"Attribute (%s) of class %s has no associated methods (did you mean to provide an "is" argument?)"

Basically, omitting "is" (or equivalent) doesn't make the attribute read-only, it makes it neither readable nor writable!

and "is" doesn't seem to be a mandatory option

Remember how I said is is just a shortcut for something else? It's the lack of getters that actually triggers the error.