iris.coord_categorisation.add_season_membership creates a boolean coordinate that cannot be saved to netcdf

🐛 Bug Report

Trying to save a cube that has had iris.coord_categorisation.add_season_membership applied to it fails because netcdf does not allow boolean types

How To Reproduce

Running

from iris import coord_categorisation
from iris.tests import stock

cube = stock.realistic_4d()
iris.save(cube, "cube_with_season_membership.nc")

gives the following error from the netcdf4 library (full traceback below)

TypeError: Illegal primitive data type, must be one of dict_keys(['S1', 'i1', 'u1', 'i2', 'u2', 'i4', 'u4', 'i8', 'u8', 'f4', 'f8']), got bool (variable 'season_membership', group '/')

Expected behaviour

It's fairly simple to work around this by converting the coordinate data to integers, but I would expect it to "just work". I can think of two solutions but both have downsides, so I wasn't sure what was best.

Change add_season_membership to return an integer 1s/0s array. This means the cube never becomes incompatible with netCDF, but might be an issue for people with code expecting a boolean array.
Change the saving behaviour in iris to convert boolean arrays to integers. This means add_season_membership can work the same but I don't know how you stop the cube becoming inconsistent between saving and reloading.

Additional context

Click to expand this section...

``` --------------------------------------------------------------------------- TypeError Traceback (most recent call last) Cell In[14], line 1 ----> 1 iris.save(cube, "cube_with_season_membership.nc") File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/io/__init__.py:497, in save(source, target, saver, **kwargs) 495 # Single cube? 496 if isinstance(source, Cube): --> 497 result = saver(source, target, **kwargs) 499 # CubeList or sequence of cubes? 500 elif isinstance(source, CubeList) or ( 501 isinstance(source, (list, tuple)) 502 and all([isinstance(i, Cube) for i in source]) 503 ): 504 # Only allow cubelist saving for those fileformats that are capable. File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:2876, in save(cube, filename, netcdf_format, local_keys, unlimited_dimensions, zlib, complevel, shuffle, fletcher32, contiguous, chunksizes, endian, least_significant_digit, packing, fill_value, compute) 2873 with Saver(filename, netcdf_format, compute=compute) as sman: 2874 # Iterate through the cubelist. 2875 for cube, packspec, fill_value in zip(cubes, packspecs, fill_values): -> 2876 sman.write( 2877 cube, 2878 local_keys, 2879 unlimited_dimensions, 2880 zlib, 2881 complevel, 2882 shuffle, 2883 fletcher32, 2884 contiguous, 2885 chunksizes, 2886 endian, 2887 least_significant_digit, 2888 packing=packspec, 2889 fill_value=fill_value, 2890 ) 2892 if iris.config.netcdf.conventions_override: 2893 # Set to the default if custom conventions are not available. 2894 conventions = cube.attributes.get( 2895 "Conventions", CF_CONVENTIONS_VERSION 2896 ) File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:698, in Saver.write(self, cube, local_keys, unlimited_dimensions, zlib, complevel, shuffle, fletcher32, contiguous, chunksizes, endian, least_significant_digit, packing, fill_value) 694 self._add_dim_coords(cube, cube_dimensions) 696 # Add the auxiliary coordinate variables and associate the data 697 # variable to them --> 698 self._add_aux_coords(cube, cf_var_cube, cube_dimensions) 700 # Add the cell_measures variables and associate the data 701 # variable to them 702 self._add_cell_measures(cube, cf_var_cube, cube_dimensions) File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:1051, in Saver._add_aux_coords(self, cube, cf_var_cube, dimension_names) 1046 location_coords: MeshNodeCoords | MeshEdgeCoords | MeshFaceCoords = getattr( 1047 mesh, f"{mesh_location}_coords" 1048 ) 1049 coords_to_add.extend(list(location_coords)) -> 1051 return self._add_inner_related_vars( 1052 cube, 1053 cf_var_cube, 1054 dimension_names, 1055 coords_to_add, 1056 ) File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:999, in Saver._add_inner_related_vars(self, cube, cf_var_cube, dimension_names, coordlike_elements) 996 cf_name = self._name_coord_map.name(element) 997 if cf_name is None: 998 # Not already present : create it --> 999 cf_name = self._create_generic_cf_array_var( 1000 cube, dimension_names, element 1001 ) 1002 self._name_coord_map.append(cf_name, element) 1004 if role_attribute_name == "cell_measures": 1005 # In the case of cell-measures, the attribute entries are not just 1006 # a var_name, but each have the form ": ". File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/saver.py:1900, in Saver._create_generic_cf_array_var(self, cube_or_mesh, cube_dim_names, element, element_dims, fill_value) 1897 cf_name = element_dims[0] 1899 # Create the CF-netCDF variable. -> 1900 cf_var = self._dataset.createVariable( 1901 cf_name, 1902 data.dtype.newbyteorder("="), 1903 element_dims, 1904 fill_value=fill_value, 1905 ) 1907 # Add the axis attribute for spatio-temporal CF-netCDF coordinates. 1908 if is_dimcoord: File ~/miniforge3/envs/core/lib/python3.11/site-packages/iris/fileformats/netcdf/_thread_safe_nc.py:229, in GroupWrapper.createVariable(self, *args, **kwargs) 221 """ 222 Calls createVariable() from netCDF4.Group/Dataset within _GLOBAL_NETCDF4_LOCK, returning VariableWrapper. 223 (...) 226 also performed within _GLOBAL_NETCDF4_LOCK. 227 """ 228 with _GLOBAL_NETCDF4_LOCK: --> 229 new_variable = self._contained_instance.createVariable( 230 *args, **kwargs 231 ) 232 return VariableWrapper.from_existing(new_variable) File src/netCDF4/_netCDF4.pyx:2967, in netCDF4._netCDF4.Dataset.createVariable() File src/netCDF4/_netCDF4.pyx:4149, in netCDF4._netCDF4.Variable.__init__() TypeError: Illegal primitive data type, must be one of dict_keys(['S1', 'i1', 'u1', 'i2', 'u2', 'i4', 'u4', 'i8', 'u8', 'f4', 'f8']), got bool (variable 'season_membership', group '/') ```

SciTools / iris