Xarray: Option to make DataArray.transpose also transpose coords

[phausamann]

I've noticed that Dataset.transpose transposes the coordinates while DataArray.transpose does not.

Example:

import xarray as xr
import numpy as np

X_da = xr.DataArray(
    np.random.random((100, 10)),
    coords={'coord_1': (['sample', 'feature'], np.ones((100, 10)))},
    dims=('sample', 'feature')
)

In []: X_da.transpose().coord_1.dims
Out[]: ('sample', 'feature')

X_ds = xr.Dataset(
    {'var_1': (['sample', 'feature'], np.random.random((100, 10)))},
    coords={'coord_1': (['sample', 'feature'], np.ones((100, 10)))},
)

In []: X_ds.transpose().coord_1.dims
Out[]: ('feature', 'sample')

This behaviour is probably intentional, but there are cases where I'd like DataArray.transpose to also transpose the coordinates. As a workaround, I have to convert to a Dataset, transpose and convert back.

I was thinking that DataArray.transpose could accept a keyword argument transpose_coords which would be False by default. I could work on a PR implementing this behavior if it's desired.

[shoyer]

To be honest, I'm not sure skipping coordinates was actually intentional. In most cases, I suspect transposing coordinates simply doesn't matter because either (1) there are no multi-dimensional coordinates or (2) xarray usually doesn't care much about dimension order (so it doesn't matter).

I would support adding a transpose_coords argument, but we should also consider switching the default behavior in the future to transpose_coords=True.

Anyone else have opinions on this API?

[NicWayand]

Also hitting this issue. (Use case: formatting netcdf files for some R code that does not have labeled indexing... ugh). Thanks @phausamann for the work around. Default transposing of coods makes sense to me.

[shoyer]

Honestly, we could probably consider the DataArray behavior to be a bug. In general DataArray methods should work exactly like Dataset methods on a Dataset with a single data variable. (In fact, this is how we implement many DataArray methods.)