def mangle(class_name: str, attribute: str) -> str:
    """
    Mangle private attributes:
        cls.attribute     => cls.attribute
        cls._attribute    => cls._attribute
        cls.__attribute   => cls._name__attribute  <-----
        cls.__attribute__ => cls.__attribute__
    """
    if attribute.startswith('__') and not attribute.endswith('__'):
        attribute = '_' + class_name + attribute
    return attribute