sect13.htm

this is the most basic to learn python

# Multiple dispatching using a table
<font color=#0000ff>from</font> __future__ <font color=#0000ff>import</font> generators
<font color=#0000ff>import</font> random

<font color=#0000ff>class</font> Outcome:
  <font color=#0000ff>def</font> __init__(self, value, name): 
    self.value = value
    self.name = name
  <font color=#0000ff>def</font> __str__(self): <font color=#0000ff>return</font> self.name 
  <font color=#0000ff>def</font> __eq__(self, other):
      <font color=#0000ff>return</font> self.value == other.value

Outcome.WIN = Outcome(0, <font color=#004488>"win"</font>)
Outcome.LOSE = Outcome(1, <font color=#004488>"lose"</font>)
Outcome.DRAW = Outcome(2, <font color=#004488>"draw"</font>)

<font color=#0000ff>class</font> Item(object):
  <font color=#0000ff>def</font> compete(self, item):
    # Use a tuple <font color=#0000ff>for</font> table lookup:
    <font color=#0000ff>return</font> outcome[self.__class__, item.__class__]
  <font color=#0000ff>def</font> __str__(self): 
    <font color=#0000ff>return</font> self.__class__.__name__ 

<font color=#0000ff>class</font> Paper(Item): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Scissors(Item): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Rock(Item): <font color=#0000ff>pass</font>

outcome = {
  (Paper, Rock): Outcome.WIN,
  (Paper, Scissors): Outcome.LOSE,
  (Paper, Paper): Outcome.DRAW,
  (Scissors, Paper): Outcome.WIN,
  (Scissors, Rock): Outcome.LOSE,
  (Scissors, Scissors): Outcome.DRAW,
  (Rock, Scissors): Outcome.WIN,
  (Rock, Paper): Outcome.LOSE,
  (Rock, Rock): Outcome.DRAW,
}

<font color=#0000ff>def</font> match(item1, item2):
  <font color=#0000ff>print</font> <font color=#004488>"%s &lt;--&gt; %s : %s"</font> % (
    item1, item2, item1.compete(item2))

# Generate the items:
<font color=#0000ff>def</font> itemPairGen(n):
  # Create a list of instances of all Items:
  Items = Item.__subclasses__()
  <font color=#0000ff>for</font> i <font color=#0000ff>in</font> range(n):
    <font color=#0000ff>yield</font> (random.choice(Items)(), 
           random.choice(Items)())

<font color=#0000ff>for</font> item1, item2 <font color=#0000ff>in</font> itemPairGen(20):
  match(item1, item2)
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">It&#146;s a tribute to the
flexibility of dictionaries that a tuple can be used as a key just as easily as
a single object.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_327">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><A NAME="_Toc462393597"></A><A NAME="_Toc476705917"></A><A NAME="_Toc534420129"></A><BR></P></DIV>
<A NAME="Heading80"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Visitor, a type of multiple dispatching</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The assumption is that you have a primary
class hierarchy that is fixed; perhaps it&#146;s from another vendor and you
can&#146;t make changes to that hierarchy. However, you&#146;d like to add new
polymorphic methods to that hierarchy, which means that normally you&#146;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&#146;t touch the base class.
How do you get around this?
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_328">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The design pattern that solves this kind
of problem is called a &#147;visitor&#148; (the final one in the <I>Design
Patterns</I> book), and it builds on the double<I> </I>dispatching scheme shown
in the last section.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_329">Add Comment</A></FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The
<A NAME="Index31"></A><A NAME="Index32"></A>visitor pattern allows you to extend
the interface of the primary type by creating a separate class hierarchy of type
<B>Visitor </B>to virtualize the operations performed upon the primary type. The
objects of the primary type simply &#147;accept&#148; the visitor, then call
the visitor&#146;s dynamically<B>-</B>bound member function.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_330">Add Comment</A></FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>#: c11:FlowerVisitors.py
# Demonstration of <font color=#004488>"visitor"</font> pattern.
<font color=#0000ff>from</font> __future__ <font color=#0000ff>import</font> generators
<font color=#0000ff>import</font> random

# The Flower hierarchy cannot be changed:
<font color=#0000ff>class</font> Flower(object):  
  <font color=#0000ff>def</font> accept(self, visitor):
    visitor.visit(self)
  <font color=#0000ff>def</font> pollinate(self, pollinator):
    <font color=#0000ff>print</font> self, <font color=#004488>"pollinated by"</font>, pollinator
  <font color=#0000ff>def</font> eat(self, eater):
    <font color=#0000ff>print</font> self, <font color=#004488>"eaten by"</font>, eater
  <font color=#0000ff>def</font> __str__(self): 
    <font color=#0000ff>return</font> self.__class__.__name__

<font color=#0000ff>class</font> Gladiolus(Flower): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Runuculus(Flower): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Chrysanthemum(Flower): <font color=#0000ff>pass</font> 

<font color=#0000ff>class</font> Visitor:
  <font color=#0000ff>def</font> __str__(self): 
    <font color=#0000ff>return</font> self.__class__.__name__

<font color=#0000ff>class</font> Bug(Visitor): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Pollinator(Bug): <font color=#0000ff>pass</font>
<font color=#0000ff>class</font> Predator(Bug): <font color=#0000ff>pass</font>

# Add the ability to do <font color=#004488>"Bee"</font> activities:
<font color=#0000ff>class</font> Bee(Pollinator):
  <font color=#0000ff>def</font> visit(self, flower):
      flower.pollinate(self)

# Add the ability to do <font color=#004488>"Fly"</font> activities:
<font color=#0000ff>class</font> Fly(Pollinator):
  <font color=#0000ff>def</font> visit(self, flower):
      flower.pollinate(self)

# Add the ability to do <font color=#004488>"Worm"</font> activities:
<font color=#0000ff>class</font> Worm(Predator):
  <font color=#0000ff>def</font> visit(self, flower):
      flower.eat(self)

<font color=#0000ff>def</font> flowerGen(n):
  flwrs = Flower.__subclasses__()
  <font color=#0000ff>for</font> i <font color=#0000ff>in</font> range(n):
    <font color=#0000ff>yield</font> random.choice(flwrs)()

# It's almost as <font color=#0000ff>if</font> I had a method to Perform
# various <font color=#004488>"Bug"</font> operations on all Flowers:
bee = Bee()
fly = Fly()
worm = Worm()
<font color=#0000ff>for</font> flower <font color=#0000ff>in</font> flowerGen(10):
  flower.accept(bee)
  flower.accept(fly)
  flower.accept(worm)
#:~</PRE></FONT></BLOCKQUOTE><DIV ALIGN="LEFT"><P><FONT FACE="Georgia">
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_331">Add Comment</A></FONT><BR></P></DIV>
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<A NAME="Heading81"></A><FONT FACE = "Verdana, Tahoma, Arial, Helvetica, Sans"><H2 ALIGN="LEFT">
Exercises</H2></FONT>
<OL>
<LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Create a business-modeling
environment with three types of <B>Inhabitant</B>: <B>Dwarf</B> (for engineers),
<B>Elf</B> (for marketers) and <B>Troll</B> (for managers). Now create a class
called <B>Project</B> that creates the different inhabitants and causes them to
<B>interact(&#160;)</B> with each other using multiple dispatching.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_332">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Modify
the above example to make the interactions more detailed. Each <B>Inhabitant</B>
can randomly produce a <B>Weapon</B> using <B>getWeapon(&#160;)</B>: a
<B>Dwarf</B> uses <B>Jargon</B> or <B>Play</B>, an <B>Elf</B> uses
<B>InventFeature</B> or <B>SellImaginaryProduct</B>, and a <B>Troll</B> uses
<B>Edict</B> and <B>Schedule</B>. You must decide which weapons
&#147;win&#148; and &#147;lose&#148; in each interaction (as in
<B>PaperScissorsRock.py</B>). Add a <B>battle(&#160;)</B> member function to
<B>Project</B> that takes two <B>Inhabitant</B>s and matches them against each
other. Now create a <B>meeting(&#160;)</B> member function for <B>Project</B>
that creates groups of <B>Dwarf</B>, <B>Elf</B> and <B>Manager</B> and battles
the groups against each other until only members of one group are left standing.
These are the &#147;winners.&#148;
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_333">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Modify
<B>PaperScissorsRock.py</B> to replace the double dispatching with a table
lookup. The easiest way to do this is to create a <B>Map </B>of <B>Map</B>s,
with the key of each <B>Map </B>the class of each object. Then you can do the
lookup by
saying:</FONT><BR><FONT FACE="Georgia">(<B>(Map)map.get(o1.getClass())).get(o2.getClass())</B></FONT><BR><FONT FACE="Georgia">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?
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_334">Add Comment</A></FONT><LI><FONT FACE="Verdana">	</FONT><FONT FACE="Georgia">Modify
Exercise 2 to use the table lookup technique described in Exercise 3.
<A HREF="http://www.mindview.net/Books/TIPython/BackTalk/FindPage/A_335">Add Comment</A></FONT></OL>
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