,np.NaN,3,4,np.NaN], index=midx)
+
+        assert_series_equal(result, expected)
+
+        # check composability of unstack
+        old_data = data.copy()
+        for _ in xrange(4):
+            data = data.unstack()
+        assert_frame_equal(old_data, data)
+
     def test_reset_index(self):
         stacked = self.frame.stack()[::2]
         stacked = DataFrame({'foo' : stacked, 'bar' : stacked})