@liyangfnst I think you're thinking of single-underscore, not double-underscore or "dunder" as it's called sometimes :)  In this instance, this is okay, it's part of how the base-class <code>validates</code> property works.  Dunder's just mean it's 'special' and isn't intended for general use by callers.  In this case it's okay because NodedevXMLBase is subclassing base.LibvirtXMLBase and overriding this class attribute.