def csp_property(key):
    """Return a new property object for a content security policy header.
    Useful if you want to add support for a csp extension in a
    subclass.
    """
    return property(lambda x: x._get_value(key), lambda x, v: x._set_value(
        key, v), lambda x: x._del_value(key), 'accessor for %r' % key)
