DataArray.set_index throws error on documented input

Problem Description

Docs for DataArray.set_index describe the main indexes argument as:

Mapping from names matching dimensions and values given by (lists of) the names of existing coordinates or variables to set as new (multi-)index.

This suggests that one can set a DataArray instance’s coordinates by passing in a dimension and a list-like object of coordinates.

MCVE

In [1]: import numpy as np

In [2]: import xarray as xr

In [3]: arr = xr.DataArray(data=np.ones((2, 3)), dims=['x', 'y'])

In [4]: arr.dims
Out[4]: ('x', 'y')

In [5]: arr.set_index({'x': range(2)})
KeyError   
...
    144         for n in var_names:
--> 145             var = variables[n]
    146             if (current_index_variable is not None and
    147                     var.dims != current_index_variable.dims):

KeyError: 0

At first, I thought it might be because coords and _coords were not being set in this case:

In [18]: arr.coords
Out[18]: 
Coordinates:
    *empty*

In [19]: arr._coords
Out[19]: OrderedDict()

but even if I set the coordinates first and then try to re-index, it fails:

In [20]: arr = xr.DataArray(data=np.ones((2, 3)), dims=['x', 'y'], coords={'x': range(2), 'y': range(3)})
In [21]: arr.set_index({'x': ['a', 'b', 'c']})
...
    144         for n in var_names:
--> 145             var = variables[n]
    146             if (current_index_variable is not None and
    147                     var.dims != current_index_variable.dims):

Expected Output

I expect my MCVE to work based on the documentation.

Problem Solution

My guess is that the issue is Xarray is using the merge_indexes function (see here) from the Dataset module, and there is no concept of a variable in a DataArray.

[gwgundersen]

Looks like the idea of a glossary is already being discussed in https://github.com/pydata/xarray/issues/2410.

[max-sixty]

Good work on finding that issue. I think even if we can get something brief in, that would be helpful.

On the specific definitions:

What do you think of the terminology “alternative” or “auxiliary” dimension? a is clearly a dimension in the sense that it has coordinates or labels for all the “tick marks” along the x dimension.

For me ‘dimension’ has a precise definition from traditional sciences, so having our ‘coordinate’ be an additional / auxiliary / alternative dimension wouldn’t be consistent with that (e.g. a 4-dimensional array would still be 4 dimensional regardless of how many coordinates it had).

At the very least, I’d love to add a lot more examples of how to actually use these things.

👍