Setting attributes to multi-index coordinate

[rafa-guedes]

I can't seem to be able to define attributes to "virtual" coordinates from multi-index coordinates. Taking from the exemple from the docs:

In [1]: import numpy as np
In [2]: import pandas as pd
In [3]: import xarray as xr
In [4]: midx = pd.MultiIndex.from_arrays([['R','R','V','V'], [.1,.2,.7,.9]], names=('band','wn'))
In [5]: mda = xr.DataArray(np.random.rand(4), coords={'spec': midx}, dims='spec')

# Setting up attrs to the full coordinate works:
In [6]: mda['spec'].attrs
Out[6]: OrderedDict()
In [7]: mda['spec'].attrs = {'spec_attr': 'some_attr'}
In [8]: mda['spec'].attrs
Out[8]: OrderedDict([('spec_attr', 'some_attr')])

# Setting attrs to the virtual coordinate does not produce any effect:
In [9]: mda['band'].attrs
Out[9]: OrderedDict()
In [10]: mda['band'].attrs = {'band_attr': 'another_attr'}
In [11]: mda['band'].attrs
Out[11]: OrderedDict()

[shoyer]

This is a little tricky: there's only a single array Variable object (the MultiIndex), so we can't save attrs on any of the levels. But xarray doesn't raise an error in this case, because it's assigning to the temporary mda['band'] object, which immediately gets thrown away. There would be the same issue if you tried to assign an attribute to ds['time.dayofyear'].

I don't think Python syntax gives us any good options for identifying these cases. It's essentially the same problem as assigning to an indexing result, e.g., ds.sel(...).attrs = ..., though it's less obvious here since you're doing indexing, not a method call.

At the very least we could document that this doesn't work.

CC @benbovy in case he has any other ideas.

[rafa-guedes]

That makes sense thanks for explaining @shoyer

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[shoyer]

This will be eventually fixed as part of #1603

[benbovy]

#5636 is a 1st step towards representing multi-index levels in separate Variable objects having their own user-defined attributes. We'll also be able in theory to propagate the attributes through operations like sel and maybe other operations like alignment.

The features added in #5636 will be integrated in a follow-up PR.