add custom data to nodes

[nschloe]

I would like to add custom data to some nodes which makes subsequent tree changes much easier for me. To this end, I tried

class RenameLambdaArgs(cst.CSTTransformer):
    def leave_Lambda(self, orig_node, updated_node):
        updated_node._foobar = ["123"]
        return updated_node

but this only gives me

E   TypeError: super(type, obj): obj must be an instance or subtype of type

Is there perhaps a free value field that I could access with with_updates or similar?