apply-visitor-pattern

Apply Visitor Pattern

Represent an operation to be performed on elements of an object structure — letting you add new operations without changing the element classes.

Why This Is Best Practice

Adopted by: Java compiler's AST visitors (the javax.lang.model API uses Visitor for all annotation processing — in every Java build), Python's ast.NodeVisitor (the foundation of Flake8, mypy, Bandit, and all Python static analysis tools), LLVM's IR pass infrastructure (every optimization is a Visitor over the IR — the core of Clang, Rust's compiler backend, and Swift), and PMD/Checkstyle (code analysis tools visited 80M+ downloads). Impact: GoF documents that without Visitor, adding a new operation to an object structure requires modifying every element class. Python's ast.NodeVisitor allows any tool to add new analysis operations (type checking, linting, security scanning) without touching the AST node classes. The entire Python static analysis ecosystem builds on this single design decision. Why best: Adding operations to each class directly — the alternative — violates OCP and bloats element classes with unrelated operations. Visitor keeps element classes focused on data representation; operations are concentrated in visitor classes.

Sources: Gamma et al. (1994) pp. 331–344; Python ast.NodeVisitor documentation; LLVM Pass documentation

Steps

Step 1: Define the visitor interface with a method for each element type

from abc import ABC, abstractmethod

class ShapeVisitor(ABC):
    @abstractmethod
    def visit_circle(self, circle: "Circle") -> None: ...

    @abstractmethod
    def visit_rectangle(self, rectangle: "Rectangle") -> None: ...

    @abstractmethod
    def visit_triangle(self, triangle: "Triangle") -> None: ...

Step 2: Add accept() to element classes — double dispatch

class Shape(ABC):
    @abstractmethod
    def accept(self, visitor: ShapeVisitor) -> None: ...

class Circle(Shape):
    def __init__(self, radius: float):
        self.radius = radius

    def accept(self, visitor: ShapeVisitor) -> None:
        visitor.visit_circle(self)   # double dispatch: runtime type of both self and visitor

class Rectangle(Shape):
    def __init__(self, width: float, height: float):
        self.width = width
        self.height = height

    def accept(self, visitor: ShapeVisitor) -> None:
        visitor.visit_rectangle(self)

Step 3: Implement concrete visitors — each adds one operation across all elements

class AreaCalculator(ShapeVisitor):
    def __init__(self):
        self.total_area = 0.0

    def visit_circle(self, circle: Circle) -> None:
        import math
        self.total_area += math.pi * circle.radius ** 2

    def visit_rectangle(self, rectangle: Rectangle) -> None:
        self.total_area += rectangle.width * rectangle.height

    def visit_triangle(self, triangle: "Triangle") -> None:
        self.total_area += 0.5 * triangle.base * triangle.height

class SVGExporter(ShapeVisitor):
    def __init__(self):
        self.output = []

    def visit_circle(self, circle: Circle) -> None:
        self.output.append(f'<circle r="{circle.radius}"/>')

    def visit_rectangle(self, rectangle: Rectangle) -> None:
        self.output.append(f'<rect width="{rectangle.width}" height="{rectangle.height}"/>')

    def visit_triangle(self, triangle: "Triangle") -> None:
        self.output.append(f'<polygon points="..."/>')

Step 4: Traverse the structure — apply any visitor to any element

shapes: list[Shape] = [Circle(5), Rectangle(3, 4), Circle(2)]

area_calc = AreaCalculator()
for shape in shapes:
    shape.accept(area_calc)
print(f"Total area: {area_calc.total_area:.2f}")

exporter = SVGExporter()
for shape in shapes:
    shape.accept(exporter)
print("\n".join(exporter.output))

Adding a new operation (e.g., JSONExporter) requires one new Visitor class — no changes to Circle, Rectangle, or Triangle.

Step 5: For recursive structures (trees), traverse in the visitor

class PrintVisitor(ast.NodeVisitor):
    def visit_FunctionDef(self, node):
        print(f"Function: {node.name}")
        self.generic_visit(node)   # recurse into children

generic_visit (Python's built-in) handles recursion, so the visitor focuses on the operation, not the traversal.

When NOT to Use

• When the element class structure changes frequently — adding a new element type requires updating every existing visitor. Visitor is best when element classes are stable and operations vary.
• When elements have very different interfaces — if the visitor's visit_X methods can't access what they need through the element's public interface, the visitor forces breaking encapsulation.

Common Mistakes

Forgetting accept() on a new element class. If a new element doesn't implement accept(), visitors skip it silently. All element classes must implement accept().

Visitor that builds state from one traversal and uses it in another. If AreaCalculator.total_area is read mid-traversal by another visitor, state is inconsistent. Each traversal should be independent.

Using Visitor when Iterator suffices. If all you need is sequential access to elements without type-specific handling, apply-iterator-pattern is simpler. Visitor pays off when behavior differs meaningfully per element type.