Missing grib / amip in CF StandardNameTable 87

[Arnoques]

Version of compliance checker running:
5.1.1

Describe the checker this affects:
CF checker

Attach a minimal CDL or NetCDF file which is able to reproduce the issue
Since I can't attach an actual file, this is a minimal python that generates a failing netCDF file

import netCDF4

root = netCDF4.Dataset('minimal.nc', 'w')
root.standard_name_vocabulary = 'CF Standard Name Table v87'  # Using older tables does not trigger the issue
scanline_var = root.createVariable(varname='latitude', datatype='i4')
scanline_var.standard_name = 'latitude'
scanline_var.units = 'degrees_north'
scanline_var[:] = 10

To Reproduce:

1. Run the above python code
2. compliance-checker --test cf:1.8 --verbose minimal.nc

Describe the issue below:
The tool outputs this internal error:

cf:1.8.check_units: list index out of range

The error arises in compliance_checker/cf/util.py line 209, when looking for grib and amip in the standards table. Version 87 does not include them, and the _get function fails for a missing non-required attribute.

I suggest adding the following fix in compliance_checker/cf/util.py::NameEntry:

        def _get(self, entrynode, attrname, required=False):
            vals = entrynode.xpath(attrname)
            if len(vals) > 1:
                raise Exception("Multiple attrs (%s) found" % attrname)
            elif required and len(vals) == 0:
                raise Exception("Required attr (%s) not found" % attrname)
+            elif len(vals) == 0:
+                return None

            return vals[0].text

[benjwadams]

Please create a pull request and I'll review this.

[Unichronic]

Can I work on this? @benjwadams

[Arnoques]

Thanks for working on it, @benjwadams!