merge drops attributes

[johnomotani]

xarray.merge() drops the attrs of Datasets being merged. They should be kept, at least if they are compatible

MCVE Code Sample

# Your code here
import xarray as xr

ds1 = xr.Dataset()
ds1.attrs['a'] = 42
ds2 = xr.Dataset()
ds2.attrs['a'] = 42

merged = xr.merge([ds1, ds2])

print(merged)

the result is

<xarray.Dataset>
Dimensions:  ()
Data variables:
    *empty*

Expected Output

<xarray.Dataset>
Dimensions:  ()
Data variables:
    *empty*
Attributes:
    a:        42

Problem Description

The current behaviour means I have to check and copy attrs to the result of merge by hand, even if the attrs of the inputs were identical or not conflicting.

I'm happy to attempt a PR to fix this.
Proposal (following pattern of compat arguments):

• add a combine_attrs argument to xarray.merge
• combine_attrs = 'drop' do not copy attrs (current behaviour)
• combine_attrs = 'identical' if attrs of all inputs are identical (using dict_equiv) then copy the attrs to the result, otherwise raise an exception
• combine_attrs = 'no_conflicts' merge the attrs of all inputs, as long as any keys shared by more than one input have the same value (if not raise an exception) [I propose this is the default behaviour]
• override copy the attrs from the first input, to the result

This proposal should also allow combine_by_coords, etc. to preserve attributes. These should probably also take a combine_attrs argument, which would be passed through to merge.

Versions

Current master of pydata/xarray on 17/3/2020

Output of `xr.show_versions()`

INSTALLED VERSIONS ------------------ commit: None python: 3.6.9 (default, Nov 7 2019, 10:44:02) [GCC 8.3.0] python-bits: 64 OS: Linux OS-release: 5.3.0-40-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 libhdf5: 1.10.2 libnetcdf: 4.6.3

xarray: 0.15.0
pandas: 1.0.2
numpy: 1.18.1
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: 2.12.0
distributed: None
matplotlib: 3.1.1
cartopy: None
seaborn: None
numbagg: None
setuptools: 45.2.0
pip: 9.0.1
conda: None
pytest: 4.4.1
IPython: 7.8.0
sphinx: 1.8.3

[max-sixty]

I think that proposal sounds pretty good!

Any other thoughts @pydata/xarray ?