Attrs for default coordinate arrays - confusing - intended?

[matthew-brett]

Sorry for the introductory question, but we (@ivanov and I) ran into this behavior while experimenting:

import numpy as np
data = np.zeros((3, 4, 5))
ds = xr.DataArray(data, dims=('i', 'j', 'k'))
print(ds['k'].attrs)

This shows {} as we might reasonably expect. But then:

ds['k'].attrs['foo'] = 'bar'
print(ds['k'].attrs)

This also gives {}, which we found surprising. We worked out why that was, after a little experimentation (the default coordinate arrays seems to get created on the fly and garbage collected immediately). But it took us a little while. Is that as intended? Is there a way of making this less confusing?

Thanks for any help.

[dcherian]

Is that as intended?

Yes, there is no underlying data stored.

Is there a way of making this less confusing?

I think we should raise an error on that attribute assignment. The message would recommend assigning values to that coordinate first ds['k'] = np.arange(ds.sizes['k'])

[matthew-brett]

Yes, thanks - an error would have been easier to understand.