sect06.htm

this is the most basic to learn python

  public Espresso(DrinkComponent):
    Decorator.__init__(self, component)

  <font color=#0000ff>def</font> getTotalCost(self):
    <font color=#0000ff>return</font> self.component.getTotalCost() + cost

  <font color=#0000ff>def</font> getDescription(self):
    <font color=#0000ff>return</font> self.component.getDescription() +
      description
</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You combine the components to
create a drink as follows, as shown in the code below:
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_219">Add Comment</A></FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: cX:decorator:alldecorators:CoffeeShop.py
# Coffee example using decorators

<font color=#0000ff>class</font> DrinkComponent:
  <font color=#0000ff>def</font> getDescription(self):
    <font color=#0000ff>return</font> self.__class__.__name__
  <font color=#0000ff>def</font> getTotalCost(self):
    <font color=#0000ff>return</font> self.__class__.cost

<font color=#0000ff>class</font> Mug(DrinkComponent):
  cost = 0.0

<font color=#0000ff>class</font> Decorator(DrinkComponent):
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    self.component = drinkComponent
  <font color=#0000ff>def</font> getTotalCost(self):
    <font color=#0000ff>return</font> self.component.getTotalCost() + \
      DrinkComponent.getTotalCost(self)
  <font color=#0000ff>def</font> getDescription(self):
    <font color=#0000ff>return</font> self.component.getDescription() + \
      ' ' + DrinkComponent.getDescription(self)

<font color=#0000ff>class</font> Espresso(Decorator):
  cost = 0.75
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Decaf(Decorator):
  cost = 0.0
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> FoamedMilk(Decorator):
  cost = 0.25
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> SteamedMilk(Decorator):
  cost = 0.25
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Whipped(Decorator):
  cost = 0.25
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Chocolate(Decorator):
  cost = 0.25
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

cappuccino = Espresso(FoamedMilk(Mug()))
<font color=#0000ff>print</font> cappuccino.getDescription().strip() + \
  <font color=#004488>": $"</font> + &#180;cappuccino.getTotalCost()&#180;

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

<font color=#0000ff>print</font> cafeMocha.getDescription().strip() + \
  <font color=#004488>": $"</font> + &#180;cafeMocha.getTotalCost()&#180;
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_220">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">If you want to describe a plain
cappuccino, you create it with</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>plainCap = Espresso(FoamedMilk(Mug()))</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Creating
a decaf Caf&eacute; Mocha with whipped cream requires an even longer
description.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_221">Add Comment</A></FONT><A NAME="_Toc534420102"></A><BR></P></DIV>
<A NAME="Heading53"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Compromise</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_222">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_223">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="CENTER"><FONT FACE="Georgia"><IMG SRC="TIPyth05.gif"></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Here is how to create a basic selection,
as well as a decorated selection:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: cX:decorator:compromise:CoffeeShop.py
# Coffee example with a compromise of basic
# combinations <font color=#0000ff>and</font> decorators

<font color=#0000ff>class</font> DrinkComponent:
  <font color=#0000ff>def</font> getDescription(self):
    <font color=#0000ff>return</font> self.__class__.__name__
  <font color=#0000ff>def</font> getTotalCost(self):
    <font color=#0000ff>return</font> self.__class__.cost

<font color=#0000ff>class</font> Espresso(DrinkComponent):
  cost = 0.75

<font color=#0000ff>class</font> EspressoConPanna(DrinkComponent):
  cost = 1.0

<font color=#0000ff>class</font> Cappuccino(DrinkComponent):
  cost = 1.0

<font color=#0000ff>class</font> CafeLatte(DrinkComponent):
  cost = 1.0

<font color=#0000ff>class</font> CafeMocha(DrinkComponent):
  cost = 1.25

<font color=#0000ff>class</font> Decorator(DrinkComponent):
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    self.component = drinkComponent
  <font color=#0000ff>def</font> getTotalCost(self):
    <font color=#0000ff>return</font> self.component.getTotalCost() + \
      DrinkComponent.getTotalCost(self)
  <font color=#0000ff>def</font> getDescription(self):
    <font color=#0000ff>return</font> self.component.getDescription() + \
      ' ' + DrinkComponent.getDescription(self)

<font color=#0000ff>class</font> ExtraEspresso(Decorator):
  cost = 0.75
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Whipped(Decorator):
  cost = 0.50
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Decaf(Decorator):
  cost = 0.0
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Dry(Decorator):
  cost = 0.0
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

<font color=#0000ff>class</font> Wet(Decorator):
  cost = 0.0
  <font color=#0000ff>def</font> __init__(self, drinkComponent):
    Decorator.__init__(self, drinkComponent)

cappuccino = Cappuccino()
<font color=#0000ff>print</font> cappuccino.getDescription() + <font color=#004488>": $"</font> + \
  &#180;cappuccino.getTotalCost()&#180;

cafeMocha = Whipped(Decaf(CafeMocha()))
<font color=#0000ff>print</font> cafeMocha.getDescription() + <font color=#004488>": $"</font> + \
  &#180;cafeMocha.getTotalCost()&#180;
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_224">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_225">Add Comment</A></FONT><A NAME="_Toc534420103"></A><BR></P></DIV>
<A NAME="Heading54"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Other considerations</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_226">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_227">Add Comment</A></FONT><A NAME="_Toc534420104"></A><BR></P></DIV>
<A NAME="Heading55"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Exercises</H2></FONT>
<OL>
<LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Add a Syrup class to the
decorator approach described above. Then create a Caf&eacute; Latte (you'll need
to use steamed milk with an espresso) with syrup.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_228">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Repeat
Exercise 1 for the compromise approach.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_229">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">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.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_230">Add Comment</A></FONT></OL>
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 Last Update:12/31/2001</P></DIV>

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