On Monday, August 26, 2002, at 09:54  AM, Yigal Duppen wrote:

>  List comprehensions (LCs) provide a much better way of getting the 
>  derived
>  list:
>  derived = [ z.getX() for z in l ]
> 
>  As you can see, this requires no anonymous functions and is perfectly 
>  OO.
> 
>  The advantage of this becomes even clearer if you want to map AND 
>  filter.
>  Let's say we only want the even values and that we have no 'odd' or 
>  'even'
>  function.
> 
>  derivedEven = filter(lambda n: n % 2 == 0, map(lambda z: z.getX(), l))
> 
>  Argl! What does this do? All we have is the 'descriptive' variable name.
> 
>  derivedEven = [ z.getX() for z in l if z.getX() % 2 == 0 ]
> 
>  Much better! But then again, LCs are also a matter of taste :)

Ah... very clear now.  I hadn't considered that LCs are useful because, 
like lambda, they don't require you to specify a specific type (such as 
in "derived = map(Foo.getX, l)"), so they can be useful in situations 
where you're trying to use polymorphism or something.  Great example.

But what -is- the problem that some people have with lambda?  (You 
mention that you don't have a problem with it, but that implies that 
others do.)

One last question -- in your final example above, you have an "if" 
statement FOLLOWING the "main" part of the list comprehension.  Is this 
a normal Python construction or something that is allowed for list 
comprehensions only?  I have seen that sort of construction in Perl:

   print "Correct" if ($correct);

But I haven't seen it in Python.
Thanks again.



Erik



--
Erik Price

email: erikprice@[...].com
jabber: erikprice@[...].org


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