def get_valid_classes_phrase(input_classes):
    """Returns a string phrase describing what types are allowed
    """
    all_classes = list(input_classes)
    all_classes = sorted(all_classes, key=lambda cls: cls.__name__)
    all_class_names = [cls.__name__ for cls in all_classes]
    if len(all_class_names) == 1:
        return 'is {0}'.format(all_class_names[0])
    return "is one of [{0}]".format(", ".join(all_class_names))