avoid casting custom indexes in Dataset.drop_attrs (#10961)

* use `xindexes` when checking for indexes

* check that the fix works

* whats-new

Author: keewis

doc/whats-new.rst

@@ -41,6 +41,8 @@ Bug Fixes
   By `Ian Hunt-Isaak <https://github.com/ianhi>`_.
 - Always normalize slices when indexing ``LazilyIndexedArray`` instances (:issue:`10941`, :pull:`10948`).
   By `Justus Magin <https://github.com/keewis>`_.
+- Avoid casting custom indexes in ``Dataset.drop_attrs`` (:pull:`10961`)
+  By `Justus Magin <https://github.com/keewis>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/core/dataset.py

@@ -10475,7 +10475,7 @@ def drop_attrs(self, *, deep: bool = True) -> Self:
         for var in self.variables:
             # variables don't have a `._replace` method, so we copy and then remove
             # attrs. If we added a `._replace` method, we could use that instead.
-            if var not in self.indexes:
+            if var not in self.xindexes:
                 self[var] = self[var].copy()
                 self[var].attrs = {}
 

xarray/tests/test_dataset.py

@@ -4868,6 +4868,19 @@ def test_drop_attrs(self) -> None:
         assert list(result.data_vars) == list(ds.data_vars)
         assert list(result.coords) == list(ds.coords)
 
+    def test_drop_attrs_custom_index(self):
+        class CustomIndex(Index):
+            @classmethod
+            def from_variables(cls, variables, *, options=None):
+                return cls()
+
+        ds = xr.Dataset(coords={"y": ("x", [1, 2])}).set_xindex("y", CustomIndex)
+        # should not raise a TypeError
+        ds.drop_attrs()
+
+        # make sure the index didn't disappear
+        assert "y" in ds.xindexes
+
     def test_assign_multiindex_level(self) -> None:
         data = create_test_multiindex()
         with pytest.raises(ValueError, match=r"cannot drop or update.*corrupt.*index "):