BUG: date_range with DateOffset with nanoseconds

doc/source/whatsnew/v2.1.0.rst

@@ -293,9 +293,9 @@ Categorical
 Datetimelike
 ^^^^^^^^^^^^
 - :meth:`DatetimeIndex.map` with ``na_action="ignore"`` now works as expected. (:issue:`51644`)
+- Bug in :func:`date_range` when ``freq`` was a :class:`DateOffset` with ``nanoseconds`` (:issue:`46877`)
 - Bug in :meth:`Timestamp.round` with values close to the implementation bounds returning incorrect results instead of raising ``OutOfBoundsDatetime`` (:issue:`51494`)
 - Bug in :meth:`arrays.DatetimeArray.map` and :meth:`DatetimeIndex.map`, where the supplied callable operated array-wise instead of element-wise (:issue:`51977`)
--
 
 Timedelta
 ^^^^^^^^^

pandas/_libs/tslibs/offsets.pyx

@@ -1217,7 +1217,10 @@ cdef class RelativeDeltaOffset(BaseOffset):
 
     @apply_wraps
     def _apply(self, other: datetime) -> datetime:
+        other_nanos = 0
         if self._use_relativedelta:
+            if isinstance(other, _Timestamp):
+                other_nanos = other.nanosecond
             other = _as_datetime(other)
 
         if len(self.kwds) > 0:
@@ -1226,17 +1229,17 @@ cdef class RelativeDeltaOffset(BaseOffset):
                 # perform calculation in UTC
                 other = other.replace(tzinfo=None)
 
-            if hasattr(self, "nanoseconds"):
-                td_nano = Timedelta(nanoseconds=self.nanoseconds)
-            else:
-                td_nano = Timedelta(0)
-
             if self.n > 0:
                 for i in range(self.n):
-                    other = other + self._offset + td_nano
+                    other = other + self._offset
             else:
                 for i in range(-self.n):
-                    other = other - self._offset - td_nano
+                    other = other - self._offset
+
+            if hasattr(self, "nanoseconds"):
+                other = self.n * Timedelta(nanoseconds=self.nanoseconds) + other
+            if other_nanos != 0:
+                other = Timedelta(nanoseconds=other_nanos) + other
 
             if tzinfo is not None and self._use_relativedelta:
                 # bring tz back from UTC calculation

pandas/core/arrays/datetimes.py

@@ -2581,7 +2581,14 @@ def _generate_range(
                 break
 
             # faster than cur + offset
-            next_date = offset._apply(cur).as_unit(unit)
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "ignore",
+                    "Discarding nonzero nanoseconds in conversion",
+                    category=UserWarning,
+                )
+                next_date = offset._apply(cur)
+            next_date = next_date.as_unit(unit)
             if next_date <= cur:
                 raise ValueError(f"Offset {offset} did not increment date")
             cur = next_date
@@ -2595,7 +2602,14 @@ def _generate_range(
                 break
 
             # faster than cur + offset
-            next_date = offset._apply(cur).as_unit(unit)
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "ignore",
+                    "Discarding nonzero nanoseconds in conversion",
+                    category=UserWarning,
+                )
+                next_date = offset._apply(cur)
+            next_date = next_date.as_unit(unit)
             if next_date >= cur:
                 raise ValueError(f"Offset {offset} did not decrement date")
             cur = next_date

pandas/tests/indexes/datetimes/test_date_range.py

@@ -790,6 +790,27 @@ def test_range_where_start_equal_end(self, inclusive_endpoints_fixture):
 
         tm.assert_index_equal(result, expected)
 
+    def test_freq_dateoffset_with_relateivedelta_nanos(self):
+        # GH 46877
+        freq = DateOffset(hours=10, days=57, nanoseconds=3)
+        result = date_range(end="1970-01-01 00:00:00", periods=10, freq=freq, name="a")
+        expected = DatetimeIndex(
+            [
+                "1968-08-02T05:59:59.999999973",
+                "1968-09-28T15:59:59.999999976",
+                "1968-11-25T01:59:59.999999979",
+                "1969-01-21T11:59:59.999999982",
+                "1969-03-19T21:59:59.999999985",
+                "1969-05-16T07:59:59.999999988",
+                "1969-07-12T17:59:59.999999991",
+                "1969-09-08T03:59:59.999999994",
+                "1969-11-04T13:59:59.999999997",
+                "1970-01-01T00:00:00.000000000",
+            ],
+            name="a",
+        )
+        tm.assert_index_equal(result, expected)
+
 
 class TestDateRangeTZ:
     """Tests for date_range with timezones"""