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
181099871,MDU6SXNzdWUxODEwOTk4NzE=,1039,NetCDF4 backend fails with single-value dimension,5184063,closed,0,1217238,,1,2016-10-05T09:03:52Z,2017-06-17T00:14:32Z,2017-06-17T00:14:32Z,NONE,,,,"Exporting and xarray object with a string dimension which only consists of a single value fails using python 3 and xarray 0.8.2.

Here is an example to reproduce the error using the toy weather data from the tutorials:

``` python
import xarray as xr
import numpy as np
import pandas as pd

times = pd.date_range('2000-01-01', '2001-12-31', name='time')
annual_cycle = np.sin(2 * np.pi * (times.dayofyear / 365.25 - 0.28))

base = 10 + 15 * annual_cycle.reshape(-1, 1)
tmin_values = base + 3 * np.random.randn(annual_cycle.size, 3)
tmax_values = base + 10 + 3 * np.random.randn(annual_cycle.size, 3)

ds = xr.Dataset({'tmin': (('time', 'location'), tmin_values),
                 'tmax': (('time', 'location'), tmax_values)},
                {'time': times, 'location': ['IA', 'IN', 'IL']})
```

The following function call fails for me with a `TypeError: len() of unsized object`:

``` python
ds.sel(location=""IA"").to_netcdf(""asdf"", engine=""netcdf4"")
```

The lower part of the traceback:

``` pytb
D:\Anaconda3\lib\site-packages\xarray\backends\netCDF4_.py in prepare_variable(self, name, variable, check_encoding)
    250 
    251         if self.format == 'NETCDF4':
--> 252             variable, datatype = _nc4_values_and_dtype(variable)
    253         else:
    254             variable = encode_nc3_variable(variable)

D:\Anaconda3\lib\site-packages\xarray\backends\netCDF4_.py in _nc4_values_and_dtype(var)
     75     if var.dtype.kind == 'U':
     76         # this entire clause should not be necessary with netCDF4>=1.0.9
---> 77         if len(var) > 0:
     78             var = var.astype('O')
     79         dtype = str

D:\Anaconda3\lib\site-packages\xarray\core\utils.py in __len__(self)
    371             return self.shape[0]
    372         except IndexError:
--> 373             raise TypeError('len() of unsized object')
    374 
    375
```

The export works fine both without the selection:

``` python
ds.to_netcdf(""asdf"", engine=""netcdf4"")
```

And with dropping the dimension after:

``` python
ds.sel(location=""IA"").drop(""location"").to_netcdf(""asdf"", engine=""netcdf4"")
```
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