tests/misc.py

from testsuite import warning

def colours_test(model):
    yield from (
        warning(element, 'bad-colour', colour_index = element.colour.index)
        for element in model.body
        if hasattr(element, 'colour') and not element.colour.is_valid
    ) 

manifest = {
    'tests': {
        'colour-validity': colours_test,
    },
    'messages': {
        'bad-colour': lambda colour_index: str.format(
            'invalid colour {}',
            colour_index,
        ),
    },
}