issues: 573768194

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
573768194 MDExOlB1bGxSZXF1ZXN0MzgyMjE1OTYw 3816 Add template xarray object kwarg to map_blocks 2448579 closed 0     9 2020-03-02T07:20:19Z 2020-05-06T16:41:59Z 2020-05-06T16:41:54Z MEMBER   0 pydata/xarray/pulls/3816
  • [x] Closes #3575
  • [x] Tests added
  • [x] Passes isort -rc . && black . && mypy . && flake8
  • [x] Fully documented, including whats-new.rst for all changes and api.rst for new API

This PR adds a template kwarg to map_blocks so that we can do more complicated things where automated inference of the template fails. template is expected to be an xarray object that looks like the result of the map_blocks computation.

@jhamman To me, this seems a lot easier than defining a dict based schema. With dask variables, the memory cost shouldn't be high. It's easy to use standard xarray operations to make something that looks like the result dataset. Here's a notebook prototyping a to_schema/from_schema approach: https://gist.github.com/dcherian/130ba22d0fbadb616837deb914eaa67e#file-map_blocks_for_metsim_test-ipynb

python ds = xr.tutorial.load_dataset("air_temperature").chunk({"lat": 5}) template = ds.isel(lat=slice(1, None, 5)) result = xr.map_blocks(lambda x: x.isel(lat=[1]), ds) # fails result = xr.map_blocks(lambda x: x.isel(lat=[1]), ds, template=template) # yay xr.testing.assert_equal(result, template) # passes Todo: - [x] add tests - [x] what if the function returns a variable not in the template? - [x] should we do anything with template.attrs?

 
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