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Factory: Encapsulating Object Creation

When you discover that you need to add new types to a system, the most sensible first step is to use polymorphism to create a common interface to those new types. This separates the rest of the code in your system from the knowledge of the specific types that you are adding. New types may be added without disturbing existing code ... or so it seems. At first it would appear that the only place you need to change the code in such a design is the place where you inherit a new type, but this is not quite true. You must still create an object of your new type, and at the point of creation you must specify the exact constructor to use. Thus, if the code that creates objects is distributed throughout your application, you have the same problem when adding new types-you must still chase down all the points of your code where type matters. It happens to be the creation of the type that matters in this case rather than the use of the type (which is taken care of by polymorphism), but the effect is the same: adding a new type can cause problems.

The solution is to force the creation of objects to occur through a common factory rather than to allow the creational code to be spread throughout your system. If all the code in your program must go through this factory whenever it needs to create one of your objects, then all you must do when you add a new object is to modify the factory.

Since every object-oriented program creates objects, and since it's very likely you will extend your program by adding new types, I suspect that factories may be the most universally useful kinds of design patterns.

Simple Factory Method

As an example, let's revisit the Shape system.

One approach is to make the factory a static method of the base class:

# Factory/shapefact1/ShapeFactory1.py
# A simple static factory method.
from __future__ import generators
import random

class Shape(object):
    # Create based on class name:
    def factory(type):
        #return eval(type + "()")
        if type == "Circle": return Circle()
        if type == "Square": return Square()
        assert 0, "Bad shape creation: " + type
    factory = staticmethod(factory)

class Circle(Shape):
    def draw(self): print("Circle.draw")
    def erase(self): print("Circle.erase")

class Square(Shape):
    def draw(self): print("Square.draw")
    def erase(self): print("Square.erase")

# Generate shape name strings:
def shapeNameGen(n):
    types = Shape.__subclasses__()
    for i in range(n):
        yield random.choice(types).__name__

shapes = \
  [ Shape.factory(i) for i in shapeNameGen(7)]

for shape in shapes:
    shape.draw()
    shape.erase()

The factory( ) takes an argument that allows it to determine what type of Shape to create; it happens to be a String in this case but it could be any set of data. The factory( ) is now the only other code in the system that needs to be changed when a new type of Shape is added (the initialization data for the objects will presumably come from somewhere outside the system, and not be a hard-coded array as in the above example).

Note that this example also shows the new Python 2.2 staticmethod( ) technique for creating static methods in a class.

I have also used a tool which is new in Python 2.2 called a generator. A generator is a special case of a factory: it's a factory that takes no arguments in order to create a new object. Normally you hand some information to a factory in order to tell it what kind of object to create and how to create it, but a generator has some kind of internal algorithm that tells it what and how to build. It "generates out of thin air" rather than being told what to create.

Now, this may not look consistent with the code you see above:

for i in shapeNameGen(7)

looks like there's an initialization taking place. This is where a generator is a bit strange - when you call a function that contains a yield statement (yield is a new keyword that determines that a function is a generator), that function actually returns a generator object that has an iterator. This iterator is implicitly used in the for statement above, so it appears that you are iterating through the generator function, not what it returns. This was done for convenience of use.

Thus, the code that you write is actually a kind of factory, that creates the generator objects that do the actual generation. You can use the generator explicitly if you want, for example:

gen = shapeNameGen(7)
print(gen.next())

So next( ) is the iterator method that's actually called to generate the next object, and it takes no arguments. shapeNameGen( ) is the factory, and gen is the generator.

Inside the generator-factory, you can see the call to __subclasses__( ), which produces a list of references to each of the subclasses of Shape (which must be inherited from object for this to work). You should be aware, however, that this only works for the first level of inheritance from Item, so if you were to inherit a new class from Circle, it wouldn't show up in the list generated by __subclasses__( ). If you need to create a deeper hierarchy this way, you must recurse the __subclasses__( ) list.

Also note that in shapeNameGen( ) the statement:

types = Shape.__subclasses__()

Is only executed when the generator object is produced; each time the next( ) method of this generator object is called (which, as noted above, may happen implicitly), only the code in the for loop will be executed, so you don't have wasteful execution (as you would if this were an ordinary function).

Preventing direct creation

To disallow direct access to the classes, you can nest the classes within the factory method, like this:

# Factory/shapefact1/NestedShapeFactory.py
import random

class Shape(object):
    types = []

def factory(type):
    class Circle(Shape):
        def draw(self): print("Circle.draw")
        def erase(self): print("Circle.erase")

    class Square(Shape):
        def draw(self): print("Square.draw")
        def erase(self): print("Square.erase")

    if type == "Circle": return Circle()
    if type == "Square": return Square()
    assert 0, "Bad shape creation: " + type

def shapeNameGen(n):
    for i in range(n):
        yield factory(random.choice(["Circle", "Square"]))

# Circle() # Not defined

for shape in shapeNameGen(7):
    shape.draw()
    shape.erase()

Polymorphic Factories

The static factory( ) method in the previous example forces all the creation operations to be focused in one spot, so that's the only place you need to change the code. This is certainly a reasonable solution, as it throws a box around the process of creating objects. However, the Design Patterns book emphasizes that the reason for the Factory Method pattern is so that different types of factories can be subclassed from the basic factory (the above design is mentioned as a special case). However, the book does not provide an example, but instead just repeats the example used for the Abstract Factory (you'll see an example of this in the next section). Here is ShapeFactory1.py modified so the factory methods are in a separate class as virtual functions. Notice also that the specific Shape classes are dynamically loaded on demand:

# Factory/shapefact2/ShapeFactory2.py
# Polymorphic factory methods.
from __future__ import generators
import random

class ShapeFactory:
    factories = {}
    def addFactory(id, shapeFactory):
        ShapeFactory.factories.put[id] = shapeFactory
    addFactory = staticmethod(addFactory)
    # A Template Method:
    def createShape(id):
        if not ShapeFactory.factories.has_key(id):
            ShapeFactory.factories[id] = \
              eval(id + '.Factory()')
        return ShapeFactory.factories[id].create()
    createShape = staticmethod(createShape)

class Shape(object): pass

class Circle(Shape):
    def draw(self): print("Circle.draw")
    def erase(self): print("Circle.erase")
    class Factory:
        def create(self): return Circle()

class Square(Shape):
    def draw(self):
        print("Square.draw")
    def erase(self):
        print("Square.erase")
    class Factory:
        def create(self): return Square()

def shapeNameGen(n):
    types = Shape.__subclasses__()
    for i in range(n):
        yield random.choice(types).__name__

shapes = [ ShapeFactory.createShape(i)
           for i in shapeNameGen(7)]

for shape in shapes:
    shape.draw()
    shape.erase()

Now the factory method appears in its own class, ShapeFactory, as the create( ) method. The different types of shapes must each create their own factory with a create( ) method to create an object of their own type. The actual creation of shapes is performed by calling ShapeFactory.createShape( ), which is a static method that uses the dictionary in ShapeFactory to find the appropriate factory object based on an identifier that you pass it. The factory is immediately used to create the shape object, but you could imagine a more complex problem where the appropriate factory object is returned and then used by the caller to create an object in a more sophisticated way. However, it seems that much of the time you don't need the intricacies of the polymorphic factory method, and a single static method in the base class (as shown in ShapeFactory1.py) will work fine.

Notice that the ShapeFactory must be initialized by loading its dictionary with factory objects, which takes place in the static initialization clause of each of the shape implementations.

Abstract Factories

The Abstract Factory pattern looks like the factory objects we've seen previously, with not one but several factory methods. Each of the factory methods creates a different kind of object. The idea is that at the point of creation of the factory object, you decide how all the objects created by that factory will be used. The example given in Design Patterns implements portability across various graphical user interfaces (GUIs): you create a factory object appropriate to the GUI that you're working with, and from then on when you ask it for a menu, button, slider, etc. it will automatically create the appropriate version of that item for the GUI. Thus you're able to isolate, in one place, the effect of changing from one GUI to another.

As another example suppose you are creating a general-purpose gaming environment and you want to be able to support different types of games. Here's how it might look using an abstract factory:

# Factory/Games.py
# An example of the Abstract Factory pattern.

class Obstacle:
    def action(self): pass

class Character:
    def interactWith(self, obstacle): pass

class Kitty(Character):
    def interactWith(self, obstacle):
        print("Kitty has encountered a",
        obstacle.action())

class KungFuGuy(Character):
    def interactWith(self, obstacle):
        print("KungFuGuy now battles a",
        obstacle.action())

class Puzzle(Obstacle):
    def action(self):
        print("Puzzle")

class NastyWeapon(Obstacle):
    def action(self):
        print("NastyWeapon")

# The Abstract Factory:
class GameElementFactory:
    def makeCharacter(self): pass
    def makeObstacle(self): pass

# Concrete factories:
class KittiesAndPuzzles(GameElementFactory):
    def makeCharacter(self): return Kitty()
    def makeObstacle(self): return Puzzle()

class KillAndDismember(GameElementFactory):
    def makeCharacter(self): return KungFuGuy()
    def makeObstacle(self): return NastyWeapon()

class GameEnvironment:
    def __init__(self, factory):
        self.factory = factory
        self.p = factory.makeCharacter()
        self.ob = factory.makeObstacle()
    def play(self):
        self.p.interactWith(self.ob)

g1 = GameEnvironment(KittiesAndPuzzles())
g2 = GameEnvironment(KillAndDismember())
g1.play()
g2.play()

In this environment, Character objects interact with Obstacle objects, but there are different types of Characters and obstacles depending on what kind of game you're playing. You determine the kind of game by choosing a particular GameElementFactory, and then the GameEnvironment controls the setup and play of the game. In this example, the setup and play is very simple, but those activities (the initial conditions and the state change) can determine much of the game's outcome. Here, GameEnvironment is not designed to be inherited, although it could very possibly make sense to do that.

This also contains examples of Double Dispatching and the Factory Method, both of which will be explained later.

Of course, the above scaffolding of Obstacle, Character and GameElementFactory (which was translated from the Java version of this example) is unnecessary - it's only required for languages that have static type checking. As long as the concrete Python classes follow the form of the required classes, we don't need any base classes:

# Factory/Games2.py
# Simplified Abstract Factory.

class Kitty:
    def interactWith(self, obstacle):
        print("Kitty has encountered a",
        obstacle.action())

class KungFuGuy:
    def interactWith(self, obstacle):
        print("KungFuGuy now battles a",
        obstacle.action())

class Puzzle:
    def action(self): print("Puzzle")

class NastyWeapon:
    def action(self): print("NastyWeapon")

# Concrete factories:
class KittiesAndPuzzles:
    def makeCharacter(self): return Kitty()
    def makeObstacle(self): return Puzzle()

class KillAndDismember:
    def makeCharacter(self): return KungFuGuy()
    def makeObstacle(self): return NastyWeapon()

class GameEnvironment:
    def __init__(self, factory):
        self.factory = factory
        self.p = factory.makeCharacter()
        self.ob = factory.makeObstacle()
    def play(self):
        self.p.interactWith(self.ob)

g1 = GameEnvironment(KittiesAndPuzzles())
g2 = GameEnvironment(KillAndDismember())
g1.play()
g2.play()

Another way to put this is that all inheritance in Python is implementation inheritance; since Python does its type-checking at runtime, there's no need to use interface inheritance so that you can upcast to the base type.

You might want to study the two examples for comparison, however. Does the first one add enough useful information about the pattern that it's worth keeping some aspect of it? Perhaps all you need is "tagging classes" like this:

class Obstacle: pass
class Character: pass
class GameElementFactory: pass

Then the inheritance serves only to indicate the type of the derived classes.

Exercises

  1. Add a class Triangle to ShapeFactory1.py
  2. Add a class Triangle to ShapeFactory2.py
  3. Add a new type of GameEnvironment called GnomesAndFairies to GameEnvironment.py
  4. Modify ShapeFactory2.py so that it uses an Abstract Factory to create different sets of shapes (for example, one particular type of factory object creates "thick shapes," another creates "thin shapes," but each factory object can create all the shapes: circles, squares, triangles etc.).