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

[leosaffin]

🐛 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.

1. 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.
2. 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 "<measure>: <varname>".

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 '/')