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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: (test) ? a:b
Date: Sat, 25 Oct 2014 14:11:56 -0400
Organization: IISS Elusive Unicorn
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On Sat, 25 Oct 2014 07:58:14 -0400, Ned Batchelder <ned@nedbatchelder.com>
declaimed the following:


>
>You mention "standard Python idioms."  I think this style of 
>conditional-via-indexing is becoming quite uncommon, and is no longer 
>one of the standard Python idioms.  This is now in the category of 
>"outdated hack."
>
	That may be true for the case where the index is based upon a Boolean
expression...

	But the general form is still viable for either numeric indices (table
of functions) and switch/case style operations (replace the list with a
dictionary).

	The Boolean application is merely a specialization of a more general
idiom.


	The "true if cond else false" just doesn't fit my mind.

	Even HP RPL (where one might expect strangeness) uses

		if cond then true else false end

though being reverse polish overall, one could write

		cond if then true else false end

as "if" merely starts the block, and it is the "then" which evaluates the
condition, whose value is still the top of stack at that point. I'm too
lazy to see if one could do

		false true cond if then else end

which does have the problem of evaluating both false and true paths -- but
if no side-effects, might work: the "then" pulls the "cond", if true the
"else" takes the "true" and the "end" drops the "false" (and if "false" the
"else" drops the "true" and the "end" takes the "false")...


	The more in-line form, usable outside of RPL appears as a function

		ifte(cond, true, false)

which wasn't wanted for Python as it would evaluate both true and false (no
lazy evaluation; one would need to make a lambda for them)


-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/