BUG: fix size()/count() when resamping empty series (pandas-dev#28427)

Author: Gabriel Corona

doc/source/whatsnew/v1.0.0.rst

@@ -210,6 +210,7 @@ Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 -
+- Bug in :meth:`Resampler.use` and `Resampler.count()` when used with an empty `Series` (:issue:`28427`)
 - Bug in :meth:`DataFrame.rolling` not allowing for rolling over datetimes when ``axis=1`` (:issue: `28192`)
 - Bug in :meth:`DataFrame.groupby` not offering selection by column name when ``axis=1`` (:issue:`27614`)
 - Bug in :meth:`DataFrameGroupby.agg` not able to use lambda function with named aggregation (:issue:`27519`)

pandas/core/resample.py

@@ -860,6 +860,25 @@ def size(self):
             from pandas import Series
 
             result = Series([], index=result.index, dtype="int64")
+        elif not len(self.ax) and isinstance(self._selected_obj, ABCSeries):
+            from pandas import Series
+
+            result = Series(
+                [], index=result.index, dtype="int64", name=self._selected_obj.name
+            )
+
+        return result
+
+    @Appender(GroupBy.size.__doc__)
+    def count(self):
+        result = self._downsample("count")
+        if not len(self.ax) and isinstance(self._selected_obj, ABCSeries):
+            from pandas import Series
+
+            result = Series(
+                [], index=result.index, dtype="int64", name=self._selected_obj.name
+            )
+
         return result
 
     def quantile(self, q=0.5, **kwargs):
@@ -907,14 +926,6 @@ def g(self, _method=method, *args, **kwargs):
     g.__doc__ = getattr(GroupBy, method).__doc__
     setattr(Resampler, method, g)
 
-# groupby & aggregate methods
-for method in ["count"]:
-
-    def h(self, _method=method):
-        return self._downsample(_method)
-
-    h.__doc__ = getattr(GroupBy, method).__doc__
-    setattr(Resampler, method, h)
 
 # series only methods
 for method in ["nunique"]:

pandas/tests/resample/test_base.py

@@ -118,6 +118,25 @@ def test_resample_empty_series(freq, empty_series, resample_method):
     assert_series_equal(result, expected, check_dtype=False)
 
 
+@pytest.mark.parametrize("freq", ["M", "D", "H"])
+@pytest.mark.parametrize("resample_method", ["count", "size"])
+def test_resample_count_empty_series(freq, resample_method):
+    # GH28427
+
+    empty_series = pd.Series([], dtype="datetime64[ns]", index=pd.DatetimeIndex([]))
+    result = getattr(empty_series.resample(freq), resample_method)()
+
+    expected = pd.Series([], dtype="int64", index=pd.DatetimeIndex([]))
+    if isinstance(empty_series.index, PeriodIndex):
+        expected.index = empty_series.index.asfreq(freq=freq)
+    else:
+        expected.index = empty_series.index._shallow_copy(freq=freq)
+
+    assert_index_equal(result.index, expected.index)
+    assert result.index.freq == expected.index.freq
+    assert_series_equal(result, expected, check_dtype=True)
+
+
 @all_ts
 @pytest.mark.parametrize("freq", ["M", "D", "H"])
 def test_resample_empty_dataframe(empty_frame, freq, resample_method):