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
387673324,MDExOlB1bGxSZXF1ZXN0MjM2MTA3NzYy,2590,use keep_attrs in binary operations II,8898319,closed,0,,,6,2018-12-05T09:47:46Z,2018-12-13T07:16:23Z,2018-12-12T17:11:22Z,CONTRIBUTOR,,0,pydata/xarray/pulls/2590," - [x] Closes #2582 
 - [x] Tests added (`test_binary_ops_keep_attrs`)

Please revise and advise.","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2590/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
385716625,MDU6SXNzdWUzODU3MTY2MjU=,2582,Keep attributes across operations,8898319,closed,0,,,6,2018-11-29T12:36:32Z,2018-12-12T17:11:22Z,2018-12-12T17:11:22Z,CONTRIBUTOR,,,,"#### The Problem
When I have two DataArrays and I use a standard operation (`` +, - ,*, /``) the attributes vanish. I think that should not be the case. Even when using as suggested the ``set_options``

```python
import numpy as np
import xarray as xr
a = xr.DataArray(np.random.randn(3,3), dims=('x','y'), name='temp', attrs={'units':'K'})
b = xr.DataArray(np.random.randn(3,3), dims=('x','y'), name='temp', attrs={'units':'K'})
print(a)
<xarray.DataArray 'temp' (x: 3, y: 3)>
array([[ 1.207407, -1.9429  ,  3.168454],
       [-0.773912, -0.121835, -0.139538],
       [ 1.823002,  0.185846,  0.53569 ]])
Dimensions without coordinates: x, y
Attributes:
    units:    K
print(a-b)
<xarray.DataArray 'temp' (x: 3, y: 3)>
array([[ 1.280892, -1.097781,  2.150318],
       [-0.208202, -0.03856 ,  0.805856],
       [ 2.192506,  1.049181,  2.277078]])
Dimensions without coordinates: x, y

with xr.set_options(keep_attrs=True):
    print(a-b)

<xarray.DataArray 'temp' (x: 3, y: 3)>
array([[ 1.280892, -1.097781,  2.150318],
       [-0.208202, -0.03856 ,  0.805856],
       [ 2.192506,  1.049181,  2.277078]])
Dimensions without coordinates: x, y
```

#### Problem description

Attributes vanish when a *normal* operation is applied!
From docs of set_options:
``keep_attrs``: rule for whether to keep attributes on xarray
  Datasets/dataarrays after operations. Either ``True`` to always keep
  attrs, ``False`` to always discard them, or ``'default'`` to use original
  logic that attrs should only be kept in unambiguous circumstances.
  Default: ``'default'``.

#### Expected Output

The Attributes should remain. Maybe keep only attributes from the left Array ?
Please adjust or advise me.

#### Output of ``xr.show_versions()``

<details>
``
xr.show_versions()
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.6.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-39-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.11.0
pandas: 0.23.4
numpy: 1.15.4
scipy: 1.1.0
netCDF4: 1.4.2
h5netcdf: None
h5py: 2.8.0
Nio: None
zarr: None
cftime: 1.0.2.1
PseudonetCDF: None
rasterio: None
iris: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.20.2
distributed: 1.24.2
matplotlib: 3.0.1
cartopy: 0.16.0
seaborn: 0.9.0
setuptools: 40.6.2
pip: 18.1
conda: 4.5.11
pytest: 4.0.0
IPython: 7.1.1
sphinx: 1.8.2
``
</details>
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