TST: unit test to assert behavior described in #2525

RELEASE.rst

@@ -231,6 +231,8 @@ pandas 0.10.0
   - DataFrame.sort_index raises more helpful exception if sorting by column
     with duplicates (GH2488_)
   - DataFrame.to_string formatters can be list, too (GH2520_)
+  - DataFrame.combine_first will always result in the union of the index and
+    columns, even if one DataFrame is length-zero (GH2525_)
 
 .. _GH407: https://github.com/pydata/pandas/issues/407
 .. _GH821: https://github.com/pydata/pandas/issues/821
@@ -345,6 +347,7 @@ pandas 0.10.0
 .. _GH2307: https://github.com/pydata/pandas/issues/2307
 .. _GH2488: https://github.com/pydata/pandas/issues/2488
 .. _GH2520: https://github.com/pydata/pandas/issues/2520
+.. _GH2525: https://github.com/pydata/pandas/issues/2525
 
 
 pandas 0.9.1

pandas/core/frame.py

@@ -3690,7 +3690,8 @@ def combine(self, other, func, fill_value=None):
     def combine_first(self, other):
         """
         Combine two DataFrame objects and default to non-null values in frame
-        calling the method. Result index will be the union of the two indexes
+        calling the method. Result index columns will be the union of the
+        respective indexes and columns
 
         Parameters
         ----------

pandas/tests/test_frame.py

@@ -6188,6 +6188,12 @@ def test_combine_first(self):
         comb = self.frame.combine_first(DataFrame(index=["faz","boo"]))
         self.assertTrue("faz" in comb.index)
 
+        # #2525
+        df = DataFrame({'a': [1]}, index=[datetime(2012,1,1)])
+        df2 = DataFrame({}, columns=['b'])
+        result = df.combine_first(df2)
+        self.assertTrue('b' in result)
+
     def test_combine_first_mixed_bug(self):
         idx = Index(['a','b','c','e'])
         ser1 = Series([5.0,-9.0,4.0,100.],index=idx)