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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
473000845 MDU6SXNzdWU0NzMwMDA4NDU= 3161 .reduce() on a DataArray with Dask distributed immediately executes the preceding portions of the computational graph zbarry 4762711 closed 0     4 2019-07-25T18:09:22Z 2019-11-13T15:48:46Z 2019-11-13T15:48:46Z NONE      

MCVE Code Sample

.mean() on a DataArray pointing to a Dask array returns a Dask array-containing DataArray as expected:

python da = xr.DataArray(np.zeros((500, 500, 500))).chunk((100, 100, 100)).mean('dim_0') da

<xarray.DataArray (dim_1: 500, dim_2: 500)> dask.array<shape=(500, 500), dtype=float64, chunksize=(100, 100)> Dimensions without coordinates: dim_1, dim_2

Calling .compute() on this result produces the expected result:

python da = xr.DataArray(np.zeros((500, 500, 500))).chunk((100, 100, 100)).mean('dim_0').compute()

<xarray.DataArray (dim_1: 500, dim_2: 500)> array([[0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], ..., [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.]]) Dimensions without coordinates: dim_1, dim_2

The .reduce() method immediately executes all of the previously queued computations leading up to the new reduce method before even calling the supplied function.

```python def func(x, axis=None): return x

da = xr.DataArray(np.zeros((500, 500, 500))).chunk((100, 100, 100)).mean('dim_0').reduce(func) ```

<xarray.DataArray (dim_1: 500, dim_2: 500)> array([[0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], ..., [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.], [0., 0., 0., ..., 0., 0., 0.]]) Dimensions without coordinates: dim_1, dim_2

Expected Output

A Dask array when .reduce(func) isn't followed up by .compute().

Problem Description

When using Dask distributed, the computational graph you are constructing is immediately executed if you call .reduce() instead of adding that function as another node in the DAG. This graph execution happens before the function you pass to reduce is called.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 21:52:21) [GCC 7.3.0] python-bits: 64 OS: Linux OS-release: 3.10.0-693.21.1.el7.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.2 xarray: 0.12.3 pandas: 0.25.0 numpy: 1.16.4 scipy: 1.3.0 netCDF4: 1.5.1.2 pydap: None h5netcdf: None h5py: 2.9.0 Nio: None zarr: None cftime: 1.0.3.4 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.1.0 distributed: 2.1.0 matplotlib: 3.1.1 cartopy: None seaborn: None numbagg: None setuptools: 41.0.1 pip: 19.2.1 conda: None pytest: 5.0.1 IPython: 7.6.1 sphinx: 2.1.2 
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   completed xarray 13221727 issue

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