Switch .dt to raise an AttributeError (#8724)

* Switch `.dt` to raise an `AttributeError`

Discussion at #8718

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

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

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: max-sixty

doc/whats-new.rst

@@ -82,6 +82,9 @@ Documentation
 Internal Changes
 ~~~~~~~~~~~~~~~~
 
+- ``DataArray.dt`` now raises an ``AttributeError`` rather than a ``TypeError``
+  when the data isn't datetime-like. (:issue:`8718`, :pull:`8724`)
+  By `Maximilian Roos <https://github.com/max-sixty>`_.
 
 .. _whats-new.2024.01.1:
 

xarray/core/accessor_dt.py

@@ -616,7 +616,11 @@ def __new__(cls, obj: T_DataArray) -> CombinedDatetimelikeAccessor:
         # appropriate. Since we're checking the dtypes anyway, we'll just
         # do all the validation here.
         if not _contains_datetime_like_objects(obj.variable):
-            raise TypeError(
+            # We use an AttributeError here so that `obj.dt` raises an error that
+            # `getattr` expects; https://github.com/pydata/xarray/issues/8718. It's a
+            # bit unusual in a `__new__`, but that's the only case where we use this
+            # class.
+            raise AttributeError(
                 "'.dt' accessor only available for "
                 "DataArray with datetime64 timedelta64 dtype or "
                 "for arrays containing cftime datetime "

xarray/tests/test_accessor_dt.py

@@ -145,7 +145,7 @@ def test_not_datetime_type(self) -> None:
         nontime_data = self.data.copy()
         int_data = np.arange(len(self.data.time)).astype("int8")
         nontime_data = nontime_data.assign_coords(time=int_data)
-        with pytest.raises(TypeError, match=r"dt"):
+        with pytest.raises(AttributeError, match=r"dt"):
             nontime_data.time.dt
 
     @pytest.mark.filterwarnings("ignore:dt.weekofyear and dt.week have been deprecated")
@@ -310,7 +310,7 @@ def test_not_datetime_type(self) -> None:
         nontime_data = self.data.copy()
         int_data = np.arange(len(self.data.time)).astype("int8")
         nontime_data = nontime_data.assign_coords(time=int_data)
-        with pytest.raises(TypeError, match=r"dt"):
+        with pytest.raises(AttributeError, match=r"dt"):
             nontime_data.time.dt
 
     @pytest.mark.parametrize(