Recap: Visitor Pattern

Operations become classes, double dispatch picks the method, and new element types cost the most.

Visitor puts each operation over a set of classes into a class of its own.

Intent. Add operations over element classes without changing those classes.
Participants. A visitor interface with one method per element class, concrete visitors with one operation each, and elements whose accept calls the method for their own class.
Double dispatch. accept chooses by the element's class; the visit call inside it chooses by the visitor's class.
In Python. ast.NodeVisitor finds the method by name, visit_ plus the class name; singledispatch and match need no accept at all.
The trade-off. A new operation is one class; a new element class edits every visitor.
When not. Element classes that change often, or only one or two operations.

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