visitor

id	behavioral/visitor
name	Visitor
aliases	double-dispatch
guide_url
problem	Run a new operation over every node of an object structure without adding a method to every node class.
symptoms	walk an AST operation per node type double dispatch add behavior across a class family
verdict	prefer-alternative
caveats	functools.singledispatch dispatches on type without touching the node classes — the visitor with the accept() plumbing deleted. The subclass form survives where the stdlib hands it to you: ast.NodeVisitor is the right tool for walking Python source.
stdlib_sightings	functools.singledispatch ast.NodeVisitor

Visitor

New operations over a node structure, without editing the nodes. Verdict: prefer an alternative — singledispatch deletes the accept() plumbing; the subclass form survives at stdlib boundaries (ast.NodeVisitor).

Where	What
pattern/	The importable code: Operation, UnhandledNodeError
docs/	Fundamentals · Implementation guide · External examples
examples/doc_exporters/	Mini-project: document exporters built on pattern/
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.behavioral.visitor.examples.doc_exporters.main