For review: PEP 308 - If-then-else expression

Erik Max Francis max at alcyone.com
Sat Feb 8 04:53:28 CET 2003


Guido van Rossum wrote:

> PEP: 308
> Title: If-then-else expression

I am +1 on the addition of a conditional expression.

>     The proposed syntax is as follows:
> 
>         <expression1> if <condition> else <expression2>

+0.8.  I want conditional expressions and will take any form I can get,
but the condition in the middle does seem a little counterintuitive.

>     Examples of how this works out:
> 
>         x if C else y if D else z <==> x if C else (y if D else z)

This is certainly the most sensible solution for chaining conditional
expressions, although chaining conditional expressions -- in whatever
language, not just Python -- is usually a sign of obfuscation.  (Still,
it should "do the right thing" even if obfuscation is what you're
actively choosing, and what is proposed abovce is the right thing.)

>         x or y if C else z        <==> (x or y) if C else z
>         x if C else y or z        <==> x if C else (y or z)
>         lambda: x if C else y     <==> lambda: (x if C else y)
>         x if C else lambda: y     <==> SyntaxError
>         x if C else y, z          <==> (x if C else y), z
>         x, y if C else z          <==> x, (y if C else z)

These are all edge cases and don't bother me.  Particularly the latter
two; if you're going to be inlining tuples in conditional expressions,
it doesn't hurt to be forced to use parentheses to disambiguate.

>     If we could live with adding a new keyword, we could use:
> 
>         if <condition> then <expression1> else <expression2>

This would get higher ratings with me than the proposed syntax, simply
because it has the left-to-right evaluation and seems more natural.  I'd
be willing to live with enforced parentheses.

But bottom line is I'll take whatever I can get.  I want a conditional
expression in Python.

As for other peoples' suggestions, I think whatever the form it takes,
the "else" portion of the conditional should be required.  No "x if y"
(or "if y then x") shortened versions; explicit is better than implicit,
if you want None or something else in that else then you should write it
out.

-- 
 Erik Max Francis / max at alcyone.com / http://www.alcyone.com/max/
 __ San Jose, CA, USA / 37 20 N 121 53 W / &tSftDotIotE
/  \ Shooters, looters / Now I got a laptop computer
\__/ Ice Cube
    Kepler's laws / http://www.alcyone.com/max/physics/kepler/
 A proof of Kepler's laws.




More information about the Python-list mailing list