groupby should not squeeze out dimensions

[aurghs]

Code Sample

arr = xr.DataArray(
    np.ones(3), 
    dims=('x',), 
    coords={
        'x': ('x', np.array([1, 3, 6])),
    }
)
list(arr.groupby('x'))

[(1, <xarray.DataArray ()>
  array(1.)
  Coordinates:
      x        int64 1), 
(3, <xarray.DataArray ()>
  array(1.)
  Coordinates:
      x        int64 3), 
(6, <xarray.DataArray ()>
  array(1.)
  Coordinates:
      x        int64 6)]

Problem description

The dimension x disappear. I have done some tests and it seems that this problem raise only with strictly ascending coordinates.
For example in this case it works correctly:

arr = xr.DataArray(
    np.ones(3), 
    dims=('x',), 
    coords={
        'x': ('x', np.array([2, 1, 0])),
    }
)
list(arr.groupby('x'))

[(0, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 0), 
(1, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 1), 
(2, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 2)]

Expected Output

arr = xr.DataArray(
    np.ones(3), 
    dims=('x',), 
    coords={
        'x': ('x', np.array([1, 3, 6])),
    }
)
list(arr.groupby('x'))

[(1, <xarray.DataArray (x: 1)>
  ar1ay([1.])
  Coordinates:
    * x        (x) int64 1), 
(3, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 3),
 (6, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 6)]

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.0.final.0 python-bits: 64 OS: Linux OS-release: 4.13.0-41-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

xarray: 0.10.4
pandas: 0.22.0
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: None
cyordereddict: None
dask: 0.17.4
distributed: 1.21.8
matplotlib: 2.2.2
cartopy: 0.16.0
seaborn: None
setuptools: 38.4.1
pip: 10.0.1
conda: None
pytest: 3.5.1
IPython: 6.2.1
sphinx: 1.7.4

[shoyer]

This was intentional behavior, but I agree that it is probably a mistake.

You can disable it by setting squeeze=False in groupby(), e.g.,

arr = xr.DataArray(
    np.ones(3), 
    dims=('x',), 
    coords={
        'x': ('x', np.array([1, 3, 6])),
    }
)
list(arr.groupby('x', squeeze=False))

[(1, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 1), (3, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 3), (6, <xarray.DataArray (x: 1)>
  array([1.])
  Coordinates:
    * x        (x) int64 6)]

This default behavior can be convenient sometimes because of fewer extra dimensions to worry about. In principle, squeezing makes sense if the grouped coordinate has all unique values, but here we aren't even doing that (I don't know why).

I have two suggested fixes:

• squeeze=True should only rely on unique values. Otherwise this really is nearly impossible to predict.
• We should deprecate squeeze=True as the default value, and change this default behavior in the next major release.