# Buggy code
def encode(self, variable, name=None):
    dims, data, attrs, encoding = unpack_for_encoding(variable)

    if encoding.get("_FillValue") is not None:
        fill_value = pop_to(encoding, attrs, "_FillValue", name=name)
        if not pd.isnull(fill_value):
            data = duck_array_ops.fillna(data, fill_value)
        variable = Variable(dims, data, attrs, encoding)

    if (
        "_FillValue" not in attrs
        and "_FillValue" not in encoding
        and np.issubdtype(data.dtype, np.floating)
    ):
        attrs["_FillValue"] = data.dtype.type(np.nan)

    return Variable(dims, data, attrs, encoding)


# Traceback (UnboundLocalError)
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-26-2052b95f2d95> in <module>()
----> 1 no2 = xr.open_dataset(product, group='/PRODUCT')

/Users/stefano/anaconda/lib/python3.6/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':

/Users/stefano/anaconda/lib/python3.6/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)

/Users/stefano/anaconda/lib/python3.6/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))

/Users/stefano/anaconda/lib/python3.6/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

/Users/stefano/anaconda/lib/python3.6/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

# Fixed code
def encode(self, variable, name=None):
    dims, data, attrs, encoding = unpack_for_encoding(variable)

    if encoding.get("_FillValue") is not None:
        fill_value = pop_to(encoding, attrs, "_FillValue", name=name)
        if not pd.isnull(fill_value):
            data = duck_array_ops.fillna(data, fill_value)

    return Variable(dims, data, attrs, encoding)