Checking non-dimensional coordinates for equality

[djhoese]

Code Sample, a copy-pastable example if possible

I'm working on a proof-of-concept for the geoxarray project where I'd like to store coordinate reference system (CRS) information in the coordinates of a DataArray or Dataset object. I'd like to avoid subclassing objects and instead depend completely on xarray accessors to implement any utilities I need.

I'm having trouble deciding what the best place is for this CRS information so that it benefits the user; .coords made the most sense. My hope was that adding two DataArrays together with two different crs coordinates would cause an error, but found out that since crs is not a dimension it doesn't get treated the same way; even when changing join method to 'exact'.

from pyproj import CRS
import xarray as xr
import dask.array as da

crs1 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=25 +lat_1=25')
crs2 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=35 +lat_1=35')


a = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs1, 'test': 1, 'test2': 2})      

b = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs2, 'test': 2, 'test2': 2})            

a + b                                                                                                                                                                              
# Results in:
# <xarray.DataArray 'zeros-e5723e7f9121b7ac546f61c19dabe786' (y: 5, x: 5)>
# dask.array<shape=(5, 5), dtype=float64, chunksize=(2, 2)>
# Coordinates:
#   * y        (y) int64 1 2 3 4 5
#   * x        (x) int64 2 3 4 5 6
#     test2    int64 2

In the above code I was hoping that because the crs coordinates are different (lat_0 and lat_1 are different and crs1 != crs2) that I could get it to raise an exception.

Any ideas for how I might be able to accomplish something like this? I'm not an expert on xarray/pandas indexes, but could this be another possible solution?

Edit: xr.merge with compat='no_conflicts' does detect this difference.

[dcherian]

It seems like we need an arithmetic_compat option similar to arithmetic_join

[shoyer]

I think the ideal solution to this would be to make CRS coordinates a virtual "index" of sorts associated with the x/y coordinates that gets checked for equality (and maybe that supports alignment).

[dcherian]

An index attribute with compat='identical' by default?