Factory in python

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Factory in python

  1. 1. Factory in Python
  2. 2. What is that crap Define an interface for creating an object, but let the subclasses decide which class to instantiate. The Factory method lets a class defer instantiation to subclasses.
  3. 3. A Simple Diagram
  4. 4. Different Factory Types x •Static x •Polymorphic x •Abstract
  5. 5. Static Factory • Forces all the creation operations to be focused in one spot Static Factory: Shape Circle Square Product
  6. 6. class Shape(object): def factory(type): if type == "Circle" : return Circle() if type == "Square" : return Square() assert 1, "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" def shape_name_gen(n): types = Shape.__subclasses__() for i in range(n): yield random.choice(types).__name__ shapes = [Shape.factory(i) for i in shape_name_gen(7)] for shape in shapes: shape.draw() shape.erase()
  7. 7. Polymorphic Factory • Make a single superclass version of the method that calls a Factory Method to handle the instantiation • The new class can be dynamically added to the factory • Factory methods are in separate class as virtual function • Different types of factories can be subclassed from the basic factory
  8. 8. class ShapeFactory: factories = {} def add_factory(id, shapefactory): ShapeFactory.factories.put[id] = shapeFactory add_factory = staticmethod(add_factory) def create_shape(id): if not ShapeFactory.factories.has_key(id): ShapeFactory.factories[id] = eval(id+'.Factory()') return ShapeFactory.factories[id].create() create_shape = staticmethod(create_shape) class Shape(object): pass class Circle(Shape): def shape_name_gen(n): def draw(self): print "Circle.draw" types = Shape.__subclasses__() def erase(self): print "Circle.erase" for i in range(n): class Factory: yield random.choice(types).__name__ def create(self): return Circle() shapes = [ShapeFactory.create_shape(i) class Square(Shape): for i in shape_name_gen(7)] def draw(self): print "Square.draw" def erase(self): print "Square.erase" for shape in shapes: class Factory: shape.draw() def create(self): return Square() shape.erase()
  9. 9. Abstract Factory • The client creates a concrete implementation of the abstract factory and then uses the generic interface to create 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.
  10. 10. class Obstacle: pass class GameElementFactory: pass class Player: pass class JavAndPuzzle(GameElementFactory): class Jav(Player): def make_player(self): return Jav() def interact_with(self, obstacle): def make_obstacle(self): return Puzzle() print("Jav is playing a" ) obstacle.action() class BenAndWeapon(GameElementFactory): class Ben(Player): def make_player(self): return Ben() def interact_with(self, obstacle): def make_obstacle(self): return Weapon() print("Ben is playing a " ) class GameEnvironment: obstacle.action() def __init__(self, factory): self.factory = factory class Puzzle(Obstacle): self.p = factory.make_player() def action(self): self.ob = factory.make_obstacle() print("Puzzle") def play(self): class Weapon(Obstacle): self.p.interact_with(self.ob) def action(self): print("Weapon") g1 = GameEnvironment(JavAndPuzzle()) g2 = GameEnvironment(BenAndWeapon()) g1.play() g2.play()
  11. 11. Limitations of Factory • The first limitation is that refactoring an existing class to use factories breaks existing clients.

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