Inconsistency in whether index is created with new dimension coordinate?

It seems like set_coords() doesn’t create an index variable. Is there a reason for this? I was surprised that the following code snippets produce different Datasets (first one has empty indexes, second one has x in indexes), even though both Datasets have a ‘dimension coordinate’ x:

(1)

import numpy as np
import xarray as xr

ds = xr.Dataset()
ds['a'] = ('x', np.linspace(0,1))
ds['b'] = ('x', np.linspace(3,4))
ds = ds.rename(b='x')
ds = ds.set_coords('x')

print(ds)
print('indexes', ds.indexes)

(2)

import numpy as np
import xarray as xr

ds = xr.Dataset()
ds['a'] = ('x', np.linspace(0,1))
ds['x'] = ('x', np.linspace(3,4))

print(ds)
print('indexes', ds.indexes)

Environment:

Output of <tt>xr.show_versions()</tt>

INSTALLED VERSIONS

commit: None python: 3.7.6 | packaged by conda-forge | (default, Jun 1 2020, 18:57:50) [GCC 7.5.0] python-bits: 64 OS: Linux OS-release: 5.4.0-47-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_GB.UTF-8 LOCALE: en_GB.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.7.4

xarray: 0.16.0 pandas: 1.1.1 numpy: 1.18.5 scipy: 1.4.1 netCDF4: 1.5.3 pydap: None h5netcdf: None h5py: 2.10.0 Nio: None zarr: None cftime: 1.2.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.23.0 distributed: 2.25.0 matplotlib: 3.2.2 cartopy: None seaborn: None numbagg: None pint: 0.13 setuptools: 49.6.0.post20200814 pip: 20.2.3 conda: 4.8.4 pytest: 5.4.3 IPython: 7.15.0 sphinx: 3.2.1

[dcherian]

I think I was trying to fix this in #4108

[johnomotani]

Thanks @benbovy - with explicit methods now to produce the result with or without index, I think we can close this now 😃