Switch to T_DataArray in .coords (#7285)

* Switch to T_DataArray in .coords

* Update coordinates.py

* Update coordinates.py

* mypy understands the type from items better apparanetly

* Update coordinates.py

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

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

* resolve DataArrayCoords generic type

* fix import

* Test adding __class_getitem__

* Update coordinates.py

* Test adding a _data slot.

* Adding class_getitem seems to work.

* test mypy on 3.8

* Update ci-additional.yaml

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

xarray/core/coordinates.py

@@ -2,7 +2,7 @@
 
 import warnings
 from contextlib import contextmanager
-from typing import TYPE_CHECKING, Any, Hashable, Iterator, Mapping, Sequence, cast
+from typing import TYPE_CHECKING, Any, Hashable, Iterator, List, Mapping, Sequence
 
 import numpy as np
 import pandas as pd
@@ -14,18 +14,27 @@
 from .variable import Variable, calculate_dimensions
 
 if TYPE_CHECKING:
+    from .common import DataWithCoords
     from .dataarray import DataArray
     from .dataset import Dataset
+    from .types import T_DataArray
 
 # Used as the key corresponding to a DataArray's variable when converting
 # arbitrary DataArray objects to datasets
 _THIS_ARRAY = ReprObject("<this-array>")
 
+# TODO: Remove when min python version >= 3.9:
+GenericAlias = type(List[int])
 
-class Coordinates(Mapping[Hashable, "DataArray"]):
-    __slots__ = ()
 
-    def __getitem__(self, key: Hashable) -> DataArray:
+class Coordinates(Mapping[Hashable, "T_DataArray"]):
+    _data: DataWithCoords
+    __slots__ = ("_data",)
+
+    # TODO: Remove when min python version >= 3.9:
+    __class_getitem__ = classmethod(GenericAlias)
+
+    def __getitem__(self, key: Hashable) -> T_DataArray:
         raise NotImplementedError()
 
     def __setitem__(self, key: Hashable, value: Any) -> None:
@@ -238,6 +247,8 @@ class DatasetCoordinates(Coordinates):
     objects.
     """
 
+    _data: Dataset
+
     __slots__ = ("_data",)
 
     def __init__(self, dataset: Dataset):
@@ -278,7 +289,7 @@ def variables(self) -> Mapping[Hashable, Variable]:
     def __getitem__(self, key: Hashable) -> DataArray:
         if key in self._data.data_vars:
             raise KeyError(key)
-        return cast("DataArray", self._data[key])
+        return self._data[key]
 
     def to_dataset(self) -> Dataset:
         """Convert these coordinates into a new Dataset"""
@@ -334,16 +345,18 @@ def _ipython_key_completions_(self):
         ]
 
 
-class DataArrayCoordinates(Coordinates):
+class DataArrayCoordinates(Coordinates["T_DataArray"]):
     """Dictionary like container for DataArray coordinates.
 
     Essentially a dict with keys given by the array's
     dimensions and the values given by corresponding DataArray objects.
     """
 
+    _data: T_DataArray
+
     __slots__ = ("_data",)
 
-    def __init__(self, dataarray: DataArray):
+    def __init__(self, dataarray: T_DataArray) -> None:
         self._data = dataarray
 
     @property
@@ -366,7 +379,7 @@ def dtypes(self) -> Frozen[Hashable, np.dtype]:
     def _names(self) -> set[Hashable]:
         return set(self._data._coords)
 
-    def __getitem__(self, key: Hashable) -> DataArray:
+    def __getitem__(self, key: Hashable) -> T_DataArray:
         return self._data._getitem_coord(key)
 
     def _update_coords(
@@ -452,7 +465,7 @@ def drop_coords(
 
 
 def assert_coordinate_consistent(
-    obj: DataArray | Dataset, coords: Mapping[Any, Variable]
+    obj: T_DataArray | Dataset, coords: Mapping[Any, Variable]
 ) -> None:
     """Make sure the dimension coordinate of obj is consistent with coords.
 

xarray/core/dataarray.py

@@ -3993,8 +3993,8 @@ def to_dict(self, data: bool = True, encoding: bool = False) -> dict[str, Any]:
         """
         d = self.variable.to_dict(data=data)
         d.update({"coords": {}, "name": self.name})
-        for k in self.coords:
-            d["coords"][k] = self.coords[k].variable.to_dict(data=data)
+        for k, coord in self.coords.items():
+            d["coords"][k] = coord.variable.to_dict(data=data)
         if encoding:
             d["encoding"] = dict(self.encoding)
         return d