[BUG] xr.concat inverts coordinates order

[clausmichele]

Following the issue #3969
Merging two datasets using xr.concat inverts the coordinates order.

MCVE Code Sample

import numpy as np
import xarray as xr

x = np.arange(0,10)
y = np.arange(0,10)
time = [0,1]
data = np.zeros((10,10), dtype=bool)
dataArray1 = xr.DataArray([data], coords={'time': [time[0]], 'y': y, 'x': x},
                             dims=['time', 'y', 'x'])
dataArray2 = xr.DataArray([data], coords={'time': [time[1]], 'y': y, 'x': x},
                             dims=['time', 'y', 'x'])
dataArray1 = dataArray1.to_dataset(name='data')
dataArray2 = dataArray2.to_dataset(name='data')

print(dataArray1)
print(xr.concat([dataArray1,dataArray2], dim='time'))

Current Output

<xarray.Dataset>
Dimensions:  (time: 1, x: 10, y: 10)
Coordinates:
  * time     (time) int64 0
  * y        (y) int64 0 1 2 3 4 5 6 7 8 9
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    data     (time, y, x) bool False False False False ... False False False
<xarray.Dataset>
Dimensions:  (time: 2, x: 10, y: 10)
Coordinates:
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9  ##Inverted x and y
  * y        (y) int64 0 1 2 3 4 5 6 7 8 9
  * time     (time) int64 0 1
Data variables:
    data     (time, y, x) bool False False False False ... False False False

Expected Output

<xarray.Dataset>
Dimensions:  (time: 1, x: 10, y: 10)
Coordinates:
  * time     (time) int64 0
  * y        (y) int64 0 1 2 3 4 5 6 7 8 9
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    data     (time, y, x) bool False False False False ... False False False
<xarray.Dataset>
Dimensions:  (time: 2, x: 10, y: 10)
Coordinates:
  * y        (y) int64 0 1 2 3 4 5 6 7 8 9
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9
  * time     (time) int64 0 1
Data variables:
    data     (time, y, x) bool False False False False ... False False False

Problem Description

The concat function should not invert the coordinates but maintain the original order.

Versions

INSTALLED VERSIONS

------------------ commit: None python: 3.6.8 (default, May 7 2019, 14:58:50) [GCC 8.3.0] python-bits: 64 OS: Linux OS-release: 4.15.0-88-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: C.UTF-8 LOCALE: en_US.UTF-8 libhdf5: None libnetcdf: None

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.18.2
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.2.0
cartopy: None
seaborn: None
numbagg: None
setuptools: 46.1.3
pip: 9.0.1
conda: None
pytest: None
IPython: 7.13.0
sphinx: 2.4.3

[keewis]

don't use the Coordinates section to check the order. Dataset objects may have multiple data variables (each with a possibly different order of dimensions) so for displaying it needs to define some kind of order.

What you want to compare is the order of dimensions referenced by a variable:

<xarray.Dataset>
Dimensions:  (time: 1, x: 10, y: 10)
Coordinates:
  * time     (time) int64 0
  * y        (y) int64 0 1 2 3 4 5 6 7 8 9
  * x        (x) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    data     (time, y, x) bool False False False False ... False False False
             ^^^^^^^^^^^^

and you will notice that the order does not change.

To make this less confusing, maybe we should always sort the coordinates when displaying them? I'm guessing that right now they are displayed in the order they were added to the coordinates.

[kmuehlbauer]

This rings a bell! It has be discussed in #2811. I've done extensive checks back then, but didn't come up with an PR.

My workaround or solution is outlined in this comment. Code might have been changed!

[clausmichele]

@keewis yes you are right, but still a consistent ordering would be less confusing, including maybe also the Dimensions field.
Now Dimensions shows first x and then y, even though the data itself has y,x ordering.

[keewis]

sorry, I forgot to remove the "closes" item in #4409. This will be fixed by #4419.

[keewis]

closed by #4419