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
599993415,MDU6SXNzdWU1OTk5OTM0MTU=,3972,Variable objects not displayable in notebook,9948595,closed,0,,,2,2020-04-15T03:32:13Z,2020-04-29T15:54:23Z,2020-04-29T15:54:23Z,CONTRIBUTOR,,,,"class `Variable` subclasses` AbstractArray` which has a `_repr_html_` method so variable objects
should display in a notebook. 

But this fails with a 
`AttributeError: 'Variable' object has no attribute 'variable'`:

```python
from os.path import join,dirname
import xarray as xr
ds=join(dirname(xr.__file__), 'tests/data/bears.nc')
xr.open_dataset(ds).variables['cross']
```

### Fix:
change line 186 in `xarray.core.formatting_html.py:: array_section`
from
```python
preview = escape(inline_variable_array_repr(obj.variable, max_width=70))
```
to (because obj *is* a variable):

```python
preview = escape(inline_variable_array_repr(obj.variable if hasattr(obj, ""variable"") else obj, max_width=70))
```
I can make a PR if you like.

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.6 | packaged by conda-forge | (default, Mar 23 2020, 23:03:20) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 5.3.0-46-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.7.4

xarray: 0.15.1
pandas: 1.0.3
numpy: 1.18.1
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: 1.1.1.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2.14.0
distributed: 2.6.0
matplotlib: 3.2.1
cartopy: None
seaborn: 0.10.0
numbagg: None
setuptools: 46.1.3.post20200325
pip: 20.0.2
conda: 4.8.3
pytest: 5.4.1
IPython: 7.13.0
sphinx: 2.2.1
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3972/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
437940426,MDU6SXNzdWU0Mzc5NDA0MjY=,2923,Calling max() on a stacked dimension raises exception,9948595,closed,0,,,1,2019-04-27T12:50:01Z,2019-06-27T16:06:55Z,2019-06-27T16:06:55Z,CONTRIBUTOR,,,,"#### Code Sample, a copy-pastable example if possible

A ""Minimal, Complete and Verifiable Example"" will make it much easier for maintainers to help you:
http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports

```python
import numpy as np
import xarray as xr
data2 = np.random.randn(3,100, 4)
da = xr.DataArray(
    data2,
    name='value',
    dims=['replicate', 'rsample', 'exp'],
    coords=dict(replicate=[0,1,2],
        exp=['a', 'b', 'c', 'd'], rsample=list(range(100))))
da2 = da.stack(sample=('replicate', 'rsample'))
da2.sample.max()
```

#### Problem description
Calling  `max()` on a stacked dimension raises the exception
`ValueError: dimensions () must have the same length as the number of data dimensions, ndim=1`

#### Expected Output
Maximum value of selected dimension. i.e an index I can use with loc
e.g. ` da2.loc['a', da2.sample.max()]` that is `da2.loc['a', (2,99)]`

So I guess I'm expecting something like:
```
<xarray.DataArray 'sample' ()>
array((2, 99), dtype=object)
```
This works: `da2.loc['a', xr.DataArray((2,99), dims=(), name='sample')]`

#### Output of ``xr.show_versions()``

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.3 (default, Mar 27 2019, 22:11:17) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.15.0-47-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_AU.UTF-8
LOCALE: en_AU.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.1

xarray: 0.12.1
pandas: 0.24.2
numpy: 1.15.4
scipy: 1.2.1
netCDF4: 1.4.2
pydap: None
h5netcdf: None
h5py: 2.9.0
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: 1.2.0
distributed: 1.27.0
matplotlib: 3.0.3
cartopy: None
seaborn: 0.9.0
setuptools: 40.6.3
pip: 18.1
conda: 4.6.14
pytest: None
IPython: 7.4.0
sphinx: 2.0.1
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2923/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue