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Decorator: Dynamic Type Selection
********************************************************************************

..  note:: I think we can rewrite this chapter to use Python decorators as
           implementation (thus the decorators chapter should precede this one).

The use of layered objects to dynamically and transparently add responsibilities
to individual objects is referred to as the *decorator* pattern.

Used when subclassing creates too many (& inflexible) classes

All decorators that wrap around the original object must have the same basic
interface

Dynamic proxy/surrogate?

This accounts for the odd inheritance structure

Tradeoff: coding is more complicated when using decorators

Basic Decorator Structure
=======================================================================

.. image:: _images/decorator.*


A Coffee Example
=======================================================================

Consider going down to the local coffee shop, *BeanMeUp*, for a coffee.  There
are typically many different drinks on offer -- espressos, lattes, teas, iced
coffees, hot chocolate to name a few, as well as a number of extras (which cost
extra too) such as whipped cream or an extra shot of espresso. You can also make
certain changes to your drink at no extra cost, such as asking for decaf coffee
instead of regular coffee.

Quite clearly if we are going to model all these drinks and combinations, there
will be sizeable class diagrams. So for clarity we will only consider a subset
of the coffees: Espresso, Espresso Con Panna, Café Late, Cappuccino and Café
Mocha. We'll include 2 extras - whipped cream ("whipped") and an extra shot of
espresso; and three changes - decaf, steamed milk ("wet") and foamed milk
("dry").

Class for Each Combination
=======================================================================

One solution is to create an individual class for every combination. Each class
describes the drink and is responsible for the cost etc. The resulting menu is
huge, and a part of the class diagram would look something like this:

.. image:: _images/coffeeExplosion.*

The key to using this method is to find the particular combination you want.
So, once you've found the drink you would like, here is how you would use it, as
shown in the **CoffeeShop** class in the following code::

    # Decorator/nodecorators/CoffeeShop.py
    # Coffee example with no decorators

    class Espresso: pass
    class DoubleEspresso: pass
    class EspressoConPanna: pass

    class Cappuccino:
        def __init__(self):
            self.cost = 1
            self.description = "Cappucino"
        def getCost(self):
            return self.cost
        def getDescription(self):
            return self.description

    class CappuccinoDecaf: pass
    class CappuccinoDecafWhipped: pass
    class CappuccinoDry: pass
    class CappuccinoDryWhipped: pass
    class CappuccinoExtraEspresso: pass
    class CappuccinoExtraEspressoWhipped: pass
    class CappuccinoWhipped: pass

    class CafeMocha: pass
    class CafeMochaDecaf: pass
    class CafeMochaDecafWhipped:
        def __init__(self):
            self.cost = 1.25
            self.description = \
              "Cafe Mocha decaf whipped cream"
        def getCost(self):
            return self.cost
        def getDescription(self):
            return self.description

    class CafeMochaExtraEspresso: pass
    class CafeMochaExtraEspressoWhipped: pass
    class CafeMochaWet: pass
    class CafeMochaWetWhipped: pass
    class CafeMochaWhipped: pass

    class CafeLatte: pass
    class CafeLatteDecaf: pass
    class CafeLatteDecafWhipped: pass
    class CafeLatteExtraEspresso: pass
    class CafeLatteExtraEspressoWhipped: pass
    class CafeLatteWet: pass
    class CafeLatteWetWhipped: pass
    class CafeLatteWhipped: pass

    cappuccino = Cappuccino()
    print((cappuccino.getDescription() + ": $" +
      `cappuccino.getCost()`))

    cafeMocha = CafeMochaDecafWhipped()
    print((cafeMocha.getDescription()
      + ": $" + `cafeMocha.getCost()`))



And here is the corresponding output::

    Cappucino: $1.0
    Cafe Mocha decaf whipped cream: $1.25

You can see that creating the particular combination you want is easy, since you
are just creating an instance of a class. However, there are a number of
problems with this approach. Firstly, the combinations are fixed statically so
that any combination a customer may wish to order needs to be created up front.
Secondly, the resulting menu is so huge that finding your particular combination
is difficult and time consuming.

The Decorator Approach
=======================================================================

Another approach would be to break the drinks down into the various components
such as espresso and foamed milk, and then let the customer combine the
components to describe a particular coffee.

In order to do this programmatically, we use the Decorator pattern.  A Decorator
adds responsibility to a component by wrapping it, but the Decorator conforms to
the interface of the component it encloses, so the wrapping is transparent.
Decorators can also be nested without the loss of this transparency.

.. image:: _images/decoratedCoffee.*


Methods invoked on the Decorator can in turn invoke methods in the component,
and can of course perform processing before or after the invocation.

So if we added **getTotalCost()** and **getDescription()** methods to the
**DrinkComponent** interface, an Espresso looks like this::

    # Decorator/alldecorators/EspressoDecorator.py

    class Espresso(Decorator):
        cost = 0.75f
        description = " espresso"
        def __init__(DrinkComponent):
            Decorator.__init__(self, component)

        def getTotalCost(self):
            return self.component.getTotalCost() + cost

        def getDescription(self):
            return self.component.getDescription() +
                description


You combine the components to create a drink as follows, as shown in the code
below::

    # Decorator/alldecorators/CoffeeShop.py
    # Coffee example using decorators

    class DrinkComponent:
        def getDescription(self):
            return self.__class__.__name__
        def getTotalCost(self):
            return self.__class__.cost

    class Mug(DrinkComponent):
        cost = 0.0

    class Decorator(DrinkComponent):
        def __init__(self, drinkComponent):
            self.component = drinkComponent
        def getTotalCost(self):
            return self.component.getTotalCost() + \
              DrinkComponent.getTotalCost(self)
        def getDescription(self):
            return self.component.getDescription() + \
              ' ' + DrinkComponent.getDescription(self)

    class Espresso(Decorator):
        cost = 0.75
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Decaf(Decorator):
        cost = 0.0
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class FoamedMilk(Decorator):
        cost = 0.25
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class SteamedMilk(Decorator):
        cost = 0.25
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Whipped(Decorator):
        cost = 0.25
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Chocolate(Decorator):
        cost = 0.25
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    cappuccino = Espresso(FoamedMilk(Mug()))
    print(cappuccino.getDescription().strip() + \)
      ": $" + `cappuccino.getTotalCost()`

    cafeMocha = Espresso(SteamedMilk(Chocolate(
      Whipped(Decaf(Mug())))))

    print(cafeMocha.getDescription().strip() + \)
      ": $" + `cafeMocha.getTotalCost()`



This approach would certainly provide the most flexibility and the smallest
menu. You have a small number of components to choose from, but assembling the
description of the coffee then becomes rather arduous.

If you want to describe a plain cappuccino, you create it with::

    plainCap = Espresso(FoamedMilk(Mug()))

Creating a decaf Cafe Mocha with whipped cream requires an even longer
description.

Compromise
=======================================================================

The previous approach takes too long to describe a coffee. There will also be
certain combinations that you will describe regularly, and it would be
convenient to have a quick way of describing them.

The 3rd approach is a mixture of the first 2 approaches, and combines
flexibility with ease of use. This compromise is achieved by creating a
reasonably sized menu of basic selections, which would often work exactly as
they are, but if you wanted to decorate them (whipped cream, decaf etc.) then
you would use decorators to make the modifications. This is the type of menu you
are presented with in most coffee shops.

.. image:: _images/compromiseDecoration.*

Here is how to create a basic selection, as well as a decorated selection::

    # Decorator/compromise/CoffeeShop.py
    # Coffee example with a compromise of basic
    # combinations and decorators

    class DrinkComponent:
        def getDescription(self):
            return self.__class__.__name__
        def getTotalCost(self):
            return self.__class__.cost

    class Espresso(DrinkComponent):
        cost = 0.75

    class EspressoConPanna(DrinkComponent):
        cost = 1.0

    class Cappuccino(DrinkComponent):
        cost = 1.0

    class CafeLatte(DrinkComponent):
        cost = 1.0

    class CafeMocha(DrinkComponent):
        cost = 1.25

    class Decorator(DrinkComponent):
        def __init__(self, drinkComponent):
            self.component = drinkComponent
        def getTotalCost(self):
            return self.component.getTotalCost() + \
              DrinkComponent.getTotalCost(self)
        def getDescription(self):
            return self.component.getDescription() + \
              ' ' + DrinkComponent.getDescription(self)

    class ExtraEspresso(Decorator):
        cost = 0.75
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Whipped(Decorator):
        cost = 0.50
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Decaf(Decorator):
        cost = 0.0
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Dry(Decorator):
        cost = 0.0
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    class Wet(Decorator):
        cost = 0.0
        def __init__(self, drinkComponent):
            Decorator.__init__(self, drinkComponent)

    cappuccino = Cappuccino()
    print(cappuccino.getDescription() + ": $" + \)
      `cappuccino.getTotalCost()`

    cafeMocha = Whipped(Decaf(CafeMocha()))
    print(cafeMocha.getDescription() + ": $" + \)
      `cafeMocha.getTotalCost()`



You can see that creating a basic selection is quick and easy, which makes sense
since they will be described regularly.  Describing a decorated drink is more
work than when using a class per combination, but clearly less work than when
only using decorators.

The final result is not too many classes, but not too many decorators either.
Most of the time it's possible to get away without using any decorators at all,
so we have the benefits of both approaches.

Other Considerations
=======================================================================

What happens if we decide to change the menu at a later stage, such as by adding
a new type of drink? If we had used the class per combination approach, the
effect of adding an extra such as syrup would be an exponential growth in the
number of classes. However, the implications to the all decorator or compromise
approaches are the same - one extra class is created.

How about the effect of changing the cost of steamed milk and foamed milk, when
the price of milk goes up? Having a class for each combination means that you
need to change a method in each class, and thus maintain many classes. By using
decorators, maintenance is reduced by defining the logic in one place.

Further Reading
=======================================================================

    Philip Eby introduces decorators: http://www.ddj.com/web-development/184406073

    Class Decorators:
    	  - http://www.informit.com/articles/article.aspx?p=1309289&seqNum=4

Exercises
=======================================================================

#.  Add a Syrup class to the decorator approach described above. Then create a
    Café Latte (you'll need to use steamed milk with an espresso) with syrup.

#.  Repeat Exercise 1 for the compromise approach.

#.  Implement the decorator pattern to create a Pizza restaurant, which has a
    set menu of choices as well as the option to design your own pizza.  Follow
    the compromise approach to create a menu consisting of a Margherita,
    Hawaiian, Regina, and Vegetarian pizzas, with toppings (decorators) of
    Garlic, Olives, Spinach, Avocado, Feta and Pepperdews. Create a Hawaiian
    pizza, as well as a Margherita decorated with Spinach, Feta, Pepperdews and
    Olives.