BUG: Fixed ignoring of nanoseconds when adding to series #47856 (#48008)

* BUG: Fixed ignoring of nanoseconds when adding to series #47856

* Update doc/source/whatsnew/v1.5.0.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* BUG: Renamed test variables #47856

* BUG: Changed added if-else for performance #47856

* Update pandas/_libs/tslibs/offsets.pyx

Co-authored-by: Matthew Roeschke <[email protected]>

* BUG: Used getattr for nano check #47856

Co-authored-by: Steven Rotondo <[email protected]>
Co-authored-by: Matthew Roeschke <[email protected]>

doc/source/whatsnew/v1.5.0.rst

@@ -896,6 +896,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.resolution` incorrectly returning "day" instead of "nanosecond" for nanosecond-resolution indexes (:issue:`46903`)
 - Bug in :class:`Timestamp` with an integer or float value and ``unit="Y"`` or ``unit="M"`` giving slightly-wrong results (:issue:`47266`)
 - Bug in :class:`.DatetimeArray` construction when passed another :class:`.DatetimeArray` and ``freq=None`` incorrectly inferring the freq from the given array (:issue:`47296`)
+- Bug when adding a :class:`DateOffset` to a :class:`Series` would not add the ``nanoseconds`` field (:issue:`47856`)
 -
 
 Timedelta

pandas/_libs/tslibs/offsets.pyx

@@ -297,8 +297,8 @@ _relativedelta_kwds = {"years", "months", "weeks", "days", "year", "month",
 
 cdef _determine_offset(kwds):
     # timedelta is used for sub-daily plural offsets and all singular
-    # offsets relativedelta is used for plural offsets of daily length or
-    # more nanosecond(s) are handled by apply_wraps
+    # offsets, relativedelta is used for plural offsets of daily length or
+    # more, nanosecond(s) are handled by apply_wraps
     kwds_no_nanos = dict(
         (k, v) for k, v in kwds.items()
         if k not in ('nanosecond', 'nanoseconds')
@@ -1163,7 +1163,12 @@ cdef class RelativeDeltaOffset(BaseOffset):
             return dt64other
         elif not self._use_relativedelta and hasattr(self, "_offset"):
             # timedelta
-            delta = Timedelta(self._offset * self.n)
+            num_nano = getattr(self, "nanoseconds", 0)
+            if num_nano != 0:
+                rem_nano = Timedelta(nanoseconds=num_nano)
+                delta = Timedelta((self._offset + rem_nano) * self.n)
+            else:
+                delta = Timedelta(self._offset * self.n)
             td = (<_Timedelta>delta)._as_reso(reso)
             return dt64other + td
         else:

pandas/tests/tseries/offsets/test_offsets.py

@@ -33,6 +33,7 @@
 
 from pandas import (
     DatetimeIndex,
+    Series,
     date_range,
 )
 import pandas._testing as tm
@@ -987,7 +988,7 @@ def test_dateoffset_add_sub(offset_kwargs, expected_arg):
     assert result == expected
 
 
-def test_dataoffset_add_sub_timestamp_with_nano():
+def test_dateoffset_add_sub_timestamp_with_nano():
     offset = DateOffset(minutes=2, nanoseconds=9)
     ts = Timestamp(4)
     result = ts + offset
@@ -1032,3 +1033,26 @@ def test_construct_int_arg_no_kwargs_assumed_days(n):
     result = Timestamp(2022, 1, 2) + offset
     expected = Timestamp(2022, 1, 2 + n)
     assert result == expected
+
+
+@pytest.mark.parametrize(
+    "offset, expected",
+    [
+        (
+            DateOffset(minutes=7, nanoseconds=18),
+            Timestamp("2022-01-01 00:07:00.000000018"),
+        ),
+        (DateOffset(nanoseconds=3), Timestamp("2022-01-01 00:00:00.000000003")),
+    ],
+)
+def test_dateoffset_add_sub_timestamp_series_with_nano(offset, expected):
+    # GH 47856
+    start_time = Timestamp("2022-01-01")
+    teststamp = start_time
+    testseries = Series([start_time])
+    testseries = testseries + offset
+    assert testseries[0] == expected
+    testseries -= offset
+    assert testseries[0] == teststamp
+    testseries = offset + testseries
+    assert testseries[0] == expected