Constrained load on MeshCoord bypasses callback

[jrackham-mo]

🐛 Bug Report

When applying a constraint on a MeshCoord and a callback which checks the loaded cube has a mesh, the callback is not applied on the returned cube.

How To Reproduce

Steps to reproduce the behaviour:

1. Define a custom callback function which raises an exception if a cube does not have a mesh.
2. Attempt to load UGrid data with a constraint on latitude (a MeshCoord) and pass the custom callback.
3. The cube is loaded, without a mesh. The MeshCoords have been converted to AuxCoords as expected, but no exception was raised.

Expected behaviour

I expected an exception to have been raised by the custom callback function, since the returned cube has no mesh.

Reproducible example

import iris
from iris.experimental.ugrid import PARSE_UGRID_ON_LOAD
import iris.exceptions
from iris import sample_data_path

def ugrid_load(uris, constraints=None):
    """Load a UGrid file. Will not load if the file contains regular data.

    Parameters
    ----------
    uris : Any
        Location of file to load. Must be a NetCDF file.
    constraints: Any | None
        One or more iris constraints. Each constraint can be either a string,
        or an instance of :class:`iris.Constraint`. If the constraint is a string
        it will be used to match against cube.name().

    Returns
    -------
    iris.cube.CubeList
        An iris :class:`iris.cube.CubeList` containing the loaded data.

    Raises
    ------
    iris.exceptions.InvalidCubeError
        If UGrid data cannot be loaded from the file.
    """
    with PARSE_UGRID_ON_LOAD.context():
        ugrid_cubelist = iris.load(uris=uris, constraints=constraints, callback=mesh_check)
    return ugrid_cubelist

def mesh_check(cube, field, filename):
    """Check if the loaded data is UGrid or not.

    A function to be passed to callback in iris.load.

    Parameters
    ----------
    cube : iris.cube.Cube
        Loaded :class:`iris.cube.CubeList` to be checked
    field : Any
        Only present to satisfy the function signature used by callback.
    filename : Any
        Original file from which the cube data was loaded

    Raises
    ------
    iris.exceptions.InvalidCubeError
        If the loaded cube does not contain UGrid data
    """
    if cube.mesh is None:
        raise iris.exceptions.InvalidCubeError(
            f"Attempting to load from file '{filename}' did not return UGrid data."
        )
    
if __name__ == "__main__":
    latitude_constraint = iris.Constraint(latitude=lambda cell: cell > 0)

    # Try with non-UGrid netcdf first to confirm it works
    non_ugrid_path = sample_data_path("atlantic_profiles.nc")
    try:
        ugrid_load(non_ugrid_path, latitude_constraint)
    except iris.exceptions.InvalidCubeError as exc:
        print(exc)

    # Now try constraining on a MeshCoord on load of a file containing UGrid data
    # Expected to raise an exception since the returned cube has no mesh due to the constraint
    # having converted the MeshCoords to AuxCoords
    mesh_C4_path = sample_data_path("mesh_C4_synthetic_float.nc")
    ugrid_cubelist = ugrid_load(mesh_C4_path, latitude_constraint)
    if len(ugrid_cubelist) != 0:
        print(f"Data successfully loaded from '{mesh_C4_path}' with constraint '{latitude_constraint}'.")
        print(ugrid_cubelist[0])
        print(f"Cube's mesh is: {ugrid_cubelist[0].mesh}")

Output:

Attempting to load from file '.../iris_sample_data/sample_data/atlantic_profiles.nc' did not return UGrid data.
Data successfully loaded from '.../iris_sample_data/sample_data/mesh_C4_synthetic_float.nc' with constraint 'Constraint(coord_values={'latitude': <function <lambda> at 0x7fd980bc0310>})'.
synthetic / (1)                     (-- : 32)
    Auxiliary coordinates:
        latitude                        x
        longitude                       x
    Attributes:
        NCO                         'netCDF Operators version 4.7.5 (Homepage = http://nco.sf.net, Code = h ...'
        history                     'Mon Apr 12 01:44:41 2021: ncap2 -s synthetic=float(synthetic) mesh_C4_synthetic.nc ...'
        nco_openmp_thread_number    1
Cube's mesh is: None

Environment

• OS & Version: RHEL 7.9
• Iris Version: 3.7.1