Fix mypy errors with newer pandas-stubs

Address 26 mypy errors that appear with pandas-stubs 2.3.3.251219, which
adds stricter type checking for pandas Index methods.

Changes by category:

1. Added type: ignore comments with explanatory notes for cases where
   pandas-stubs is stricter than actual pandas behavior:
   - CFTimeIndex.__add__/__radd__ return Self instead of overloaded types
   - Index.get_indexer accepts ndarray/list, not just Index
   - CategoricalIndex.remove_unused_categories missing from stubs
   - Series.where accepts broader argument types
   - ExtensionArray.astype accepts ExtensionDtype
   - Series[datetime].__setitem__ accepts np.nan (converts to NaT)
   - MultiIndex.rename accepts list of names

2. Added explicit type annotations to help mypy infer correct types:
   - coordinates.py: codes as list[np.ndarray]
   - dataset.py: arrays and extension_arrays list types
   - indexes.py: xr_index variable with PandasIndex | PandasMultiIndex

3. Removed redundant casts and fixed variable shadowing:
   - Removed unnecessary cast in remove_unused_levels_categories
   - Converted dict_keys to list for reorder_levels
   - Fixed test_backends.py variable naming (tdf -> tdf_series)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: max-sixty

xarray/coding/cftimeindex.py

@@ -514,12 +514,14 @@ def shift(  # type: ignore[override,unused-ignore]
             f"'freq' must be of type str or datetime.timedelta, got {type(freq)}."
         )
 
-    def __add__(self, other) -> Self:
+    # pandas-stubs defines many overloads for Index.__add__/__radd__ with specific
+    # return types, but CFTimeIndex legitimately returns Self for all cases
+    def __add__(self, other) -> Self:  # type: ignore[override,unused-ignore]
         if isinstance(other, pd.TimedeltaIndex):
             other = other.to_pytimedelta()
         return type(self)(np.array(self) + other)
 
-    def __radd__(self, other) -> Self:
+    def __radd__(self, other) -> Self:  # type: ignore[override,unused-ignore]
         if isinstance(other, pd.TimedeltaIndex):
             other = other.to_pytimedelta()
         return type(self)(other + np.array(self))

xarray/core/coordinates.py

@@ -169,7 +169,8 @@ def to_index(self, ordered_dims: Sequence[Hashable] | None = None) -> pd.Index:
 
             for i, index in enumerate(indexes):
                 if isinstance(index, pd.MultiIndex):
-                    codes, levels = index.codes, index.levels
+                    codes: list[np.ndarray] = list(index.codes)
+                    levels = index.levels
                 else:
                     code, level = pd.factorize(index)
                     codes = [code]

xarray/core/dataset.py

@@ -7402,8 +7402,8 @@ def from_dataframe(cls, dataframe: pd.DataFrame, sparse: bool = False) -> Self:
                 "cannot convert a DataFrame with a non-unique MultiIndex into xarray"
             )
 
-        arrays = []
-        extension_arrays = []
+        arrays: list[tuple[Hashable, np.ndarray]] = []
+        extension_arrays: list[tuple[Hashable, pd.Series]] = []
         for k, v in dataframe.items():
             if not is_allowed_extension_array(v) or isinstance(
                 v.array, UNSUPPORTED_EXTENSION_ARRAY_TYPES

xarray/core/extension_array.py

@@ -104,7 +104,9 @@ def as_extension_array(
             [array_or_scalar], dtype=dtype
         )
     else:
-        return array_or_scalar.astype(dtype, copy=copy)  # type: ignore[union-attr]
+        # pandas-stubs is overly strict about astype's dtype parameter and return type;
+        # ExtensionArray.astype accepts ExtensionDtype and returns ExtensionArray
+        return array_or_scalar.astype(dtype, copy=copy)  # type: ignore[union-attr,return-value,arg-type,unused-ignore]
 
 
 @implements(np.result_type)
@@ -192,10 +194,11 @@ def __extension_duck_array__where(
     # pd.where won't broadcast 0-dim arrays across a scalar-like series; scalar y's must be preserved
     if hasattr(y, "shape") and len(y.shape) == 1 and y.shape[0] == 1:
         y = y[0]  # type: ignore[index]
-    return cast(T_ExtensionArray, pd.Series(x).where(condition, y).array)  # type: ignore[arg-type]
+    # pandas-stubs has strict overloads for Series.where that don't cover all valid arg types
+    return cast(T_ExtensionArray, pd.Series(x).where(condition, y).array)  # type: ignore[arg-type,call-overload,unused-ignore]
 
 
-def _replace_duck(args, replacer: Callable[[PandasExtensionArray], list]) -> list:
+def _replace_duck(args, replacer: Callable[[PandasExtensionArray], Any]) -> list:
     args_as_list = list(args)
     for index, value in enumerate(args_as_list):
         if isinstance(value, PandasExtensionArray):

xarray/core/indexes.py

@@ -628,7 +628,8 @@ def get_indexer_nd(index: pd.Index, labels, method=None, tolerance=None) -> np.n
     flat_labels = np.ravel(labels)
     if flat_labels.dtype == "float16":
         flat_labels = flat_labels.astype("float64")
-    flat_indexer = index.get_indexer(flat_labels, method=method, tolerance=tolerance)
+    # pandas-stubs expects Index for get_indexer, but ndarray works at runtime
+    flat_indexer = index.get_indexer(flat_labels, method=method, tolerance=tolerance)  # type: ignore[arg-type,unused-ignore]
     indexer = flat_indexer.reshape(labels.shape)
     return indexer
 
@@ -978,14 +979,15 @@ def remove_unused_levels_categories(index: T_PDIndex) -> T_PDIndex:
     Remove unused levels from MultiIndex and unused categories from CategoricalIndex
     """
     if isinstance(index, pd.MultiIndex):
-        new_index = cast(pd.MultiIndex, index.remove_unused_levels())
+        new_index = index.remove_unused_levels()
         # if it contains CategoricalIndex, we need to remove unused categories
         # manually. See https://github.com/pandas-dev/pandas/issues/30846
         if any(isinstance(lev, pd.CategoricalIndex) for lev in new_index.levels):
             levels = []
             for i, level in enumerate(new_index.levels):
                 if isinstance(level, pd.CategoricalIndex):
-                    level = level[new_index.codes[i]].remove_unused_categories()
+                    # pandas-stubs is missing remove_unused_categories on CategoricalIndex
+                    level = level[new_index.codes[i]].remove_unused_categories()  # type: ignore[attr-defined,unused-ignore]
                 else:
                     level = level[new_index.codes[i]]
                 levels.append(level)
@@ -1229,7 +1231,7 @@ def reorder_levels(
         its corresponding coordinates.
 
         """
-        index = cast(pd.MultiIndex, self.index.reorder_levels(level_variables.keys()))
+        index = self.index.reorder_levels(list(level_variables.keys()))
         level_coords_dtype = {k: self.level_coords_dtype[k] for k in index.names}
         return self._replace(index, level_coords_dtype=level_coords_dtype)
 
@@ -1378,28 +1380,29 @@ def sel(self, labels, method=None, tolerance=None) -> IndexSelResult:
                     indexer = DataArray(indexer, coords=coords, dims=label.dims)
 
         if new_index is not None:
+            xr_index: PandasIndex | PandasMultiIndex
             if isinstance(new_index, pd.MultiIndex):
                 level_coords_dtype = {
                     k: self.level_coords_dtype[k] for k in new_index.names
                 }
-                new_index = self._replace(
+                xr_index = self._replace(
                     new_index, level_coords_dtype=level_coords_dtype
                 )
                 dims_dict = {}
-                drop_coords = []
+                drop_coords: list[Hashable] = []
             else:
-                new_index = PandasIndex(
+                xr_index = PandasIndex(
                     new_index,
                     new_index.name,
                     coord_dtype=self.level_coords_dtype[new_index.name],
                 )
-                dims_dict = {self.dim: new_index.index.name}
+                dims_dict = {self.dim: xr_index.index.name}
                 drop_coords = [self.dim]
 
             # variable(s) attrs and encoding metadata are propagated
             # when replacing the indexes in the resulting xarray object
-            new_vars = new_index.create_variables()
-            indexes = cast(dict[Any, Index], dict.fromkeys(new_vars, new_index))
+            new_vars = xr_index.create_variables()
+            indexes = cast(dict[Any, Index], dict.fromkeys(new_vars, xr_index))
 
             # add scalar variable for each dropped level
             variables = new_vars

xarray/core/missing.py

@@ -588,7 +588,8 @@ def _localize(obj: T, indexes_coords: SourceDest) -> tuple[T, SourceDest]:
         minval = np.nanmin(new_x_loaded)
         maxval = np.nanmax(new_x_loaded)
         index = x.to_index()
-        imin, imax = index.get_indexer([minval, maxval], method="nearest")
+        # pandas-stubs expects Index for get_indexer, but list works at runtime
+        imin, imax = index.get_indexer([minval, maxval], method="nearest")  # type: ignore[arg-type,unused-ignore]
         indexes[dim] = slice(max(imin - 2, 0), imax + 2)
         indexes_coords[dim] = (x[indexes[dim]], new_x)
     return obj.isel(indexes), indexes_coords  # type: ignore[attr-defined]

xarray/tests/test_backends.py

@@ -7500,10 +7500,10 @@ def test_write_file_from_np_str(str_type: type[str | np.str_], tmpdir: str) -> N
     )
     tdf.index.name = "scenario"
     tdf.columns.name = "year"
-    tdf = cast(pd.DataFrame, tdf.stack())
-    tdf.name = "tas"
+    tdf_series = cast(pd.Series, tdf.stack())
+    tdf_series.name = "tas"
 
-    txr = tdf.to_xarray()
+    txr = tdf_series.to_xarray()
 
     txr.to_netcdf(tmpdir.join("test.nc"))
 

xarray/tests/test_dataarray.py

@@ -3931,7 +3931,8 @@ def test_to_and_from_dict_with_nan_nat(self) -> None:
         y = np.random.randn(10, 3)
         y[2] = np.nan
         t = pd.Series(pd.date_range("20130101", periods=10))
-        t[2] = np.nan
+        # pandas-stubs doesn't allow np.nan for datetime Series, but it converts to NaT
+        t[2] = np.nan  # type: ignore[call-overload,unused-ignore]
         lat = [77.7, 83.2, 76]
         da = DataArray(y, {"t": t, "lat": lat}, dims=["t", "lat"])
         roundtripped = DataArray.from_dict(da.to_dict())

xarray/tests/test_dataset.py

@@ -3497,7 +3497,8 @@ def test_rename_multiindex(self) -> None:
         midx_coords = Coordinates.from_pandas_multiindex(midx, "x")
         original = Dataset({}, midx_coords)
 
-        midx_renamed = midx.rename(["a", "c"])
+        # pandas-stubs expects Hashable for rename, but list of names works for MultiIndex
+        midx_renamed = midx.rename(["a", "c"])  # type: ignore[call-overload,unused-ignore]
         midx_coords_renamed = Coordinates.from_pandas_multiindex(midx_renamed, "x")
         expected = Dataset({}, midx_coords_renamed)
 
@@ -5602,7 +5603,8 @@ def test_to_and_from_dict_with_nan_nat(
         y = np.random.randn(10, 3)
         y[2] = np.nan
         t = pd.Series(pd.date_range("20130101", periods=10))
-        t[2] = np.nan
+        # pandas-stubs doesn't allow np.nan for datetime Series, but it converts to NaT
+        t[2] = np.nan  # type: ignore[call-overload,unused-ignore]
 
         lat = [77.7, 83.2, 76]
         ds = Dataset(

xarray/tests/test_duck_array_ops.py

@@ -1150,6 +1150,7 @@ def test_extension_array_attr():
     assert (roundtripped == wrapped).all()
 
     interval_array = pd.arrays.IntervalArray.from_breaks([0, 1, 2, 3], closed="right")
-    wrapped = PandasExtensionArray(interval_array)
+    # pandas-stubs types PandasExtensionArray too narrowly; IntervalArray is valid
+    wrapped = PandasExtensionArray(interval_array)  # type: ignore[arg-type,unused-ignore]
     assert_array_equal(wrapped.left, interval_array.left, strict=True)
     assert wrapped.closed == interval_array.closed