Backport PR pandas-dev#52841: BUG: dt.round with equal/higher freq resolution would not noop

Author: mroeschke

doc/source/whatsnew/v2.0.1.rst

@@ -27,6 +27,7 @@ Bug fixes
 ~~~~~~~~~
 - Bug in :attr:`Series.dt.days` that would overflow ``int32`` number of days (:issue:`52391`)
 - Bug in :class:`arrays.DatetimeArray` constructor returning an incorrect unit when passed a non-nanosecond numpy datetime array (:issue:`52555`)
+- Bug in :func:`Series.dt.round` when passing a ``freq`` of equal or higher resolution compared to the :class:`Series` would raise a ``ZeroDivisionError`` (:issue:`52761`)
 - Bug in :func:`Series.median` with :class:`ArrowDtype` returning an approximate median (:issue:`52679`)
 - Bug in :func:`api.interchange.from_dataframe` was unnecessarily raising on categorical dtypes (:issue:`49889`)
 - Bug in :func:`api.interchange.from_dataframe` was unnecessarily raising on large string dtypes (:issue:`52795`)

pandas/core/arrays/datetimelike.py

@@ -2005,8 +2005,12 @@ def _round(self, freq, mode, ambiguous, nonexistent):
 
         values = self.view("i8")
         values = cast(np.ndarray, values)
-        nanos = to_offset(freq).nanos  # raises on non-fixed frequencies
-        nanos = delta_to_nanoseconds(to_offset(freq), self._creso)
+        offset = to_offset(freq)
+        offset.nanos  # raises on non-fixed frequencies
+        nanos = delta_to_nanoseconds(offset, self._creso)
+        if nanos == 0:
+            # GH 52761
+            return self
         result_i8 = round_nsint64(values, mode, nanos)
         result = self._maybe_mask_results(result_i8, fill_value=iNaT)
         result = result.view(self._ndarray.dtype)

pandas/tests/series/accessors/test_dt_accessor.py

@@ -381,6 +381,17 @@ def test_dt_round_tz_nonexistent(self, method, ts_str, freq):
         with pytest.raises(pytz.NonExistentTimeError, match="2018-03-11 02:00:00"):
             getattr(ser.dt, method)(freq, nonexistent="raise")
 
+    @pytest.mark.parametrize("freq", ["ns", "U", "1000U"])
+    def test_dt_round_nonnano_higher_resolution_no_op(self, freq):
+        # GH 52761
+        ser = Series(
+            ["2020-05-31 08:00:00", "2000-12-31 04:00:05", "1800-03-14 07:30:20"],
+            dtype="datetime64[ms]",
+        )
+        expected = ser.copy()
+        result = ser.dt.round(freq)
+        tm.assert_series_equal(result, expected)
+
     def test_dt_namespace_accessor_categorical(self):
         # GH 19468
         dti = DatetimeIndex(["20171111", "20181212"]).repeat(2)