Comparison with masked array yields object-array with nans for masked values

[gerritholl]

Code Sample, a copy-pastable example if possible

$ cat mwe.py 
#!/usr/bin/env python3.6
import xarray
import numpy

da = xarray.DataArray(numpy.arange(5))
ma = numpy.ma.masked_array(numpy.arange(5), [True, False, False, False, True])
print(da>ma)
$ ./mwe.py 
<xarray.DataArray (dim_0: 5)>
array([nan, False, False, False, nan], dtype=object)
Dimensions without coordinates: dim_0

Problem description

A comparison between a DataArray and a masked_array results in an array with dtype object instead of an array with dtype bool. This is problematic, because code should be able to assume that x > y returns something with a bool dtype.

Expected Output

I would expect the masked array to be dropped (which it is) and an array to be returned equivalent to the comparison da>ma.data

<xarray.DataArray (dim_0: 5)>
array([False, False, False, False, False], dtype=bool)
Dimensions without coordinates: dim_0

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.1.final.0 python-bits: 64 OS: Linux OS-release: 2.6.32-696.6.3.el6.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8

xarray: 0.10.0+dev12.gf882a58
pandas: 0.21.0
numpy: 1.13.3
scipy: 1.0.0
netCDF4: 1.3.1
h5netcdf: None
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.16.0
matplotlib: 2.1.0
cartopy: None
seaborn: 0.8.1
setuptools: 38.2.4
pip: 9.0.1
conda: 4.3.16
pytest: 3.1.2
IPython: 6.1.0
sphinx: 1.6.2
None

[shoyer]

Yes, I suppose we can consider this a bug. The right behavior would be to convert the masked array into an xarray object before the operation, e.g.,

In [14]: da > xarray.DataArray(ma)
Out[14]:
<xarray.DataArray (dim_0: 5)>
array([False, False, False, False, False], dtype=bool)
Dimensions without coordinates: dim_0

We currently don't have any masked array specific logic for arithmetic so this would require special case handling.

[gerritholl]

See also: #2377.

[stale]

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