BUG: resample and apply modify the index type for empty Series (#17149)

Author: discort

doc/source/whatsnew/v0.21.0.txt

@@ -348,6 +348,7 @@ Groupby/Resample/Rolling
 - Bug in :func:`infer_freq` causing indices with 2-day gaps during the working week to be wrongly inferred as business daily (:issue:`16624`)
 - Bug in ``.rolling(...).quantile()`` which incorrectly used different defaults than :func:`Series.quantile()` and :func:`DataFrame.quantile()` (:issue:`9413`, :issue:`16211`)
 - Bug in ``groupby.transform()`` that would coerce boolean dtypes back to float (:issue:`16875`)
+- Bug in ``Series.resample(...).apply()`` where an empty ``Series`` modified the source index and did not return the name of a ``Series`` (:issue:`14313`)
 
 Sparse
 ^^^^^^

pandas/core/resample.py

@@ -17,7 +17,7 @@
 from pandas.core.indexes.period import PeriodIndex, period_range
 import pandas.core.common as com
 import pandas.core.algorithms as algos
-from pandas.core.dtypes.generic import ABCDataFrame
+from pandas.core.dtypes.generic import ABCDataFrame, ABCSeries
 
 import pandas.compat as compat
 from pandas.compat.numpy import function as nv
@@ -439,6 +439,11 @@ def _wrap_result(self, result):
         if isinstance(result, com.ABCSeries) and self._selection is not None:
             result.name = self._selection
 
+        if isinstance(result, ABCSeries) and result.empty:
+            obj = self.obj
+            result.index = obj.index._shallow_copy(freq=to_offset(self.freq))
+            result.name = getattr(obj, 'name', None)
+
         return result
 
     def pad(self, limit=None):

pandas/tests/test_resample.py

@@ -852,6 +852,16 @@ def test_resample_loffset_arg_type(self):
                 assert_frame_equal(result_agg, expected)
                 assert_frame_equal(result_how, expected)
 
+    def test_apply_to_empty_series(self):
+        # GH 14313
+        series = self.create_series()[:0]
+
+        for freq in ['M', 'D', 'H']:
+            result = series.resample(freq).apply(lambda x: 1)
+            expected = series.resample(freq).apply(np.sum)
+
+            assert_series_equal(result, expected, check_dtype=False)
+
 
 class TestDatetimeIndex(Base):
     _index_factory = lambda x: date_range
@@ -2794,6 +2804,14 @@ def test_evenly_divisible_with_no_extra_bins(self):
         result = df.resample('7D').sum()
         assert_frame_equal(result, expected)
 
+    def test_apply_to_empty_series(self):
+        # GH 14313
+        series = self.create_series()[:0]
+
+        for freq in ['M', 'D', 'H']:
+            with pytest.raises(TypeError):
+                series.resample(freq).apply(lambda x: 1)
+
 
 class TestTimedeltaIndex(Base):
     _index_factory = lambda x: timedelta_range