The attrs attribute contains a dictionary with invisible, required, and readonly keys (all of which are optional). The values are domains that may refer to the fields that exist on the form (and really only those, so there are no dotted paths), and the whole dictionary is evaluated according to the rules for client-side Python, as described earlier in the Passing parameters to forms and actions – Context recipe of this chapter. So, for example, you can access the context in the right-hand operand.