BUG pandas-dev#14962

Author: discort

pandas/tseries/resample.py

@@ -699,6 +699,10 @@ def _downsample(self, how, **kwargs):
         if not len(ax):
             # reset to the new freq
             obj = obj.copy()
+            if how == "size" and isinstance(obj, pd.DataFrame):
+                obj = obj.groupby(
+                    self.grouper, axis=self.axis).aggregate(how, **kwargs)
+
             obj.index.freq = self.freq
             return obj
 

pandas/tseries/tests/test_resample.py

@@ -26,7 +26,8 @@
                                      DatetimeIndexResampler)
 from pandas.tseries.tdi import timedelta_range, TimedeltaIndex
 from pandas.util.testing import (assert_series_equal, assert_almost_equal,
-                                 assert_frame_equal, assert_index_equal)
+                                 assert_frame_equal, assert_index_equal,
+                                 assertIsInstance)
 from pandas._period import IncompatibleFrequency
 
 bday = BDay()
@@ -824,6 +825,14 @@ def test_resample_loffset_arg_type(self):
                 assert_frame_equal(result_agg, expected)
                 assert_frame_equal(result_how, expected)
 
+    def test_resample_empty_dataframe_with_size(self):
+        # GH 14962
+        df1 = pd.DataFrame(dict(a=range(100)),
+                           index=pd.date_range('1/1/2000', periods=100, freq="M"))
+        df2 = df1[df1.a < 0]
+        result = df2.resample("Q").size()
+        assertIsInstance(result, pd.Series)
+
 
 class TestDatetimeIndex(Base, tm.TestCase):
     _multiprocess_can_split_ = True