beginner, idiomatic python

Erik Max Francis max at alcyone.com
Fri Aug 24 07:34:32 CEST 2007


bambam wrote:

> Excellent. By symmetry, I see that "list" casts the set back into a list.
> 
> I wonder why list has not been extended with the same (difference,
> interesection) methods?  Casting to set looks a little kludgy:
> 
> c = list(set(a)-set(b))
> I wonder if that is clearer than the explicit loop?

This isn't a "cast" in the sense of some less-strongly-typed languages; 
it's just a conversion.  The `list` function/type iterates over its 
argument and turns it into a list.  Sets are iterable, so that's all 
that's really going on here.

The reason that lists don't have set-like methods is because lists 
aren't sets -- lists can contain duplicate elements, whereas sets 
cannot.  You should use the proper type for your needs; if you want to 
take two lists, remove duplicate elements, and then end up with a list, 
then the sets-difference-and-then-make-a-list mechanism is appropriate.

-- 
Erik Max Francis && max at alcyone.com && http://www.alcyone.com/max/
  San Jose, CA, USA && 37 20 N 121 53 W && AIM, Y!M erikmaxfrancis
   It [freedom] must be demanded by the oppressed.
    -- Dr. Martin Luther King, Jr.



More information about the Python-list mailing list