Multidimensional dask coordinates unexpectedly computed

MCVE Code Sample

from dask.diagnostics import ProgressBar
import xarray as xr
import numpy as np
import dask.array as da

a = xr.DataArray(da.zeros((10, 10), chunks=2), dims=('y', 'x'), coords={'y': np.arange(10), 'x': np.arange(10), 'lons': (('y', 'x'), da.zeros((10, 10), chunks=2))}) 
b = xr.DataArray(da.zeros((10, 10), chunks=2), dims=('y', 'x'), coords={'y': np.arange(10), 'x': np.arange(10), 'lons': (('y', 'x'), da.zeros((10, 10), chunks=2))}) 

with ProgressBar():
    c = a + b

Output:

[########################################] | 100% Completed |  0.1s

Problem Description

Using arrays with 2D dask array coordinates results in the coordinates being computed for any binary operations (anything combining two or more DataArrays). I use ProgressBar in the above example to show when coordinates are being computed.

In my own work, when I learned that 2D dask coordinates were possible, I started adding longitude and latitude coordinates. These are rather large and can take a while to load/compute so I was surprised that simple operations (ex. a.fillna(b)) were causing things to be computed and taking a long time.

Is this computation by design or a possible bug?

Expected Output

No output from the ProgressBar, hoping that no coordinates would be computed/loaded.

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None python: 3.6.7 | packaged by conda-forge | (default, Feb 28 2019, 02:16:08) [GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.6.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.4 libnetcdf: 4.6.2

xarray: 0.12.1 pandas: 0.24.2 numpy: 1.14.3 scipy: 1.3.0 netCDF4: 1.5.1.2 pydap: None h5netcdf: 0.7.4 h5py: 2.9.0 Nio: None zarr: 2.3.2 cftime: 1.0.3.4 nc_time_axis: None PseudonetCDF: None rasterio: 1.0.22 cfgrib: None iris: None bottleneck: 1.2.1 dask: 2.0.0 distributed: 2.0.0 matplotlib: 3.1.0 cartopy: 0.17.1.dev147+HEAD.detached.at.5e624fe seaborn: None setuptools: 41.0.1 pip: 19.1.1 conda: None pytest: 4.6.3 IPython: 7.5.0 sphinx: 2.1.2

[dhirschfeld]

FYI: @djhoese, you can inline code snippits using the permanent link to the source: https://github.com/pydata/xarray/blob/e5bb647637063153a7feb750793d6fd8fb58dda8/xarray/core/variable.py#L1223

[djhoese]

Ah, good call. The transpose currently in xarray would still be a problem though.