keep_attrs not respected for unary operators

[gerritholl]

The xarray global option keep_attrs (introduced in #2482 ) is not respected for unary operators.

MCVE Code Sample

import xarray as xr
x = xr.DataArray([1, 2, 3], attrs={"A": "B"})
with xr.set_options(keep_attrs=True):
    y = ~x
print(x.attrs, y.attrs)

Expected Output

I expect

{'A': 'B'} {'A': 'B'}

Problem Description

I get:

{'A': 'B'} {}

I get the same for the other unary operators +x, -x, and abs(x).

Versions

Tested with latest xarray master (see below for details).

Output of xr.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.8.2 | packaged by conda-forge | (default, Mar 23 2020, 18:16:37)
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.12.14-lp150.12.82-default
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.4

xarray: 0.15.2.dev64+g2542a63f
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: 2.4.0
cftime: 1.1.1.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.3
cfgrib: None
iris: None
bottleneck: None
dask: 2.14.0
distributed: 2.14.0
matplotlib: 3.2.1
cartopy: 0.17.0
seaborn: None
numbagg: None
pint: None
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: installed
pytest: 5.4.1
IPython: 7.13.0
sphinx: None

[TomNicholas]

Thanks for flagging this.

Question for other devs: Where are these unary operators on xarray objects defined? If I search __neg__ I don't get anything, are they inherited somehow?

[keewis]

they're defined mentioned here:

xarray/xarray/core/ops.py

Line 25 in 2542a63

UNARY_OPS = ["neg", "pos", "abs", "invert"]

and injected:

xarray/xarray/core/ops.py

Lines 326 to 328 in 2542a63

	# patch in standard special operations
	for name in UNARY_OPS:
	setattr(cls, op_str(name), cls._unary_op(get_op(name)))