def METHOD_NAME(instance):
    """Returns the list of SampleType UIDs the instance is assigned to
    This is a KeywordIndex, so it will be indexed as a list, even if only one
    SampleType can be assigned to the instance. Moreover, if the instance has
    no SampleType assigned, it returns a tuple with a None value. This allows
    searches for `MissingValue` entries too.
    """
    sample_type = instance.getSampleType()
    return to_keywords_list(sample_type, api.get_uid)