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

[Andrew Dalke]

David Eppstein:
> As far as I can remember, the last time I wanted to use that construct,
> I couldn't, because b was false.  Hah!  Found it:
>
>         if i < 0:
>             col = None
>         else:
>             col = cols[i]

Ooo, I like this example.  The tricks I've used elsewhere don't work here,
so this is indeed one where the if/else expression would make things clearer
and where short-circuit evaluation is needed and appropriate.

Hmmm, well, here's an ugly trick,

    col = [None, cols[max(i, 0)]][i>=0]

and here's another

   col = [[None] + cols][max(i+1, 0)]

but neither have the grace of

   col = None if i < 0 else cols[i]

OTOH, I still say this is easier to debug, because I can do

         if i < 0:
             print "Negative?", i
             col = None
         else:
             print "i", i, "neighbors", cols[max(i-1, 0):i+1]
             col = cols[i]

                    Andrew
                    dalke at dalkescientific.com