PEP 308: A PEP Writer's Experience - PRO

[Robin Munn]

Paul Rubin <phr-n2003b at NOSPAMnightsong.com> wrote:
> Michael Chermside <mcherm at destiny.com> writes:
>> There are lots of arguments in favor of a ternary expression,
>> but there are just two which I feel are quite convincing.
>> 
>> [1] It makes expressions more powerful.
>> [2] It distinguishes choice from branching
> 
> I think Andrew Koenig pointed out a third argument which many of us
> had perhaps underestimated: it can prevent bugs.  A lot of the
> workarounds attempted for its absence are flat out wrong.  He gives an
> example showing finding the maximum of x and y with
> 
>    x > y and x or y
> 
> which looks reasonable and might even pass the initial unit tests of a
> real program, but gives the wrong answer (zero) if x is 0 and y is
> negative.  A conditional expression like
> 
>    x if (x > y) else y
> 
> is much harder to get wrong.

There is a beautiful irony here: while demonstrating how easy it is to
get the "C and x or y" idiom wrong, you actually got it wrong. :-) You
wrote that:

    x > y and x or y

gives the wrong answer (zero) if x is 0 and y is negative. Actually in
that case what you get is this:

    x > y  and x or      y
    (True) and 0 or (true value)

which evaluates to:

    x or      y
    0 or (true value)

which evaluates to:

         y
    (true value)

So the result of the expression, in the case where x = 0 and y = -5,
say, is -5. But the expression was *supposed* to find the larger of x
and y, which would have been 0.

Yes, the "C and x or y" idiom is indeed far too confusing and should
therefore be avoided for practical use.

-- 
Robin Munn <rmunn at pobox.com>
http://www.rmunn.com/
PGP key ID: 0x6AFB6838    50FF 2478 CFFB 081A 8338  54F7 845D ACFD 6AFB 6838