BUG: Set frequency for empty Series

closes #14320

Author: Sahil Dua <[email protected]>

Closes #14458 from sahildua2305/frequency-series-fix and squashes the following commits:

384e666 [Sahil Dua] BUG: Set frequency for empty Series

Author: sahildua2305

doc/source/whatsnew/v0.20.0.txt

@@ -605,6 +605,9 @@ Bug Fixes
 
 
 - Bug in the display of ``.info()`` where a qualifier (+) would always be displayed with a ``MultiIndex`` that contains only non-strings (:issue:`15245`)
+
+- Bug in ``.asfreq()``, where frequency was not set for empty ``Series` (:issue:`14320`)
+
 - Bug in ``pd.read_msgpack()`` in which ``Series`` categoricals were being improperly processed (:issue:`14901`)
 - Bug in ``Series.ffill()`` with mixed dtypes containing tz-aware datetimes. (:issue:`14956`)
 

pandas/tests/series/test_timeseries.py

@@ -260,6 +260,14 @@ def test_asfreq(self):
                           index=[-1.0, 2.0, 1.0, 0.0]).sort_index()
         assert_series_equal(result, expected)
 
+    def test_asfreq_datetimeindex_empty_series(self):
+        # GH 14320
+        expected = Series(index=pd.DatetimeIndex(
+            ["2016-09-29 11:00"])).asfreq('H')
+        result = Series(index=pd.DatetimeIndex(["2016-09-29 11:00"]),
+                        data=[3]).asfreq('H')
+        self.assert_index_equal(expected.index, result.index)
+
     def test_diff(self):
         # Just run the function
         self.ts.diff()

pandas/tseries/resample.py

@@ -1382,16 +1382,18 @@ def asfreq(obj, freq, method=None, how=None, normalize=False, fill_value=None):
         if how is None:
             how = 'E'
 
-        new_index = obj.index.asfreq(freq, how=how)
         new_obj = obj.copy()
-        new_obj.index = new_index
-        return new_obj
+        new_obj.index = obj.index.asfreq(freq, how=how)
+
+    elif len(obj.index) == 0:
+        new_obj = obj.copy()
+        new_obj.index = obj.index._shallow_copy(freq=to_offset(freq))
+
     else:
-        if len(obj.index) == 0:
-            return obj.copy()
         dti = date_range(obj.index[0], obj.index[-1], freq=freq)
         dti.name = obj.index.name
-        rs = obj.reindex(dti, method=method, fill_value=fill_value)
+        new_obj = obj.reindex(dti, method=method, fill_value=fill_value)
         if normalize:
-            rs.index = rs.index.normalize()
-        return rs
+            new_obj.index = new_obj.index.normalize()
+
+    return new_obj