Fix false timedelta decoding SerializationWarning and improve warning message (#10072)

* Fix false timedelta serialization warning

* Optional[PDDatetimeUnitOptions] -> PDDatetimeUnitOptions | None

Co-authored-by: Mathias Hauser <mathause@users.noreply.github.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: Mathias Hauser <mathause@users.noreply.github.com>
Co-authored-by: Kai Mühlbauer <kai.muehlbauer@uni-bonn.de>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 4 people

doc/whats-new.rst

@@ -62,6 +62,11 @@ Bug fixes
 - Fix DataArray().drop_attrs(deep=False) and add support for attrs to
   DataArray()._replace(). (:issue:`10027`, :pull:`10030`). By `Jan
   Haacker <https://github.com/j-haacker>`_.
+- Prevent false resolution change warnings from being emitted when decoding
+  timedeltas encoded with floating point values, and make it clearer how to
+  silence this warning message in the case that it is rightfully emitted
+  (:issue:`10071`, :pull:`10072`).  By `Spencer Clark
+  <https://github.com/spencerkclark>`_.
 - Fix ``isel`` for multi-coordinate Xarray indexes (:issue:`10063`, :pull:`10066`).
   By `Benoit Bovy <https://github.com/benbovy>`_.
 - Fix dask tokenization when opening each node in :py:func:`xarray.open_datatree`

xarray/coding/times.py

@@ -488,7 +488,7 @@ def _decode_datetime_with_pandas(
         flat_num_dates = flat_num_dates.astype(np.float64)
 
     timedeltas = _numbers_to_timedelta(
-        flat_num_dates, time_unit, ref_date.unit, "datetime"
+        flat_num_dates, time_unit, ref_date.unit, "datetimes"
     )
 
     # add timedeltas to ref_date
@@ -582,6 +582,7 @@ def _numbers_to_timedelta(
     time_unit: NPDatetimeUnitOptions,
     ref_unit: PDDatetimeUnitOptions,
     datatype: str,
+    target_unit: PDDatetimeUnitOptions | None = None,
 ) -> np.ndarray:
     """Transform numbers to np.timedelta64."""
     # keep NaT/nan mask
@@ -605,13 +606,23 @@ def _numbers_to_timedelta(
             flat_num, cast(PDDatetimeUnitOptions, time_unit)
         )
         if time_unit != new_time_unit:
-            msg = (
-                f"Can't decode floating point {datatype} to {time_unit!r} without "
-                f"precision loss, decoding to {new_time_unit!r} instead. "
-                f"To silence this warning use time_unit={new_time_unit!r} in call to "
-                f"decoding function."
-            )
-            emit_user_level_warning(msg, SerializationWarning)
+            if target_unit is None or np.timedelta64(1, target_unit) > np.timedelta64(
+                1, new_time_unit
+            ):
+                if datatype == "datetimes":
+                    kwarg = "decode_times"
+                    coder = "CFDatetimeCoder"
+                else:
+                    kwarg = "decode_timedelta"
+                    coder = "CFTimedeltaCoder"
+                formatted_kwarg = f"{kwarg}={coder}(time_unit={new_time_unit!r})"
+                message = (
+                    f"Can't decode floating point {datatype} to {time_unit!r} "
+                    f"without precision loss; decoding to {new_time_unit!r} "
+                    f"instead. To silence this warning pass {formatted_kwarg} "
+                    f"to your opening function."
+                )
+                emit_user_level_warning(message, SerializationWarning)
             time_unit = new_time_unit
 
     # Cast input ordinals to integers and properly handle NaN/NaT
@@ -640,7 +651,9 @@ def decode_cf_timedelta(
         _check_timedelta_range(np.nanmin(num_timedeltas), unit, time_unit)
         _check_timedelta_range(np.nanmax(num_timedeltas), unit, time_unit)
 
-    timedeltas = _numbers_to_timedelta(num_timedeltas, unit, "s", "timedelta")
+    timedeltas = _numbers_to_timedelta(
+        num_timedeltas, unit, "s", "timedeltas", target_unit=time_unit
+    )
     pd_timedeltas = pd.to_timedelta(ravel(timedeltas))
 
     if np.isnat(timedeltas).all():

xarray/tests/test_coding_times.py

@@ -1337,7 +1337,7 @@ def test_coding_float_datetime_warning(
     values = np.array([num], dtype="float32")
     with pytest.warns(
         SerializationWarning,
-        match=f"Can't decode floating point datetime to {time_unit!r}",
+        match=f"Can't decode floating point datetimes to {time_unit!r}",
     ):
         decode_cf_datetime(values, units, calendar, time_unit=time_unit)
 
@@ -1907,3 +1907,11 @@ def test_lazy_decode_timedelta_error() -> None:
     )
     with pytest.raises(OutOfBoundsTimedelta, match="overflow"):
         decoded.load()
+
+
+def test_decode_floating_point_timedelta_no_serialization_warning() -> None:
+    attrs = {"units": "seconds"}
+    encoded = Variable(["time"], [0, 0.1, 0.2], attrs=attrs)
+    decoded = conventions.decode_cf_variable("foo", encoded, decode_timedelta=True)
+    with assert_no_warnings():
+        decoded.load()