BUG: Append the empty frame with columns, #3121

Author: waitingkuo

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 +5108,84 @@ 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')
+
+        assert_almost_equal(offset_monthly['A'], rule_monthly['A'])
+
+        filled = rule_monthly.asfreq('B', method='pad')
+        # TODO: actually check that this worked.
+
+        # don't forget!
+        filled_dep = rule_monthly.asfreq('B', method='pad')
+
+        # test does not blow up on length-0 DataFrame
+        zero_length = self.tsframe.reindex([])
+        result = zero_length.asfreq('BM')
+        self.assert_(result is not zero_length)
+
+    def test_asfreq_datetimeindex(self):
+        df = DataFrame({'A': [1, 2, 3]},
+                       index=[datetime(2011, 11, 01), datetime(2011, 11, 2),
+                              datetime(2011, 11, 3)])
+        df = df.asfreq('B')
+        self.assert_(isinstance(df.index, DatetimeIndex))
+
+        ts = df['A'].asfreq('B')
+        self.assert_(isinstance(ts.index, DatetimeIndex))
+
+    def test_at_time_between_time_datetimeindex(self):
+        index = pan.date_range("2012-01-01", "2012-01-05", freq='30min')
+        df = DataFrame(randn(len(index), 5), index=index)
+        akey = time(12, 0, 0)
+        bkey = slice(time(13, 0, 0), time(14, 0, 0))
+        ainds = [24, 72, 120, 168]
+        binds = [26, 27, 28, 74, 75, 76, 122, 123, 124, 170, 171, 172]
+
+        result = df.at_time(akey)
+        expected = df.ix[akey]
+        expected2 = df.ix[ainds]
+        assert_frame_equal(result, expected)
+        assert_frame_equal(result, expected2)
+        self.assert_(len(result) == 4)
+
+        result = df.between_time(bkey.start, bkey.stop)
+        expected = df.ix[bkey]
+        expected2 = df.ix[binds]
+
+        # Non-Empty df with columns append empty df
+
     def test_asfreq(self):
         offset_monthly = self.tsframe.asfreq(datetools.bmonthEnd)
         rule_monthly = self.tsframe.asfreq('BM')