Bug: side effect on method GroupBy.first

[aurghs]

Code Sample, a copy-pastable example if possible

arr = xr.DataArray(
    np.arange(5), 
    dims=('x',), 
    coords={
        'x': ('x', np.array([1, 1, 1, 2, 2])),
    }
)

gr = arr.groupby('x')
gr.first()

arr

<xarray.DataArray (x: 5)>
array([0, 1, 2, 3, 4])
Coordinates:
  * x        (x) int64 1 2

Problem description

A side effect of the GroupBy.first method call is that it substitutes the original array coordinates with the grouped ones .

Expected Output

arr

<xarray.DataArray (x: 5)>
array([0, 1, 2, 3, 4])
Coordinates:

• x (x) int64 1 1 1 2 2

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.0.final.0 python-bits: 64 OS: Linux OS-release: 4.13.0-41-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

xarray: 0.10.4
pandas: 0.22.0
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: None
cyordereddict: None
dask: 0.17.4
distributed: 1.21.8
matplotlib: 2.2.2
cartopy: 0.16.0
seaborn: None
setuptools: 38.4.1
pip: 10.0.1
conda: None
pytest: 3.5.1
IPython: 6.2.1
sphinx: 1.7.4

[shoyer]

Thanks for the report -- this is pretty bad!

It appears to be triggered by any reductions (including mean(), etc.) on a DataArray groupby object, if the dimension coordinate includes duplicate entries.