issue_comments

Alright, I see. In that case we should not create an entirely new syntax for this - unless there are more people than just me interested in this feature. I can just use my own little convenience function that generates a series of array indices that can be fed to isel, something like this:

python def wrap_iselkw(ds, dim, istart, istop): """Returns a kw dict for indices from `istart` to `istop` that wrap around dimension `dim`""" n = len(ds[dim]) if istart > istop: istart -= n return { dim : np.mod(np.arange(istart, istop), n) }

@jhamman You guys seem to be always ahead of me - which is great! (I just did not notice it this time before I raised the issue...)

But it would actually not be that hard to make selecting regions across the boundary of a circular dimension more convenient in xray.

It would of course be great if it worked with real coordinate (longitude) values:

ds.sel(lon=slice(-90, 15))

which, however, only works if the axis explicitly covers this range (e.g. has range -180E to 180E). If it ranges, say, from 0E to 360E, an empty DataArray is returned. There is no nice way of resolving this, I guess, because you would need to hard-code 360 or have it as a clumsy, non-CF-standard parameter and transform all values to some common range before trying to find the requested region.

But it should be less problematic with

ds.isel(lon=slice(-100, 200))

i.e. isel with negative slice start. As it is now, this also returns an empty DataArray for me (xray 0.6.0), which is in a way consistent with the sel behaviour. But asking for negative indices is actually a quite pythonic way of implying circular. So could this be implemented, now that netCDF4 supports non-contiguous indexing?