Switch to T_DataArray and T_Dataset in concat (#6784)

* Switch to T_DataArray in concat

* Switch tp T_Dataset in concat

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

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

* Update concat.py

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

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

* cast types

* Update concat.py

* Update concat.py

* Update concat.py

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

Author: Illviljan

xarray/core/concat.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, Hashable, Iterable, overload
+from typing import TYPE_CHECKING, Any, Hashable, Iterable, cast, overload
 
 import pandas as pd
 
@@ -14,42 +14,41 @@
     merge_attrs,
     merge_collected,
 )
+from .types import T_DataArray, T_Dataset
 from .variable import Variable
 from .variable import concat as concat_vars
 
 if TYPE_CHECKING:
-    from .dataarray import DataArray
-    from .dataset import Dataset
     from .types import CombineAttrsOptions, CompatOptions, ConcatOptions, JoinOptions
 
 
 @overload
 def concat(
-    objs: Iterable[Dataset],
-    dim: Hashable | DataArray | pd.Index,
+    objs: Iterable[T_Dataset],
+    dim: Hashable | T_DataArray | pd.Index,
     data_vars: ConcatOptions | list[Hashable] = "all",
     coords: ConcatOptions | list[Hashable] = "different",
     compat: CompatOptions = "equals",
     positions: Iterable[Iterable[int]] | None = None,
     fill_value: object = dtypes.NA,
     join: JoinOptions = "outer",
     combine_attrs: CombineAttrsOptions = "override",
-) -> Dataset:
+) -> T_Dataset:
     ...
 
 
 @overload
 def concat(
-    objs: Iterable[DataArray],
-    dim: Hashable | DataArray | pd.Index,
+    objs: Iterable[T_DataArray],
+    dim: Hashable | T_DataArray | pd.Index,
     data_vars: ConcatOptions | list[Hashable] = "all",
     coords: ConcatOptions | list[Hashable] = "different",
     compat: CompatOptions = "equals",
     positions: Iterable[Iterable[int]] | None = None,
     fill_value: object = dtypes.NA,
     join: JoinOptions = "outer",
     combine_attrs: CombineAttrsOptions = "override",
-) -> DataArray:
+) -> T_DataArray:
     ...
 
 
@@ -402,7 +401,7 @@ def process_subset_opt(opt, subset):
 
 # determine dimensional coordinate names and a dict mapping name to DataArray
 def _parse_datasets(
-    datasets: Iterable[Dataset],
+    datasets: Iterable[T_Dataset],
 ) -> tuple[dict[Hashable, Variable], dict[Hashable, int], set[Hashable], set[Hashable]]:
 
     dims: set[Hashable] = set()
@@ -429,16 +428,16 @@ def _parse_datasets(
 
 
 def _dataset_concat(
-    datasets: list[Dataset],
-    dim: str | DataArray | pd.Index,
+    datasets: list[T_Dataset],
+    dim: str | T_DataArray | pd.Index,
     data_vars: str | list[str],
     coords: str | list[str],
     compat: CompatOptions,
     positions: Iterable[Iterable[int]] | None,
     fill_value: object = dtypes.NA,
     join: JoinOptions = "outer",
     combine_attrs: CombineAttrsOptions = "override",
-) -> Dataset:
+) -> T_Dataset:
     """
     Concatenate a sequence of datasets along a new or existing dimension
     """
@@ -482,7 +481,8 @@ def _dataset_concat(
 
     # case where concat dimension is a coordinate or data_var but not a dimension
     if (dim in coord_names or dim in data_names) and dim not in dim_names:
-        datasets = [ds.expand_dims(dim) for ds in datasets]
+        # TODO: Overriding type because .expand_dims has incorrect typing:
+        datasets = [cast(T_Dataset, ds.expand_dims(dim)) for ds in datasets]
 
     # determine which variables to concatenate
     concat_over, equals, concat_dim_lengths = _calc_concat_over(
@@ -590,7 +590,7 @@ def get_indexes(name):
             # preserves original variable order
             result_vars[name] = result_vars.pop(name)
 
-    result = Dataset(result_vars, attrs=result_attrs)
+    result = type(datasets[0])(result_vars, attrs=result_attrs)
 
     absent_coord_names = coord_names - set(result.variables)
     if absent_coord_names:
@@ -618,16 +618,16 @@ def get_indexes(name):
 
 
 def _dataarray_concat(
-    arrays: Iterable[DataArray],
-    dim: str | DataArray | pd.Index,
+    arrays: Iterable[T_DataArray],
+    dim: str | T_DataArray | pd.Index,
     data_vars: str | list[str],
     coords: str | list[str],
     compat: CompatOptions,
     positions: Iterable[Iterable[int]] | None,
     fill_value: object = dtypes.NA,
     join: JoinOptions = "outer",
     combine_attrs: CombineAttrsOptions = "override",
-) -> DataArray:
+) -> T_DataArray:
     from .dataarray import DataArray
 
     arrays = list(arrays)
@@ -650,7 +650,8 @@ def _dataarray_concat(
             if compat == "identical":
                 raise ValueError("array names not identical")
             else:
-                arr = arr.rename(name)
+                # TODO: Overriding type because .rename has incorrect typing:
+                arr = cast(T_DataArray, arr.rename(name))
         datasets.append(arr._to_temp_dataset())
 
     ds = _dataset_concat(