BUG: Set frequency for empty Series; fixes #14320

Add test to verify frequency for empty series

Update test to compare empty series behaviour
against normal series

Add entry in whatsnew 0.20.0

Author: sahildua2305

doc/source/whatsnew/v0.20.0.txt

@@ -79,3 +79,5 @@ Performance Improvements
 
 Bug Fixes
 ~~~~~~~~~
+
+- Bug in ``asfreq``, where frequency wasn't set for empty series (:issue:`14320`)

pandas/tests/frame/test_timeseries.py

@@ -323,6 +323,11 @@ def test_asfreq_datetimeindex(self):
         ts = df['A'].asfreq('B')
         tm.assertIsInstance(ts.index, DatetimeIndex)
 
+    def test_asfreq_datetimeindex_empty_series(self):
+        empty_series = Series(index=pd.DatetimeIndex([]), data=[]).asfreq('H')
+        normal_series = Series(index=pd.DatetimeIndex(["2016-09-29 11:00"]), data=[3]).asfreq('H')
+        self.assertEqual(empty_series.index.freq, normal_series.index.freq)
+
     def test_first_last_valid(self):
         N = len(self.frame.index)
         mat = randn(N)

pandas/tseries/resample.py

@@ -1344,7 +1344,10 @@ def asfreq(obj, freq, method=None, how=None, normalize=False):
         return new_obj
     else:
         if len(obj.index) == 0:
-            return obj.copy()
+            new_index = obj.index._shallow_copy(freq=to_offset(freq))
+            new_obj = obj.copy()
+            new_obj.index = new_index
+            return new_obj
         dti = date_range(obj.index[0], obj.index[-1], freq=freq)
         dti.name = obj.index.name
         rs = obj.reindex(dti, method=method)