conditional expressions

Bruce Dawson bruce_deletethis_dawson at cygnus-software.com
Mon Sep 23 08:18:09 CEST 2002


thp at cs.ucr.edu wrote:

> Terry Reedy <tjreedy at udel.edu> wrote:
> 
> + "daniel w. moore" <dan65536 at hotmail.com> wrote in message
> + news:33b491f.0209181820.17b6ea70 at posting.google.com...
> +> It's occurred to me after some brainstorming on the subject that the
> +> C/perl construct
> +>
> +>   a ? b : c
> +>
> +> can be arranged like this in python:
> +>
> +>   (a and [b] or [c])[0]
> 
> + If you *know* that b will evaluate as True (perhaps because it is a
> + non-null constant, then this can be simplified to 'a and b or c'.  I
> + first used this several years ago.
> 
> But why use grotesque hacks in the first place?  "a ? b : c" is a
> well-established syntax for conditional expressions.  Why doesn't
> Python simply adopt it?
> 
> Tom Payne


I assume that there are reasons why it could clutter or make ambiguous 
the syntax - but I'm not expert.

I find the (a and [b] or [c])[0] construct to obscure to be useful - the 
space it saves is wasted by the comment I have to put in to explain it.

It occurs to me that the bool() built in function allows for what is - 
in my opinion - a cleaner solution. Easier for mortals to comprehend.

Basically, construct a two entry list of the two items and then index 
that with your expression, converted to bool. The bool function is 
essential since there was previously no generic way of getting the 
true/false value of an expression.

For example:

Python 2.2.1 (#34, Sep 14 2002, 09:40:33) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
 >>>
 >>> a = "Hello"
 >>> b = "Goodbye"
 >>> myString = ""
 >>> [a,b][bool(myString)]
'Hello'
 >>> myString = "Something"
 >>> [a,b][bool(myString)]
'Goodbye'




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