html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/1781#issuecomment-354370636,https://api.github.com/repos/pydata/xarray/issues/1781,354370636,MDEyOklzc3VlQ29tbWVudDM1NDM3MDYzNg==,3698640,2017-12-28T22:55:28Z,2017-12-28T22:55:28Z,CONTRIBUTOR,"I've got a reproducible example of this (sorry for the length):

```python
In [1]: import xarray as xr
   ...: import numpy as np
   ...: import pandas as pd

   ...: import netCDF4
```

```python
In [2]: xr.show_versions()
```

    INSTALLED VERSIONS
    ------------------
    commit: None
    python: 3.5.4.final.0
    python-bits: 64
    OS: Darwin
    OS-release: 17.3.0
    machine: x86_64
    processor: 
    byteorder: little
    LC_ALL: None
    LANG: en_US.UTF-8
    LOCALE: en_US.UTF-8
    
    xarray: 0.10.0
    pandas: 0.21.0
    numpy: 1.13.3
    scipy: 1.0.0
    netCDF4: 1.3.1
    h5netcdf: None
    Nio: None
    bottleneck: 1.2.1
    cyordereddict: None
    dask: 0.16.0
    matplotlib: 2.1.0
    cartopy: None
    seaborn: 0.8.1
    setuptools: 36.5.0.post20170921
    pip: 9.0.1
    conda: None
    pytest: 3.3.1
    IPython: 6.2.1
    sphinx: None

```python
In [3]: nds = netCDF4.Dataset('test.nc', 'w', format='NETCDF4')
```

```python
In [4]: nds.createDimension('X', 3)
   ...: nds.createVariable('X', str, ('X', ), fill_value='invalid')
   ...: nds.createVariable('xname', str, ('X', ), fill_value='invalid')
   ...: nds.createVariable('var1', np.float32, ('X', ), fill_value=np.nan)
   ...: 
   ...: nds.variables['X'][:] = np.array(['first', 'second', 'third'])
   ...: nds.variables['xname'][:] = np.array(['the_first', 'invalid', 'the_third'])
   ...: nds.variables['var1'][:] = np.random.random((3, ))
```

```python
In [5]: nds.variables['X']
```

    <class 'netCDF4._netCDF4.Variable'>
    vlen X(X)
        _FillValue: invalid
    vlen data type: <class 'str'>
    unlimited dimensions: 
    current shape = (3,)


```python
In [6]: nds.variables['X'][:]
```

    array(['first', 'second', 'third'], dtype=object)

```python
In [7]: nds.variables['xname']
```

    <class 'netCDF4._netCDF4.Variable'>
    vlen xname(X)
        _FillValue: invalid
    vlen data type: <class 'str'>
    unlimited dimensions: 
    current shape = (3,)

```python
In [8]: nds.variables['xname'][:]
```

    array(['the_first', 'invalid', 'the_third'], dtype=object)

```python
In [9]: nds.variables['var1']
```

    <class 'netCDF4._netCDF4.Variable'>
    float32 var1(X)
        _FillValue: nan
    unlimited dimensions: 
    current shape = (3,)
    filling on

```python
In [10]: nds.variables['var1'][:]
```

    array([ 0.58315825,  0.82443351,  0.39345944], dtype=float32)

```python
In [11]: nds.close()
```

```python
In [12]: ds = xr.open_dataset('test.nc')
```

    ---------------------------------------------------------------------------

    UnboundLocalError                         Traceback (most recent call last)

    <ipython-input-14-1a1dee73db10> in <module>()
    ----> 1 ds = xr.open_dataset('test.nc')
    

    ~/miniconda2/envs/spatial3/lib/python3.5/site-packages/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, autoclose, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables)
        303             lock = _default_lock(filename_or_obj, engine)
        304         with close_on_error(store):
    --> 305             return maybe_decode_store(store, lock)
        306     else:
        307         if engine is not None and engine != 'scipy':


    ~/miniconda2/envs/spatial3/lib/python3.5/site-packages/xarray/backends/api.py in maybe_decode_store(store, lock)
        223             store, mask_and_scale=mask_and_scale, decode_times=decode_times,
        224             concat_characters=concat_characters, decode_coords=decode_coords,
    --> 225             drop_variables=drop_variables)
        226 
        227         _protect_dataset_variables_inplace(ds, cache)


    ~/miniconda2/envs/spatial3/lib/python3.5/site-packages/xarray/conventions.py in decode_cf(obj, concat_characters, mask_and_scale, decode_times, decode_coords, drop_variables)
       1153     vars, attrs, coord_names = decode_cf_variables(
       1154         vars, attrs, concat_characters, mask_and_scale, decode_times,
    -> 1155         decode_coords, drop_variables=drop_variables)
       1156     ds = Dataset(vars, attrs=attrs)
       1157     ds = ds.set_coords(coord_names.union(extra_coords).intersection(vars))


    ~/miniconda2/envs/spatial3/lib/python3.5/site-packages/xarray/conventions.py in decode_cf_variables(variables, attributes, concat_characters, mask_and_scale, decode_times, decode_coords, drop_variables)
       1086             k, v, concat_characters=concat_characters,
       1087             mask_and_scale=mask_and_scale, decode_times=decode_times,
    -> 1088             stack_char_dim=stack_char_dim)
       1089         if decode_coords:
       1090             var_attrs = new_vars[k].attrs


    ~/miniconda2/envs/spatial3/lib/python3.5/site-packages/xarray/conventions.py in decode_cf_variable(name, var, concat_characters, mask_and_scale, decode_times, decode_endianness, stack_char_dim)
        998         if (has_fill or scale_factor is not None or add_offset is not None):
        999             if has_fill and np.array(fill_value).dtype.kind in ['U', 'S', 'O']:
    -> 1000                 if string_encoding is not None:
       1001                     raise NotImplementedError(
       1002                         'variable %r has a _FillValue specified, but '


    UnboundLocalError: local variable 'string_encoding' referenced before assignment


This seems to be produced by the fact that `string_encoding` is not defined for unicode-encoded variable-length string types. Could the problem be the lack of `'O'` and `'U'` in conventions.py line [952](https://github.com/pydata/xarray/blob/6eac8574f85623b18b59ba1ac9cf95e09c87980b/xarray/conventions.py#L952)? I'll try my hand at a PR.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,282061228