BUG: Append the empty frame with columns, #3121

pandas/core/common.py

@@ -1667,6 +1667,9 @@ def _concat_compat(to_concat, axis=0):
     # filter empty arrays
     to_concat = [x for x in to_concat if x.shape[axis] > 0]
 
+    # return the empty np array, if nothing to concatenate, #3121
+    if not to_concat: return np.array([], dtype=object)
+
     is_datetime64 = [x.dtype == _NS_DTYPE for x in to_concat]
     if all(is_datetime64):
         # work around NumPy 1.6 bug

pandas/tests/test_frame.py

@@ -5108,6 +5108,36 @@ def test_append_list_of_series_dicts(self):
         expected = df.append(DataFrame(dicts), ignore_index=True)
         assert_frame_equal(result, expected)
 
+    def test_append_empty_dataframe(self):
+
+        # Empty df append empty df
+        df1 = DataFrame([])
+        df2 = DataFrame([])
+        result = df1.append(df2)
+        expected = df1.copy()
+        assert_frame_equal(result, expected)
+
+        # Non-empty df append empty df
+        df1 = DataFrame(np.random.randn(5, 2))
+        df2 = DataFrame()
+        result = df1.append(df2)
+        expected = df1.copy()
+        assert_frame_equal(result, expected)
+
+        # Empty df with columns append empty df
+        df1 = DataFrame(columns=['bar', 'foo'])
+        df2 = DataFrame()
+        result = df1.append(df2)
+        expected = df1.copy()
+        assert_frame_equal(result, expected)
+
+        # Non-Empty df with columns append empty df
+        df1 = DataFrame(np.random.randn(5, 2), columns=['bar', 'foo'])
+        df2 = DataFrame()
+        result = df1.append(df2)
+        expected = df1.copy()
+        assert_frame_equal(result, expected)
+
     def test_asfreq(self):
         offset_monthly = self.tsframe.asfreq(datetools.bmonthEnd)
         rule_monthly = self.tsframe.asfreq('BM')