TypeError when merging masked scalar coord

[trexfeathers]

The handling of scalar coordinates in _merge.py includes the build_indexes step. This cannot handle the scalar point being a masked value since it attempts to look the value up in the dictionary name_index_by_scalar, resulting in TypeError: unhashable type: 'MaskedConstant'.

This was discovered when attempting to load a NetCDF file that had previously been converted from NIMROD to NetCDF format. NetCDF had recognized a fill value for a scalar coordinate and therefore saved it as masked. load_cube includes a merge step, resulting in the error. This is demonstrated in the code below.

The presence of a masked point for a scalar coordinate is a possible scenario and should therefore be handled rather than producing an error.

import numpy as np
import iris
from iris import coords

my_cube = iris.cube.Cube(data=[1, 2])

# Generate a single-point array with a value NetCDF will recognise as a fill
# value and convert to a mask.
# Use the array as a scalar coordinate on my_cube.
my_array = np.array([-32767], dtype=np.int16)
my_scalar_coord = coords.DimCoord(points=my_array, standard_name="realization")
my_cube.add_aux_coord(my_scalar_coord)

# Save my_cube in NetCDF format.
iris.save(my_cube, "masked_scalar.nc")
# Attempt to load back the saved file. Results in the TypeError.
iris.load_cube("masked_scalar.nc")

[github-actions]

In order to maintain a backlog of relevant issues, we automatically label them as stale after 500 days of inactivity.

If this issue is still important to you, then please comment on this issue and the stale label will be removed.

Otherwise this issue will be automatically closed in 28 days time.

[schlunma]

I just discovered a very similar problem: if the input cubes for merging have a scalar coordinate that uses a masked array, the scalar coordinate of the merged cube is not a masked array anymore.

MWE:

import numpy as np
import iris
import iris.coords
import iris.cube

# Setup cubes
scalar_coord = iris.coords.AuxCoord(np.ma.masked_array([0]), var_name='x')
merge_coord_1 = iris.coords.AuxCoord([1], var_name='merge_coord')
merge_coord_2 = iris.coords.AuxCoord([2], var_name='merge_coord')
cube_1 = iris.cube.Cube(1, var_name='cube',
                        aux_coords_and_dims=[(scalar_coord, ()), (merge_coord_1, ())])
cube_2 = iris.cube.Cube(2, var_name='cube',
                        aux_coords_and_dims=[(scalar_coord, ()), (merge_coord_2, ())])

# Create merged cube
cubes = iris.cube.CubeList([cube_1, cube_2])
merged_cube = cubes.merge_cube()

# Print scalar coordinates
print("Scalar coordinate of input cubes:")
print(cube_1.coord('x'))
print(cube_2.coord('x'))
print("")
print("Scalar coordinate of merged cube:")
print(merged_cube.coord('x'))

gives

Scalar coordinate of input cubes:
AuxCoord(masked_array(data=[0],
             mask=False,
       fill_value=999999), standard_name=None, units=Unit('unknown'), var_name='x')
AuxCoord(masked_array(data=[0],
             mask=False,
       fill_value=999999), standard_name=None, units=Unit('unknown'), var_name='x')

Scalar coordinate of merged cube:
DimCoord(array([0]), standard_name=None, units=Unit('unknown'), var_name='x')

As you can see, the scalar coordinate of the merged cube is not a masked_array anymore.

[github-actions]

In order to maintain a backlog of relevant issues, we automatically label them as stale after 500 days of inactivity.

If this issue is still important to you, then please comment on this issue and the stale label will be removed.

Otherwise this issue will be automatically closed in 28 days time.

[schlunma]

Still relevant