[Bug]: Singletons breaks default sort_ascending=True in swap_lon_axis()

[tomvothecoder]

What happened?

Using xcdat v0.4.0, I just got an error when trying to to swap the longitude axes. I wonder if the issue is that coord_keys = get_dim_coords(ds, "X").coords.keys() on L236 of swap_lon_axis() returns not just the coordinate key for the "X" dimension but also "height" since this is a 2-meter air temperature.

Hey @mzelinka and @pochedls, this issue is related to height being a singleton coordinate. Singleton coordinate variables are included alongside other coordinate variables (refer to code example below). There is an open xarray issue regarding singletons here: pydata/xarray#6196

import xcdat as xc

fnames = "/p/css03/esgf_publish/CMIP6/CMIP/BCC/BCC-CSM2-MR/historical/r1i1p1f1/day/tasmax/gn/v20181126/*.nc"

ds = xc.open_mfdataset(fnames)

print(xc.get_dim_coords(ds, axis="X"))

# Notice how `height` is included in the `lon` coordinate var.

# <xarray.DataArray 'lon' (lon: 320)>
# array([  0.   ,   1.125,   2.25 , ..., 356.625, 357.75 , 358.875])
# Coordinates:
#   * lon      (lon) float64 0.0 1.125 2.25 3.375 4.5 ... 355.5 356.6 357.8 358.9
#     height   float64 2.0
# Attributes:
#     bounds:         lon_bnds
#     units:          degrees_east
#     axis:           X
#     long_name:      Longitude
#     standard_name:  longitude

How it affects swap_lon_axis()

In swap_lon_axis(), we are getting all of the coordinates keys using lon, which includes height.
This causes swap_lon_axis(ds, sort_ascending=True) to break on this file.

xcdat/xcdat/axis.py

Lines 235 to 236 in d5d4fd7

	ds = dataset.copy()
	coord_keys = get_dim_coords(ds, "X").coords.keys()

xcdat/xcdat/axis.py

Lines 259 to 260 in d5d4fd7

	if sort_ascending:
	ds = ds.sortby(list(coord_keys), ascending=True)

Temporary workaround:

• Set sort_ascending=False in swap_lon_axis() and perform sorting manually

The fix for swap_lon_axis() is to ignore singleton coordinate keys.

Originally posted by @tomvothecoder in #296 (reply in thread)

What did you expect to happen?

swap_lon_axis() should disregard singletons such as height when sorting coordinates.

Minimal Complete Verifiable Example

No response

Relevant log output

No response

Anything else we need to know?

No response

Environment

xcdat=0.4.0