[Bug]: rename_vars to dimension coordinate does not create an index

What happened?

We used Data{set,Array}.rename{_vars}({coord: dim_coord}) to make a coordinate a dimension coordinate (instead of set_index). This results in the coordinate correctly being displayed as a dimension coordinate (with the *) but it does not create an index, such that further operations like sel fail with a strange KeyError.

What did you expect to happen?

I expect one of two things to be true:

1. rename{_vars} does not allow setting dimension coordinates (raises Error and tells you to use set_index)
2. rename{_vars} checks for this occasion and sets the index correctly

Minimal Complete Verifiable Example

import xarray as xr

data = xr.DataArray([5, 6, 7], coords={"c": ("x", [1, 2, 3])}, dims="x")
# <xarray.DataArray (x: 3)>
# array([5, 6, 7])
# Coordinates:
#     c        (x) int64 1 2 3
# Dimensions without coordinates: x

data_renamed = data.rename({"c": "x"})
# <xarray.DataArray (x: 3)>
# array([5, 6, 7])
# Coordinates:
#   * x        (x) int64 1 2 3

data_renamed.indexes
# Empty
data_renamed.sel(x=2)
# KeyError: 'no index found for coordinate x'

# if we use set_index it works
data_indexed = data.set_index({"x": "c"})
# looks the same as data_renamed!
# <xarray.DataArray (x: 3)>
# array([1, 2, 3])
# Coordinates:
#   * x        (x) int64 1 2 3

data_indexed.indexes
# x: Int64Index([1, 2, 3], dtype='int64', name='x')

Relevant log output

No response

Anything else we need to know?

No response

Environment

INSTALLED VERSIONS

commit: None python: 3.9.1 (default, Jan 13 2021, 15:21:08) [GCC 4.8.5 20150623 (Red Hat 4.8.5-44)] python-bits: 64 OS: Linux OS-release: 3.10.0-1160.49.1.el7.x86_64 machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: (‘en_US’, ‘UTF-8’) libhdf5: 1.12.0 libnetcdf: 4.7.4

xarray: 0.20.2 pandas: 1.3.5 numpy: 1.21.5 scipy: 1.7.3 netCDF4: 1.5.8 pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: 1.5.1.1 nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: None distributed: None matplotlib: 3.5.1 cartopy: None seaborn: None numbagg: None fsspec: None cupy: None pint: None sparse: None setuptools: 49.2.1 pip: 22.0.2 conda: None pytest: 6.2.5 IPython: 8.0.0 sphinx: None

[benbovy]

This has been discussed in #4825.

A third option for rename{_vars} would be to rename the coordinate and its index (if any), regardless of whether the old and new names correspond to existing dimensions. We plan to drop the concept of a “dimension coordinate” with an implicit index in favor of indexes explicitly part of Xarray’s data model (see https://github.com/pydata/xarray/projects/1), so that it will be possible to set indexes for non-dimension coordinates and/or set dimension coordinates without indexes.

Re your example, in #5692 data.rename({"c": "x"}) does not implicitly create anymore an indexed coordinate (no *):

data_renamed
# <xarray.DataArray (x: 3)>
# array([5, 6, 7])
# Coordinates:
#     x        (x) int64 1 2 3

Instead, it should be possible to directly set an index for the c coordinate without the need to rename it, e.g.,

# API has still to be defined
data_indexed = data.set_index("c", index_cls=xr.PandasIndex)

data_indexed.sel(c=[1, 2])
# <xarray.DataArray (x: 2)>
# array([5, 6])
# Coordinates:
#   * c       (x) int64 1 2

[headtr1ck]

Then feel free to close this issue with your PR 😃