Add getitem to array protocol (#8406)

* Update _typing.py

* Update _typing.py

* Update test_namedarray.py

* fixes

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

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

* Update _typing.py

* Update _typing.py

---------

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

xarray/namedarray/_typing.py

@@ -29,7 +29,7 @@ class Default(Enum):
 _T = TypeVar("_T")
 _T_co = TypeVar("_T_co", covariant=True)
 
-
+_dtype = np.dtype
 _DType = TypeVar("_DType", bound=np.dtype[Any])
 _DType_co = TypeVar("_DType_co", covariant=True, bound=np.dtype[Any])
 # A subset of `npt.DTypeLike` that can be parametrized w.r.t. `np.generic`
@@ -69,9 +69,16 @@ def dtype(self) -> _DType_co:
 _Dims = tuple[_Dim, ...]
 
 _DimsLike = Union[str, Iterable[_Dim]]
-_AttrsLike = Union[Mapping[Any, Any], None]
 
-_dtype = np.dtype
+# https://data-apis.org/array-api/latest/API_specification/indexing.html
+# TODO: np.array_api was bugged and didn't allow (None,), but should!
+# https://github.com/numpy/numpy/pull/25022
+# https://github.com/data-apis/array-api/pull/674
+_IndexKey = Union[int, slice, "ellipsis"]
+_IndexKeys = tuple[Union[_IndexKey], ...]  #  tuple[Union[_IndexKey, None], ...]
+_IndexKeyLike = Union[_IndexKey, _IndexKeys]
+
+_AttrsLike = Union[Mapping[Any, Any], None]
 
 
 class _SupportsReal(Protocol[_T_co]):
@@ -113,6 +120,25 @@ class _arrayfunction(
     Corresponds to np.ndarray.
     """
 
+    @overload
+    def __getitem__(
+        self, key: _arrayfunction[Any, Any] | tuple[_arrayfunction[Any, Any], ...], /
+    ) -> _arrayfunction[Any, _DType_co]:
+        ...
+
+    @overload
+    def __getitem__(self, key: _IndexKeyLike, /) -> Any:
+        ...
+
+    def __getitem__(
+        self,
+        key: _IndexKeyLike
+        | _arrayfunction[Any, Any]
+        | tuple[_arrayfunction[Any, Any], ...],
+        /,
+    ) -> _arrayfunction[Any, _DType_co] | Any:
+        ...
+
     @overload
     def __array__(self, dtype: None = ..., /) -> np.ndarray[Any, _DType_co]:
         ...
@@ -165,6 +191,14 @@ class _arrayapi(_array[_ShapeType_co, _DType_co], Protocol[_ShapeType_co, _DType
     Corresponds to np.ndarray.
     """
 
+    def __getitem__(
+        self,
+        key: _IndexKeyLike
+        | Any,  # TODO: Any should be _arrayapi[Any, _dtype[np.integer]]
+        /,
+    ) -> _arrayapi[Any, Any]:
+        ...
+
     def __array_namespace__(self) -> ModuleType:
         ...
 

xarray/tests/test_namedarray.py

@@ -28,6 +28,7 @@
         _AttrsLike,
         _DimsLike,
         _DType,
+        _IndexKeyLike,
         _Shape,
         duckarray,
     )
@@ -58,6 +59,19 @@ class CustomArrayIndexable(
     ExplicitlyIndexed,
     Generic[_ShapeType_co, _DType_co],
 ):
+    def __getitem__(
+        self, key: _IndexKeyLike | CustomArrayIndexable[Any, Any], /
+    ) -> CustomArrayIndexable[Any, _DType_co]:
+        if isinstance(key, CustomArrayIndexable):
+            if isinstance(key.array, type(self.array)):
+                # TODO: key.array is duckarray here, can it be narrowed down further?
+                # an _arrayapi cannot be used on a _arrayfunction for example.
+                return type(self)(array=self.array[key.array])  # type: ignore[index]
+            else:
+                raise TypeError("key must have the same array type as self")
+        else:
+            return type(self)(array=self.array[key])
+
     def __array_namespace__(self) -> ModuleType:
         return np