pydata
diff --git a/‎xarray/backends/api.py
Lines changed: 259 additions & 105 deletions b/‎xarray/backends/api.py
Lines changed: 259 additions & 105 deletions
diff --git a/‎xarray/backends/common.py
Lines changed: 8 additions & 6 deletions b/‎xarray/backends/common.py
Lines changed: 8 additions & 6 deletions
diff --git a/‎xarray/core/alignment.py
Lines changed: 5 additions & 3 deletions b/‎xarray/core/alignment.py
Lines changed: 5 additions & 3 deletions
diff --git a/‎xarray/core/combine.py
Lines changed: 39 additions & 32 deletions b/‎xarray/core/combine.py
Lines changed: 39 additions & 32 deletions
diff --git a/‎xarray/core/computation.py
Lines changed: 26 additions & 9 deletions b/‎xarray/core/computation.py
Lines changed: 26 additions & 9 deletions
@@ -1,8 +1,10 @@
+from __future__ import annotations
+
 import logging
 import os
 import time
 import traceback
-from typing import Any, Dict, Tuple, Type, Union
+from typing import Any
 
 import numpy as np
 
@@ -369,13 +371,13 @@ class BackendEntrypoint:
       method is not mandatory.
     """
 
-    open_dataset_parameters: Union[Tuple, None] = None
+    open_dataset_parameters: tuple | None = None
     """list of ``open_dataset`` method parameters"""
 
     def open_dataset(
         self,
-        filename_or_obj: str,
-        drop_variables: Tuple[str] = None,
+        filename_or_obj: str | os.PathLike,
+        drop_variables: tuple[str] | None = None,
         **kwargs: Any,
     ):
         """
@@ -384,12 +386,12 @@ def open_dataset(
 
         raise NotImplementedError
 
-    def guess_can_open(self, filename_or_obj):
+    def guess_can_open(self, filename_or_obj: str | os.PathLike):
         """
         Backend open_dataset method used by Xarray in :py:func:`~xarray.open_dataset`.
         """
 
         return False
 
 
-BACKEND_ENTRYPOINTS: Dict[str, Type[BackendEntrypoint]] = {}
+BACKEND_ENTRYPOINTS: dict[str, type[BackendEntrypoint]] = {}
@@ -30,6 +30,7 @@
 if TYPE_CHECKING:
     from .dataarray import DataArray
     from .dataset import Dataset
+    from .types import JoinOptions
 
 DataAlignable = TypeVar("DataAlignable", bound=DataWithCoords)
 
@@ -557,7 +558,7 @@ def align(self) -> None:
 
 def align(
     *objects: DataAlignable,
-    join="inner",
+    join: JoinOptions = "inner",
     copy=True,
     indexes=None,
     exclude=frozenset(),
@@ -590,6 +591,7 @@ def align(
         - "override": if indexes are of same size, rewrite indexes to be
           those of the first object with that dimension. Indexes for the same
           dimension must have the same size in all objects.
+
     copy : bool, optional
         If ``copy=True``, data in the return values is always copied. If
         ``copy=False`` and reindexing is unnecessary, or can be performed with
@@ -764,7 +766,7 @@ def align(
 
 def deep_align(
     objects,
-    join="inner",
+    join: JoinOptions = "inner",
     copy=True,
     indexes=None,
     exclude=frozenset(),
@@ -834,7 +836,7 @@ def is_alignable(obj):
         if key is no_key:
             out[position] = aligned_obj
         else:
-            out[position][key] = aligned_obj
+            out[position][key] = aligned_obj  # type: ignore[index]  # maybe someone can fix this?
 
     # something went wrong: we should have replaced all sentinel values
     for arg in out:
 
@@ -1,7 +1,9 @@
+from __future__ import annotations
+
 import itertools
 import warnings
 from collections import Counter
-from typing import Iterable, Sequence, Union
+from typing import TYPE_CHECKING, Iterable, Literal, Sequence, Union
 
 import pandas as pd
 
@@ -12,6 +14,9 @@
 from .merge import merge
 from .utils import iterate_nested
 
+if TYPE_CHECKING:
+    from .types import CombineAttrsOptions, CompatOptions, JoinOptions
+
 
 def _infer_concat_order_from_positions(datasets):
     return dict(_infer_tile_ids_from_nested_list(datasets, ()))
@@ -188,10 +193,10 @@ def _combine_nd(
     concat_dims,
     data_vars="all",
     coords="different",
-    compat="no_conflicts",
+    compat: CompatOptions = "no_conflicts",
     fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="drop",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "drop",
 ):
     """
     Combines an N-dimensional structure of datasets into one by applying a
@@ -250,10 +255,10 @@ def _combine_all_along_first_dim(
     dim,
     data_vars,
     coords,
-    compat,
+    compat: CompatOptions,
     fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="drop",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "drop",
 ):
 
     # Group into lines of datasets which must be combined along dim
@@ -276,12 +281,12 @@ def _combine_all_along_first_dim(
 def _combine_1d(
     datasets,
     concat_dim,
-    compat="no_conflicts",
+    compat: CompatOptions = "no_conflicts",
     data_vars="all",
     coords="different",
     fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="drop",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "drop",
 ):
     """
     Applies either concat or merge to 1D list of datasets depending on value
@@ -336,8 +341,8 @@ def _nested_combine(
     coords,
     ids,
     fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="drop",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "drop",
 ):
 
     if len(datasets) == 0:
@@ -377,15 +382,13 @@ def _nested_combine(
 
 def combine_nested(
     datasets: DATASET_HYPERCUBE,
-    concat_dim: Union[
-        str, DataArray, None, Sequence[Union[str, "DataArray", pd.Index, None]]
-    ],
+    concat_dim: (str | DataArray | None | Sequence[str | DataArray | pd.Index | None]),
     compat: str = "no_conflicts",
     data_vars: str = "all",
     coords: str = "different",
     fill_value: object = dtypes.NA,
-    join: str = "outer",
-    combine_attrs: str = "drop",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "drop",
 ) -> Dataset:
     """
     Explicitly combine an N-dimensional grid of datasets into one by using a
@@ -603,9 +606,9 @@ def _combine_single_variable_hypercube(
     fill_value=dtypes.NA,
     data_vars="all",
     coords="different",
-    compat="no_conflicts",
-    join="outer",
-    combine_attrs="no_conflicts",
+    compat: CompatOptions = "no_conflicts",
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "no_conflicts",
 ):
     """
     Attempt to combine a list of Datasets into a hypercube using their
@@ -659,15 +662,15 @@ def _combine_single_variable_hypercube(
 
 # TODO remove empty list default param after version 0.21, see PR4696
 def combine_by_coords(
-    data_objects: Sequence[Union[Dataset, DataArray]] = [],
-    compat: str = "no_conflicts",
-    data_vars: str = "all",
+    data_objects: Iterable[Dataset | DataArray] = [],
+    compat: CompatOptions = "no_conflicts",
+    data_vars: Literal["all", "minimal", "different"] | list[str] = "all",
     coords: str = "different",
     fill_value: object = dtypes.NA,
-    join: str = "outer",
-    combine_attrs: str = "no_conflicts",
-    datasets: Sequence[Dataset] = None,
-) -> Union[Dataset, DataArray]:
+    join: JoinOptions = "outer",
+    combine_attrs: CombineAttrsOptions = "no_conflicts",
+    datasets: Iterable[Dataset] = None,
+) -> Dataset | DataArray:
     """
 
     Attempt to auto-magically combine the given datasets (or data arrays)
@@ -695,7 +698,7 @@ def combine_by_coords(
 
     Parameters
     ----------
-    data_objects : sequence of xarray.Dataset or sequence of xarray.DataArray
+    data_objects : Iterable of Datasets or DataArrays
         Data objects to combine.
 
     compat : {"identical", "equals", "broadcast_equals", "no_conflicts", "override"}, optional
@@ -711,18 +714,19 @@ def combine_by_coords(
           must be equal. The returned dataset then contains the combination
           of all non-null values.
         - "override": skip comparing and pick variable from first dataset
+
     data_vars : {"minimal", "different", "all" or list of str}, optional
         These data variables will be concatenated together:
 
-        * "minimal": Only data variables in which the dimension already
+        - "minimal": Only data variables in which the dimension already
           appears are included.
-        * "different": Data variables which are not equal (ignoring
+        - "different": Data variables which are not equal (ignoring
           attributes) across all datasets are also concatenated (as well as
           all for which dimension already appears). Beware: this option may
           load the data payload of data variables into memory if they are not
           already loaded.
-        * "all": All data variables will be concatenated.
-        * list of str: The listed data variables will be concatenated, in
+        - "all": All data variables will be concatenated.
+        - list of str: The listed data variables will be concatenated, in
           addition to the "minimal" data variables.
 
         If objects are DataArrays, `data_vars` must be "all".
@@ -745,6 +749,7 @@ def combine_by_coords(
         - "override": if indexes are of same size, rewrite indexes to be
           those of the first object with that dimension. Indexes for the same
           dimension must have the same size in all objects.
+
     combine_attrs : {"drop", "identical", "no_conflicts", "drop_conflicts", \
                      "override"} or callable, default: "drop"
         A callable or a string indicating how to combine attrs of the objects being
@@ -762,6 +767,8 @@ def combine_by_coords(
         If a callable, it must expect a sequence of ``attrs`` dicts and a context object
         as its only parameters.
 
+    datasets : Iterable of Datasets
+
     Returns
     -------
     combined : xarray.Dataset or xarray.DataArray
 
@@ -37,7 +37,7 @@
     from .coordinates import Coordinates
     from .dataarray import DataArray
     from .dataset import Dataset
-    from .types import T_Xarray
+    from .types import CombineAttrsOptions, JoinOptions, T_Xarray
 
 _NO_FILL_VALUE = utils.ReprObject("<no-fill-value>")
 _DEFAULT_NAME = utils.ReprObject("<default-name>")
@@ -184,7 +184,7 @@ def _enumerate(dim):
         return str(alt_signature)
 
 
-def result_name(objects: list) -> Any:
+def result_name(objects: Iterable[Any]) -> Any:
     # use the same naming heuristics as pandas:
     # https://github.com/blaze/blaze/issues/458#issuecomment-51936356
     names = {getattr(obj, "name", _DEFAULT_NAME) for obj in objects}
@@ -196,7 +196,7 @@ def result_name(objects: list) -> Any:
     return name
 
 
-def _get_coords_list(args) -> list[Coordinates]:
+def _get_coords_list(args: Iterable[Any]) -> list[Coordinates]:
     coords_list = []
     for arg in args:
         try:
@@ -209,23 +209,39 @@ def _get_coords_list(args) -> list[Coordinates]:
 
 
 def build_output_coords_and_indexes(
-    args: list,
+    args: Iterable[Any],
     signature: _UFuncSignature,
     exclude_dims: AbstractSet = frozenset(),
-    combine_attrs: str = "override",
+    combine_attrs: CombineAttrsOptions = "override",
 ) -> tuple[list[dict[Any, Variable]], list[dict[Any, Index]]]:
     """Build output coordinates and indexes for an operation.
 
     Parameters
     ----------
-    args : list
+    args : Iterable
         List of raw operation arguments. Any valid types for xarray operations
         are OK, e.g., scalars, Variable, DataArray, Dataset.
     signature : _UfuncSignature
         Core dimensions signature for the operation.
     exclude_dims : set, optional
         Dimensions excluded from the operation. Coordinates along these
         dimensions are dropped.
+    combine_attrs : {"drop", "identical", "no_conflicts", "drop_conflicts", \
+                     "override"} or callable, default: "drop"
+        A callable or a string indicating how to combine attrs of the objects being
+        merged:
+
+        - "drop": empty attrs on returned Dataset.
+        - "identical": all attrs must be the same on every object.
+        - "no_conflicts": attrs from all objects are combined, any that have
+          the same name must also have the same value.
+        - "drop_conflicts": attrs from all objects are combined, any that have
+          the same name but different values are dropped.
+        - "override": skip comparing and copy attrs from the first dataset to
+          the result.
+
+        If a callable, it must expect a sequence of ``attrs`` dicts and a context object
+        as its only parameters.
 
     Returns
     -------
@@ -267,10 +283,10 @@ def apply_dataarray_vfunc(
     func,
     *args,
     signature,
-    join="inner",
+    join: JoinOptions = "inner",
     exclude_dims=frozenset(),
     keep_attrs="override",
-):
+) -> tuple[DataArray, ...] | DataArray:
     """Apply a variable level function over DataArray, Variable and/or ndarray
     objects.
     """
@@ -295,6 +311,7 @@ def apply_dataarray_vfunc(
     data_vars = [getattr(a, "variable", a) for a in args]
     result_var = func(*data_vars)
 
+    out: tuple[DataArray, ...] | DataArray
     if signature.num_outputs > 1:
         out = tuple(
             DataArray(
@@ -829,7 +846,7 @@ def apply_ufunc(
     output_core_dims: Sequence[Sequence] | None = ((),),
     exclude_dims: AbstractSet = frozenset(),
     vectorize: bool = False,
-    join: str = "exact",
+    join: JoinOptions = "exact",
     dataset_join: str = "exact",
     dataset_fill_value: object = _NO_FILL_VALUE,
     keep_attrs: bool | str | None = None,