[Info] PEP 308 accepted - new conditional expressions

[Bengt Richter]

On Wed, 12 Oct 2005 02:15:40 -0400, Chris Smith <smitty_one_each at bigfoot.com> wrote:

>>>>>> "Sebastian" == Sebastian Bassi <sbassi at gmail.com> writes:
>
>    Sebastian> On 9/30/05, Reinhold Birkenfeld <reinhold-birkenfeld-nospam at wolke7.net> wrote:
>    >> after Guido's pronouncement yesterday, in one of the next
>    >> versions of Python there will be a conditional expression with
>    >> the following syntax: X if C else Y
>
>    Sebastian> I don't understand why there is a new expression, if
>    Sebastian> this could be accomplished with:
>
>    Sebastian> if C: X else: Y
>
>    Sebastian> What is the advantage with the new expression?
>
>One very frequent use case is building a string.
But what you show doesn't require choice of which to _evaluate_,
since you are merely choosing between immutable constants, and there is
no side effect or computation overhead to avoid by not evaluating
the unselected expression. So

    "the answer is " + ('no', 'yes')[X==0]

would do as well.

>Instead of:
>
>if X==0:
>   filler="yes"
>else:
>   filler="no"
>return "the answer is %s" % filler
>
>
>What I really want to do is take four lines of conditional, and put
>them into one, as well as blow off dealing with a 'filler' variable:
>
>return "the answer is " + "yes" if X==0 else "no"
As has been pointed out, though legal there is some doubt as to whether
you meant what you wrote ;-)
>
>
>Or whatever the final release syntax is.
>Conditional expressions are a nice way to shim something in place, on
>an acute basis.  Chronic use of them could lead to a full-on plate of
>spaghetti, where you really wanted code.
>They can be impenitrable.  Whenever I'm dealing with them in C/C++, I
>always line the ?, the :, and the ; characters vertically, which may
>seem a bit excessive in terms of whitespace, but provides a nice
>hieroglyph over on the right side of the screen, to make things
>obvious.
>Best,
>Chris

Regards,
Bengt Richter