list comprehensions to effect visitors

Bruce Eckel Bruce at EckelObjects.com
Fri Dec 7 03:49:35 CET 2001


>I thought "list comprehensions" was a typo
>(should've been "...incomprehensions") until I discovered that 
>the leftmost element was evaluated anew for each element.  
>Suddenly it became easy to invoke the same method on a whole 
>collection of objects, without lambda:
>
>    [foo.asTabSeparatedValue() for foo in selection]

Whoa! (slaps forehead). I've wanted to do this very thing a bunch
of times. And the gods of Python clearly anticipated my needs.

>With list comprehensions many applications of the visitor design 
>pattern can be reduced to a single statement.

Based on this, I fooled around with my visitor example until I got
it to work in some fashion with list comprehensions. It cleaned up
nicely. Note the last line, in particular:

# Using list comprehensions to effect visitors.

class Flower:  
  def pollinate(self, pollinator):
    print `self`, "pollinated by", `pollinator`
  def eat(self, predator):
    print `self`, "eaten by", `predator`
  def __repr__(self): 
    return self.__class__.__name__

class Gladiolus(Flower): pass
class Runuculus(Flower): pass
class Chrysanthemum(Flower): pass 

class Bug:
  def __repr__(self): 
    return self.__class__.__name__
class Pollinator(Bug): pass
class Predator(Bug): pass
class Bee(Pollinator): pass
class Fly(Pollinator): pass
class Worm(Predator): pass

import random
rgen = random.Random()
flwrs = [Gladiolus(),Runuculus(),Chrysanthemum()]
flowers = [rgen.choice(flwrs) for i in range(10)]
bee = Bee()
fly = Fly()
worm = Worm()
[(f.pollinate(bee), f.pollinate(fly), f.eat(worm)) for f in
flowers]


At this point I'm trying to figure out
(A) what does multiple dispatching mean in Python?
(B) does Python already, in effect, *do* multiple dispatching, or
perhaps just remove the need for it?

>Sorry for the spewage.  It's just that I see many posts from you
>today, and am assuming you're working on "Thinking in Python"
>(or something like it) and am hoping that you plan to cover 
>the foregoing therein.  If you do, I'll have an *intelligible*
>explanation of list comprehensions :)

I am in fact feverishly translating "Thinking in Patterns" into
"Thinking in Python" to prepare for my tutorial at the Python
conference, which is due monday. I was certain it would be an
enormous job as it was with C++ and Java to write these examples,
but as usual Python changes everything, and I might even hit the
deadline.

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