issues: 506914634

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
506914634 MDU6SXNzdWU1MDY5MTQ2MzQ= 3398 Mean called on groupby object adds dimensions to undesired variables 22488770 closed 0     3 2019-10-14T23:03:04Z 2019-10-16T14:30:38Z 2019-10-16T14:30:38Z CONTRIBUTOR      

MCVE Code Sample

```python 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

python <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

python <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 
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