"?:", "a and b or c" or "iif"

[Michael Vanier]

"Fred L. Drake" <fdrake at cnri.reston.va.us> writes:

> news.eunet.no writes:
>  > def iif(x, a, b):
>  >   if x:
>  >     return a
>  >   else:
>  >     return b
> 
>   Assuming you're emulating "x ? a : b"....
>   This isn't quite it.  Using C's ?:, only one of a or b is evaluated
> when the expression is evaluated.  iif(x, a, b) evaluates both a and
> b.  Sometimes this is acceptable, but not if either a or b has side
> effects or is expensive to evaluate.
>   I don't think there's a good way to do this in Python right now; we
> have to use an if in the context where we need the result:
> 
>      if x:
>          result = a
>      else:
>          result = b
> 
> (Unless, of course, we happen to like some of those hideous constructs 
> needed to sidestep the problem.)
>   Perhaps Python 2 can fix this shortcoming; I'd often like to use
> something equivalent to ?:.
> 
> 
>   -Fred
> 
> --
> Fred L. Drake, Jr.	     <fdrake at acm.org>
> Corporation for National Research Initiatives

This sounds like another job for bytecodehacks :-)  

I personally detest the ?: operator in C, mainly because people who use it
tend to write fun code like this:

a = (b ? (c ? d : (e ? f : g)) : (h ? i : j));

If something like this is coded, I *strongly* recommend that it not be an
operator but something like an "iif"-type function.  Of course, to do that,
you need to be able to define functions with lazy evaluation.  THAT would be a
neat thing to have in python 2.0! E.g.:


# Warning: pseudo-python ahead!

def iif(condition, consequent, alternative):
	lazy consequent, alternative

	if (condition):
		return consequent
	else:
		return alternative

With-lazy-evaluation-can-true-macros-be-far-away-ly y'rs,

Mike


-------------------------------------------------------------------------
Mike Vanier	mvanier at bbb.caltech.edu
Department of Computation and Neural Systems, Caltech 216-76
Will optimize nonlinear functions with complex parameter spaces for food.