Adapt map_blocks to use new Coordinates API

xarray/core/coordinates.py

@@ -213,7 +213,7 @@ class Coordinates(AbstractCoordinates):
         :py:class:`~xarray.Coordinates` object is passed, its indexes
         will be added to the new created object.
     indexes: dict-like, optional
-        Mapping of where keys are coordinate names and values are
+        Mapping where keys are coordinate names and values are
         :py:class:`~xarray.indexes.Index` objects. If None (default),
         pandas indexes will be created for each dimension coordinate.
         Passing an empty dictionary will skip this default behavior.

xarray/core/dataarray.py

@@ -80,7 +80,7 @@
     try:
         from dask.dataframe import DataFrame as DaskDataFrame
     except ImportError:
-        DaskDataFrame = None  # type: ignore
+        DaskDataFrame = None
     try:
         from dask.delayed import Delayed
     except ImportError:

xarray/core/dataset.py

@@ -171,7 +171,7 @@
     try:
         from dask.dataframe import DataFrame as DaskDataFrame
     except ImportError:
-        DaskDataFrame = None  # type: ignore
+        DaskDataFrame = None
 
 
 # list of attributes of pd.DatetimeIndex that are ndarrays of time info

xarray/core/parallel.py

@@ -4,19 +4,29 @@
 import itertools
 import operator
 from collections.abc import Hashable, Iterable, Mapping, Sequence
-from typing import TYPE_CHECKING, Any, Callable
+from typing import TYPE_CHECKING, Any, Callable, Literal, TypedDict
 
 import numpy as np
 
 from xarray.core.alignment import align
+from xarray.core.coordinates import Coordinates
 from xarray.core.dataarray import DataArray
 from xarray.core.dataset import Dataset
+from xarray.core.indexes import Index
+from xarray.core.merge import merge
 from xarray.core.pycompat import is_dask_collection
 
 if TYPE_CHECKING:
     from xarray.core.types import T_Xarray
 
 
+class ExpectedDict(TypedDict):
+    shapes: dict[Hashable, int]
+    coords: set[Hashable]
+    data_vars: set[Hashable]
+    indexes: dict[Hashable, Index]
+
+
 def unzip(iterable):
     return zip(*iterable)
 
@@ -31,7 +41,9 @@ def assert_chunks_compatible(a: Dataset, b: Dataset):
 
 
 def check_result_variables(
-    result: DataArray | Dataset, expected: Mapping[str, Any], kind: str
+    result: DataArray | Dataset,
+    expected: ExpectedDict,
+    kind: Literal["coords", "data_vars"],
 ):
     if kind == "coords":
         nice_str = "coordinate"
@@ -254,7 +266,7 @@ def _wrapper(
         args: list,
         kwargs: dict,
         arg_is_array: Iterable[bool],
-        expected: dict,
+        expected: ExpectedDict,
     ):
         """
         Wrapper function that receives datasets in args; converts to dataarrays when necessary;
@@ -345,33 +357,45 @@ def _wrapper(
         for arg in aligned
     )
 
+    merged_coordinates = merge([arg.coords for arg in aligned]).coords
+
     _, npargs = unzip(
         sorted(list(zip(xarray_indices, xarray_objs)) + others, key=lambda x: x[0])
     )
 
     # check that chunk sizes are compatible
     input_chunks = dict(npargs[0].chunks)
-    input_indexes = dict(npargs[0]._indexes)
     for arg in xarray_objs[1:]:
         assert_chunks_compatible(npargs[0], arg)
         input_chunks.update(arg.chunks)
-        input_indexes.update(arg._indexes)
 
+    coordinates: Coordinates
     if template is None:
         # infer template by providing zero-shaped arrays
         template = infer_template(func, aligned[0], *args, **kwargs)
-        template_indexes = set(template._indexes)
-        preserved_indexes = template_indexes & set(input_indexes)
-        new_indexes = template_indexes - set(input_indexes)
-        indexes = {dim: input_indexes[dim] for dim in preserved_indexes}
-        indexes.update({k: template._indexes[k] for k in new_indexes})
+        template_coords = set(template.coords)
+        preserved_coord_vars = template_coords & set(merged_coordinates)
+        new_coord_vars = template_coords - set(merged_coordinates)
+
+        preserved_coords = merged_coordinates.to_dataset()[preserved_coord_vars]
+        # preserved_coords contains all coordinates bariables that share a dimension
+        # with any index variable in preserved_indexes
+        # Drop any unneeded vars in a second pass, this is required for e.g.
+        # if the mapped function were to drop a non-dimension coordinate variable.
+        preserved_coords = preserved_coords.drop_vars(
+            tuple(k for k in preserved_coords.variables if k not in template_coords)
+        )
+
+        coordinates = merge(
+            (preserved_coords, template.coords.to_dataset()[new_coord_vars])
+        ).coords
         output_chunks: Mapping[Hashable, tuple[int, ...]] = {
             dim: input_chunks[dim] for dim in template.dims if dim in input_chunks
         }
 
     else:
         # template xarray object has been provided with proper sizes and chunk shapes
-        indexes = dict(template._indexes)
+        coordinates = template.coords
         output_chunks = template.chunksizes
         if not output_chunks:
             raise ValueError(
@@ -473,6 +497,9 @@ def subset_dataset_to_block(
 
         return (Dataset, (dict, data_vars), (dict, coords), dataset.attrs)
 
+    # variable names that depend on the computation. Currently, indexes
+    # cannot be modified in the mapped function, so we exclude thos
+    computed_variables = set(template.variables) - set(coordinates.xindexes)
     # iterate over all possible chunk combinations
     for chunk_tuple in itertools.product(*ichunk.values()):
         # mapping from dimension name to chunk index
@@ -485,29 +512,32 @@ def subset_dataset_to_block(
             for isxr, arg in zip(is_xarray, npargs)
         ]
 
-        # expected["shapes", "coords", "data_vars", "indexes"] are used to
         # raise nice error messages in _wrapper
-        expected = {}
-        # input chunk 0 along a dimension maps to output chunk 0 along the same dimension
-        # even if length of dimension is changed by the applied function
-        expected["shapes"] = {
-            k: output_chunks[k][v] for k, v in chunk_index.items() if k in output_chunks
-        }
-        expected["data_vars"] = set(template.data_vars.keys())  # type: ignore[assignment]
-        expected["coords"] = set(template.coords.keys())  # type: ignore[assignment]
-        expected["indexes"] = {
-            dim: indexes[dim][_get_chunk_slicer(dim, chunk_index, output_chunk_bounds)]
-            for dim in indexes
+        expected: ExpectedDict = {
+            # input chunk 0 along a dimension maps to output chunk 0 along the same dimension
+            # even if length of dimension is changed by the applied function
+            "shapes": {
+                k: output_chunks[k][v]
+                for k, v in chunk_index.items()
+                if k in output_chunks
+            },
+            "data_vars": set(template.data_vars.keys()),
+            "coords": set(template.coords.keys()),
+            "indexes": {
+                dim: coordinates.xindexes[dim][
+                    _get_chunk_slicer(dim, chunk_index, output_chunk_bounds)
+                ]
+                for dim in coordinates.xindexes
+            },
         }
 
         from_wrapper = (gname,) + chunk_tuple
         graph[from_wrapper] = (_wrapper, func, blocked_args, kwargs, is_array, expected)
 
         # mapping from variable name to dask graph key
         var_key_map: dict[Hashable, str] = {}
-        for name, variable in template.variables.items():
-            if name in indexes:
-                continue
+        for name in computed_variables:
+            variable = template.variables[name]
             gname_l = f"{name}-{gname}"
             var_key_map[name] = gname_l
 
@@ -543,12 +573,7 @@ def subset_dataset_to_block(
         },
     )
 
-    # TODO: benbovy - flexible indexes: make it work with custom indexes
-    # this will need to pass both indexes and coords to the Dataset constructor
-    result = Dataset(
-        coords={k: idx.to_pandas_index() for k, idx in indexes.items()},
-        attrs=template.attrs,
-    )
+    result = Dataset(coords=coordinates, attrs=template.attrs)
 
     for index in result._indexes:
         result[index].attrs = template[index].attrs

xarray/tests/test_dask.py

@@ -1367,6 +1367,25 @@ def test_map_blocks_da_ds_with_template(obj):
     assert_identical(actual, template)
 
 
+def test_map_blocks_roundtrip_string_index():
+    ds = xr.Dataset(
+        {"data": (["label"], [1, 2, 3])}, coords={"label": ["foo", "bar", "baz"]}
+    ).chunk(label=1)
+    assert ds.label.dtype == np.dtype("<U3")
+
+    mapped = ds.map_blocks(lambda x: x, template=ds)
+    assert mapped.label.dtype == ds.label.dtype
+
+    mapped = ds.map_blocks(lambda x: x, template=None)
+    assert mapped.label.dtype == ds.label.dtype
+
+    mapped = ds.data.map_blocks(lambda x: x, template=ds.data)
+    assert mapped.label.dtype == ds.label.dtype
+
+    mapped = ds.data.map_blocks(lambda x: x, template=None)
+    assert mapped.label.dtype == ds.label.dtype
+
+
 def test_map_blocks_template_convert_object():
     da = make_da()
     func = lambda x: x.to_dataset().isel(x=[1])