OrderedDict([('PRODUCT', <class 'netCDF4._netCDF4.Group'>
              group /PRODUCT:
                  dimensions(sizes): scanline(5640), ground_pixel(450), corner(4), time(1), polynomial_exponents(6), layer(34), vertices(2)
                  variables(dimensions): int32 scanline(scanline), int32 ground_pixel(ground_pixel), int32 time(time), int32 corner(corner), int32 polynomial_exponents(polynomial_exponents), int32 layer(layer), int32 vertices(vertices), float32 latitude(time,scanline,ground_pixel), float32 longitude(time,scanline,ground_pixel), int32 delta_time(time,scanline), <class 'str'> time_utc(time,scanline), uint8 qa_value(time,scanline,ground_pixel), float32 nitrogendioxide_tropospheric_column(time,scanline,ground_pixel), float32 nitrogendioxide_tropospheric_column_precision(time,scanline,ground_pixel), float32 averaging_kernel(time,scanline,ground_pixel,layer), float32 air_mass_factor_troposphere(time,scanline,ground_pixel), float32 air_mass_factor_total(time,scanline,ground_pixel), int32 tm5_tropopause_layer_index(time,scanline,ground_pixel), float32 tm5_constant_a(layer,vertices), float32 tm5_constant_b(layer,vertices)
                  groups: SUPPORT_DATA),
             ('METADATA', <class 'netCDF4._netCDF4.Group'>
              group /METADATA:
                  dimensions(sizes): 
                  variables(dimensions): 
                  groups: QA_STATISTICS, ALGORITHM_SETTINGS, GRANULE_DESCRIPTION, ISO_METADATA, EOP_METADATA, ESA_METADATA)])

# opening the product with xarray raises an UnboundLocalError exception
no2 = xr.open_dataset(product, group='/PRODUCT')

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

<xarray.Dataset>
Dimensions:                                                (nitrogendioxide_stratospheric_column_histogram_axis: 100, nitrogendioxide_stratospheric_column_pdf_axis: 400, nitrogendioxide_total_column_histogram_axis: 100, nitrogendioxide_total_column_pdf_axis: 400, nitrogendioxide_tropospheric_column_histogram_axis: 100, nitrogendioxide_tropospheric_column_pdf_axis: 400, vertices: 2)
[...]