Editing Visitor pattern (section)

=== Python ===
Python does not support method overloading in the classical sense (polymorphic behavior according to type of passed parameters), so the "visit" methods for the different model types need to have different names.

==== 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>

==== Output ====
<syntaxhighlight lang="output">
Visiting front left wheel.
Visiting front right wheel.
Visiting back left wheel.
Visiting back right wheel.
Visiting body.
Visiting engine.
Visiting car.
Kicking my front left wheel.
Kicking my front right wheel.
Kicking my back left wheel.
Kicking my back right wheel.
Moving my body.
Starting my engine.
Starting my car.
</syntaxhighlight>

==== Abstraction ====
Using Python 3 or above allows to make a general implementation of the accept method:

<syntaxhighlight lang="python">
class Visitable:
    def accept(self, visitor):
        lookup = "visit_" + self.__qualname__.replace(".", "_")
        return getattr(visitor, lookup)(self)
</syntaxhighlight>

One could extend this to iterate over the class's method resolution order if they would like to fall back on already-implemented classes. They could also use the subclass hook feature to define the lookup in advance.