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[WIP] Add map_blocks. #3258

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2 changes: 2 additions & 0 deletions xarray/__init__.py
Original file line number Diff line number Diff line change
Expand Up @@ -34,6 +34,8 @@
from .coding.cftime_offsets import cftime_range
from .coding.cftimeindex import CFTimeIndex

from .core.parallel import map_blocks

from .util.print_versions import show_versions

from . import tutorial
Expand Down
86 changes: 86 additions & 0 deletions xarray/core/parallel.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,86 @@
try:
import dask
import dask.array
from dask.highlevelgraph import HighLevelGraph

except ImportError:
pass

from .dataarray import DataArray


def get_chunk_slices(dataset):
chunk_slices = {}
for dim, chunks in dataset.chunks.items():
slices = []
start = 0
for chunk in chunks:
stop = start + chunk
slices.append(slice(start, stop))
start = stop
chunk_slices[dim] = slices

return chunk_slices


def map_blocks(func, darray):
"""
A version of dask's map_blocks for DataArrays.

Parameters
----------
func: callable
User-provided function that should accept DataArrays corresponding to one chunk.
darray: DataArray
Chunks of this array will be provided to 'func'. The function must not change
shape of the provided DataArray.

Returns
-------
DataArray

See Also
--------
dask.array.map_blocks
"""

def _wrapper(darray):
result = func(darray)
if not isinstance(result, type(darray)):
raise ValueError("Result is not the same type as input.")
if result.shape != darray.shape:
raise ValueError("Result does not have the same shape as input.")
return result

meta_array = DataArray(darray.data._meta, dims=darray.dims)
result_meta = func(meta_array)

name = "%s-%s" % (darray.name or func.__name__, dask.base.tokenize(darray))

slicers = get_chunk_slices(darray._to_temp_dataset())
dask_keys = list(dask.core.flatten(darray.__dask_keys__()))

graph = {
(name,)
+ (*key[1:],): (
_wrapper,
(
DataArray,
key,
{
dim_name: darray[dim_name][slicers[dim_name][index]]
for dim_name, index in zip(darray.dims, key[1:])
},
darray.dims,
),
)
for key in dask_keys
}

graph = HighLevelGraph.from_collections(name, graph, dependencies=[darray])

return DataArray(
dask.array.Array(graph, name, chunks=darray.chunks, meta=result_meta),
dims=darray.dims,
coords=darray.coords,
)
22 changes: 22 additions & 0 deletions xarray/tests/test_dask.py
Original file line number Diff line number Diff line change
Expand Up @@ -878,3 +878,25 @@ def test_dask_layers_and_dependencies():
assert set(x.foo.__dask_graph__().dependencies).issuperset(
ds.__dask_graph__().dependencies
)


def test_map_blocks():
darray = xr.DataArray(
dask.array.ones((10, 20), chunks=[4, 5]),
dims=["x", "y"],
coords={"x": np.arange(10), "y": np.arange(100, 120)},
)
darray.name = None

def good_func(darray):
return darray * darray.x + 5 * darray.y

def bad_func(darray):
return (darray * darray.x + 5 * darray.y)[:1, :1]

actual = xr.map_blocks(good_func, darray)
expected = good_func(darray)
xr.testing.assert_equal(expected, actual)

with raises_regex(ValueError, "not have the same shape"):
xr.map_blocks(bad_func, darray).compute()