PEP 308: an additional "select/case" survey option

[Clark C. Evans]

On Wed, Mar 05, 2003 at 03:57:50PM +0000, Stephen Horne wrote:
| If I want to write, for example...
| 
|   a =   (if (b != 0) then a/b else 1000)
|       + (if (d != 0) then c/d else 1000)
|       + (if (h != 0) then g/h else 1000))
| 
| ... then readability is best served by keeping each conditional on a
| single line so that vertical alignment can emphasize the pattern.

Ok.

   a = ((when b != 0: a/b
         else: 1000)
      + (when d != 0: c/d
         else: 1000)
      + (when h != 0: g/h
         else: 1000))

Which I admit isn't very pretty. However, if you assume that
in the select/when proposal a missing else clause causes
non-matches to evaluate to None, you get...

   a = ( ((when b != 0: a/b) or 1000))
       + ((when d != 0: c/d) or 1000))
       + ((when h != 0: g/h) or 1000)))

I guess this is one extra level of parenthesis... but then
again the if/then proposal could be limited to such a scope
(not having an else).

| That said, I think that some form of 'select' could have merit - but
| as an alternative to, rather than a replacement for, a simpler
| conditional expression notation.

Hmm.  As soon as you add "else" to the if clause, I think
it should be time to consider how to make it more flexible.

Best,

Clark















    def sum(lst):
        res = 0
        for itm in lst: 
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        return res
    
    def div(numerator, denominator, ifzero = None):
        if denominator != 0: return numerator/denominator
        return ifzero

    a = sum([div(x,y) for x,y in ((a,b),(c,d),(g,h))])