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
315948443,MDU6SXNzdWUzMTU5NDg0NDM=,2067,A way to generate automatically-numbered coords,1411280,closed,0,,,4,2018-04-19T16:19:53Z,2020-04-19T05:57:17Z,2020-04-19T05:57:17Z,NONE,,,,"It is great that `xarray` supports dimensions without coords, but sometimes I think it would be useful to be able to easily *opt into* autogenerated coords from 0 to `n-1`.  This can be useful to obtain DataArrays for pointwise indexing:

```python
import xarray as xr
ds = xr.Dataset()
# a list of selected indices for each layer
ds['selected'] = (['layer', 'selected-i'], [
    [0, 1, 2],
    [1, 5, 3],
])

# normally, concatenation would drop layer data
print(xr.concat(ds['selected'], dim='selected-i'))
# <xarray.DataArray 'selected' (selected-i: 6)>
# array([0, 1, 2, 1, 5, 3])
# Dimensions without coordinates: selected-i

# if you generate coords from 0 to n-1 for layer, however, the resulting DataArray
# contains 'layer' indices for use in pointwise indexing
print(xr.concat(ds
                .assign_coords(layer=list(range(ds.sizes['layer'])))
                ['selected'], dim='selected-i'))
# <xarray.DataArray 'selected' (selected-i: 6)>
# array([0, 1, 2, 1, 5, 3])
# Coordinates:
#     layer    (selected-i) int64 0 0 0 1 1 1
# Dimensions without coordinates: selected-i
```

My issue with the above is that `layer=list(range(ds.sizes['layer']))` is verbose and fails to be DRY. My thought for such an API is that xarray could maybe have a special constant for auto-assignment, usable in any method that takes input coords:

```python
print(xr.concat(ds
                .assign_coords(layer=xr.AUTO)
                ['selected'], dim='selected-i'))
```

(Additionally, perhaps `xr.AUTO` could be a function/class, so that `xr.AUTO(start)` produces indices starting at `start`.)","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2067/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue