beginner, idiomatic python

Erik Max Francis max at
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 &&
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