list comprehensions to effect visitors

Bruce Eckel Bruce at
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), for f in

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

Most current information can be found at:
Bruce Eckel
Contains free electronic books: "Thinking in Java 2e" & "Thinking
in C++ 2e"
Please subscribe to my free newsletter -- just send any email to:
join-eckel-oo-programming at
My schedule can be found at:

More information about the Python-list mailing list