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dictionary_switches / src / Visitor.rst

Visitor

The visitor pattern is implemented using multiple dispatching, but people often confuse the two, because they look at the implementation rather than the intent.

The assumption is that you have a primary class hierarchy that is fixed; perhaps it's from another vendor and you can't make changes to that hierarchy. However, your intent is that you'd like to add new polymorphic methods to that hierarchy, which means that normally you'd have to add something to the base class interface. So the dilemma is that you need to add methods to the base class, but you can't touch the base class. How do you get around this?

The design pattern that solves this kind of problem is called a "visitor" (the final one in the Design Patterns book), and it builds on the double dispatching scheme shown in the last section.

The visitor pattern allows you to extend the interface of the primary type by creating a separate class hierarchy of type Visitor to virtualize the operations performed upon the primary type. The objects of the primary type simply "accept" the visitor, then call the visitor's dynamically-bound member function:

# visitor/FlowerVisitors.py
# Demonstration of "visitor" pattern.
from __future__ import generators
import random

# The Flower hierarchy cannot be changed:
class Flower(object):
    def accept(self, visitor):
        visitor.visit(self)
    def pollinate(self, pollinator):
        print(self, "pollinated by", pollinator)
    def eat(self, eater):
        print(self, "eaten by", eater)
    def __str__(self):
        return self.__class__.__name__

class Gladiolus(Flower): pass
class Runuculus(Flower): pass
class Chrysanthemum(Flower): pass

class Visitor:
    def __str__(self):
        return self.__class__.__name__

class Bug(Visitor): pass
class Pollinator(Bug): pass
class Predator(Bug): pass

# Add the ability to do "Bee" activities:
class Bee(Pollinator):
    def visit(self, flower):
        flower.pollinate(self)

# Add the ability to do "Fly" activities:
class Fly(Pollinator):
    def visit(self, flower):
        flower.pollinate(self)

# Add the ability to do "Worm" activities:
class Worm(Predator):
    def visit(self, flower):
        flower.eat(self)

def flowerGen(n):
    flwrs = Flower.__subclasses__()
    for i in range(n):
        yield random.choice(flwrs)()

# It's almost as if I had a method to Perform
# various "Bug" operations on all Flowers:
bee = Bee()
fly = Fly()
worm = Worm()
for flower in flowerGen(10):
    flower.accept(bee)
    flower.accept(fly)
    flower.accept(worm)

Exercises

  1. Create a business-modeling environment with three types of Inhabitant: Dwarf (for engineers), Elf (for marketers) and Troll (for managers). Now create a class called Project that creates the different inhabitants and causes them to interact( ) with each other using multiple dispatching.
  2. Modify the above example to make the interactions more detailed. Each Inhabitant can randomly produce a Weapon using getWeapon( ): a Dwarf uses Jargon or Play, an Elf uses InventFeature or SellImaginaryProduct, and a Troll uses Edict and Schedule. You must decide which weapons "win" and "lose" in each interaction (as in PaperScissorsRock.py). Add a battle( ) member function to Project that takes two Inhabitants and matches them against each other. Now create a meeting( ) member function for Project that creates groups of Dwarf, Elf and Manager and battles the groups against each other until only members of one group are left standing. These are the "winners."
  3. Modify PaperScissorsRock.py to replace the double dispatching with a table lookup. The easiest way to do this is to create a Map of Maps, with the key of each Map the class of each object. Then you can do the lookup by saying: ((Map)map.get(o1.getClass())).get(o2.getClass()) Notice how much easier it is to reconfigure the system. When is it more appropriate to use this approach vs. hard-coding the dynamic dispatches? Can you create a system that has the syntactic simplicity of use of the dynamic dispatch but uses a table lookup?
  4. Modify Exercise 2 to use the table lookup technique described in Exercise 3.
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