best way to replace a dimension

[Ryan Abernathey]

Hi Folks,

I need a function that takes a dataarray and replaces a dimension with a coordinate from a different dataset. I would like some advice on the best way to do this.

Performance is important, and checking is not. I want to bypass all reindexing, aligning, etc and operate at a low level.

Here's what I have now. I'm sure there is a better way.

Thanks,

Ryan

def _replace_dim(da, olddim, newdim):

    """Replace a dimension with a new dimension

    PARAMETERS

    ----------

    da : xarray.DataArray

    olddim : str

        name of the dimension to replace

    newdim : xarray.DataArray

        dimension to replace it with

    RETURNS

    -------

    da_new : xarray.DataArray

    """

    # get a dictionary of coords

    # can't use da.coords because it is an immutable

    # xarray.core.coordinates.DataArrayCoordinates object

    coords = {dim: da[dim] for dim in da.dims}

    # add the new dimension to it

    coords[newdim.name] = newdim

    # delete the old dimension

    del coords[olddim]

    # redefine dims

    dims = list(da.dims)

    dims[dims.index(olddim)] = newdim.name

    # define a new dataset

    return xr.DataArray(da.data, coords, dims)

[Joe Hamman]

Ryan, I'm wondering if something like this would work:

new = da.rename({olddim, newdim.name})
new.coords[newdim.name] = newdim

Joe

[Stephan Hoyer]

It's still pretty awkward, but I think something like the following would work:

def _replace_dim(da, olddim, newdim):

    renamed = da.rename({olddim: newdim.name})

    # note that alignment along a dimension is skipped when you are overriding

    # the relevant coordinate values

    renamed.coords[newdim.name] = newdim

    return renamed

One advantage of this version is that the "rename" operation ensures that the dimension gets swapped out on every coordinate that might be using it, too (in your version these would raise an error).

This comes up with some regularity, so possibly it's worth adding a built-in method to take care of this.

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[Ryan Abernathey]

Thanks for the advice. This is much better, because it also keep the auxiliary coordinates in place.

Since there might be broader interest, I will open an issue.

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[Ryan Abernathey]

Wow, incredible agreement in the solutions proposed by Joe and Stephan!