def __check_boolean(items):
    """Checks a list of strings whether items are valid boolean values (1 or 0).

    Args:
        items (list of [str,]): The list of items to test.

    Returns:
        list of [bool,]: The list of items converted to boolean values. Returns
        None ifany one test fails.
    """
    for i, item in enumerate(items):
        if item == '1':
            items[i] = True
        elif item == '0':
            items[i] = False
        else:
            print('Failed on item: "{}"'.format(item))
            return None
    return items
