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PEP 308: ternary operator

Terry Hancock hancock at anansispaceworks.com
Thu Feb 20 01:25:19 EST 2003 
(I've not been reading c.l.py for a few days, so I'm not sure which branch 
of this thread to reply to)

I have always hated C's ternary operator syntax!  I never used it that 
much, and every time I did I had to look up the syntax.  I REALLY hope 
Python does not simply adopt this syntax, as I think it will go on 
torturing newbies and casual programmers like myself if it does.  Here's 
why it bothers me:

1) Is it <cond>? <a>:<b> or <cond>: <a>?<b>
I can never remember this!

2) Is it <a> ? <cond>: <b> or <cond>:<a>?<b>, or how about <a>:<cond>?<b>
They all make exactly the same amount of sense to me. (Which is to say, not 
much).

Note that I am not objecting to the functionality, but rather the symbols 
used to encode it.  I find them highly arbitrary.

Likewise, I find the current kludge of using <cond> and <a> or <b> to be 
annoyingly idiomatic. It works by a kind of trick, and not for the reason 
it reads well. It gets you when you're not expecting it, and surely that's 
a good reason not to use it.  Again, this is for newbies and casual 
programmers, but my understanding of Python is that it's supposed to be 
newbie-friendly.

I also find the (if <cond>: <a> else <b>) syntax confusing because it looks 
too much like the statement syntax, and I just can't warm up to the 
parentheses.  I especially don't like the overloaded meanings of the 
keywords.

I therefore would like to voice support for either of the following 
"new-keyword" options:

d = <a> when <cond> else <b>

This is my favorite -- thank you M. Pinard! I like the chaining which is 
superior to the C alternative of nesting the expressions.  I also think 
this one reads the nicest, and does not cause confusion.  Although: "where" 
is just as good as "when", BTW, and has the advantage of being used in 
nearly the same way in IDL.  Not that we care, but I think it's true.  I 
don't really prefer either over the other.

The main thing I like is that it properly places the most common action 
(d=<a>) at the beginning where it reads nicely. Then we add the exceptional 
case onto the end. It has the feel of the try-except syntax in miniature.  
Each successive "when" introduces a fall-back.

d = <cond> then <a> else <b>

This is a concession. If you must have the <cond> <a> <b> order, please use 
this one.  However, I don't like it as much, because:

1) It does not read as an expression in English.

2) "then", while not a Python keyword is part of an "if" block in so many 
other languages (especially Perl and shell languages), I feel it could be 
confusing to people switching back and forth. "when" does appears as a 
keyword in some languages, but is much more distinct (and also usually 
appears in expression syntaxes, I think -- just like "where" in IDL).

Neither of these objections is a killer though -- I could learn the 
then-else syntax and be happy with it.

I think that the improved clarity is worth the hassle of introducing a 
keyword.

I would also like to say that I find a greater need for what the ternary 
operator does in Python than I do in C.  I'm not sure why, but it comes up 
more often. Maybe I'm just remembering using it in Zope (where it is 
definitely useful in DTML -- and probably ZPT too).  So far I've gotten 
away with the <cond> and <a> or <b> syntax, but I don't like the potential 
pitfalls -- it always adds an extra amount of testing to make sure I'm not 
fooling myself.

Anyway, that's my two cents on it.

Cheers,
Terry

-- 
Anansi Spaceworks
http://www.anansispaceworks.com
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