Editing Visitor pattern (section)

==== Sources ====
<syntaxhighlight lang="python">
"""
Visitor pattern example.
"""

from abc import ABCMeta, abstractmethod

NOT_IMPLEMENTED = "You should implement this."

class CarElement(metaclass=ABCMeta):
    @abstractmethod
    def accept(self, visitor):
        raise NotImplementedError(NOT_IMPLEMENTED)

class Body(CarElement):
    def accept(self, visitor):
        visitor.visitBody(self)

class Engine(CarElement):
    def accept(self, visitor):
        visitor.visitEngine(self)

class Wheel(CarElement):
    def __init__(self, name):
        self.name = name
    def accept(self, visitor):
        visitor.visitWheel(self)

class Car(CarElement):
    def __init__(self):
        self.elements = [
            Wheel("front left"), Wheel("front right"),
            Wheel("back left"), Wheel("back right"),
            Body(), Engine()
        ]

    def accept(self, visitor):
        for element in self.elements:
            element.accept(visitor)
        visitor.visitCar(self)

class CarElementVisitor(metaclass=ABCMeta):
    @abstractmethod
    def visitBody(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitEngine(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitWheel(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)
    @abstractmethod
    def visitCar(self, element):
        raise NotImplementedError(NOT_IMPLEMENTED)

class CarElementDoVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Moving my body.")
    def visitCar(self, car):
        print("Starting my car.")
    def visitWheel(self, wheel):
        print("Kicking my {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Starting my engine.")

class CarElementPrintVisitor(CarElementVisitor):
    def visitBody(self, body):
        print("Visiting body.")
    def visitCar(self, car):
        print("Visiting car.")
    def visitWheel(self, wheel):
        print("Visiting {} wheel.".format(wheel.name))
    def visitEngine(self, engine):
        print("Visiting engine.")

car = Car()
car.accept(CarElementPrintVisitor())
car.accept(CarElementDoVisitor())
</syntaxhighlight>