Mean called on groupby object adds dimensions to undesired variables

[andrewpauling]

MCVE Code Sample

import numpy as np
import xarray as xr
import cftime

# create time coordinate
tdays = np.arange(0, 730)
time = cftime.num2date(tdays, 'days since 0001-01-01 00:00:00',
                       calendar='noleap')

# create spatial coordinate
lev = np.arange(100)

# Create dummy data
x = np.random.rand(time.size, lev.size)
y = np.random.rand(lev.size)

# Create sample Dataset
ds = xr.Dataset({'sample_data': (['time', 'lev'],  x),
                 'independent_data': (['lev'], y)},
                coords={'time': (['time'], time),
                        'lev': (['lev'], lev)})

# Perform groupby and mean
ds2 = ds.groupby('time.month').mean(dim='time')

Actual Output

<xarray.Dataset>
Dimensions:           (lev: 100, month: 12)
Coordinates:
  * lev               (lev) int64 0 1 2 3 4 5 6 7 8 ... 92 93 94 95 96 97 98 99
  * month             (month) int64 1 2 3 4 5 6 7 8 9 10 11 12
Data variables:
    sample_data       (month, lev) float64 0.5143 0.554 0.5027 ... 0.5246 0.5435
    independent_data  (month, lev) float64 0.01667 0.4687 ... 0.1015 0.7459

Expected Output

<xarray.Dataset>
Dimensions:           (lev: 100, month: 12)
Coordinates:
  * lev               (lev) int64 0 1 2 3 4 5 6 7 8 ... 92 93 94 95 96 97 98 99
  * month             (month) int64 1 2 3 4 5 6 7 8 9 10 11 12
Data variables:
    sample_data       (month, lev) float64 0.5143 0.554 0.5027 ... 0.5246 0.5435
    independent_data  (lev) float64 0.01667 0.4687 ... 0.1015 0.7459

Problem Description

The variable independent_data above initially has no time dimension but, after performing groupby('time.month').mean(dim='time') on the Dataset, it now has a month dimension that is meaningless.
Preferably, it should leave the independent_data variable untouched.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 (default, Mar 27 2019, 16:54:48) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.7.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: 1.10.5 libnetcdf: 4.6.2

xarray: 0.12.2
pandas: 0.24.2
numpy: 1.16.4
scipy: 1.3.0
netCDF4: 1.5.1.2
pydap: installed
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.1.0
cartopy: 0.17.0
seaborn: None
numbagg: None
setuptools: 41.0.1
pip: 19.1.1
conda: None
pytest: None
IPython: 7.2.0
sphinx: 2.1.2

[jhamman]

@andrewpauling - Can you confirm this is still an issue with xarray v0.14 (released today)?

[andrewpauling]

@jhamman yes, I just updated to v0.14 and the issue is still present

[dcherian]

Closing as dupe of #2145