PEP 308: A PEP Writer's Experience - PRO

Andrew Dalke adalke at mindspring.com
Sun Feb 9 01:24:12 CET 2003


Michael Chermside
> Suppose you want to write a line of logging
> into your program:
>
>        print "x = " + x.getDisplayString()
>
> but you have to allow for the fact that x might be None. With
> a ternary expression, you can write this:
>
>        print "x = " + x.getDisplayString() if x else "None"
  ....
> But without a ternary expression, you need to do this instead:

Or do what's often done and write a dummy display
used for the null case.

class DummyDisplay:
  def getDisplayString(self):
    return "None"

This is very nice because there are no branches in the
code flow, except once at the very top.  And the fewer
branches the better.

> Similar examples could be found using a ternary expression in
> lambda, in a list comprehension, and anywhere else that
> expressions are used.

I just posted a really hairy list comp w/ if/else expression.
It looks like

 indicies = [(long_function_name(atom->realAtom(), atomCount,
reactionMapping)
                     if atom != OXYGEN else -1)
                        for atom in (atom1, atom2, atom3, atom4)]

Given the code base from which I extracted it, it is
likely that the programmers would have thought about this
sort of construct, and deemed it 'cool'.

I would prefer

 indicies = []
for atom in (atom1, atom2, atom3, atom4):
  if atom != OXYGEN:
    indicies.append(long_function_name(atom.realAtom(),
                                                        atomCount,
reactionMapping))
 else:
   indicies.append(-1)


In the analysis from which this came, I estimated that, in my target
user base, the ?: would be used appropriately about once every
2,000 lines of Python code, and likely to use it inappropriately at
a more frequent rate than that.

>  Being able to spell the first one as:
>
>        x = a if condition else b
>
> makes something which is conceptually different LOOK different
> -- and that's an important feature in a programming language.

Good point.  Still, I think (and I now have the numbers to back me
up) that amount non-expert developers the misuse rate is
higher than the proper use rate and that in any case the use
rate is very low -- too low to warrant a fundamental/low-level
construct like this which introduces a choice with no good,
unambiguous guidelines of when it is proper use.

(See my post with the buy/bonus buy/werewolf buy for just
such an example.)

                    Andrew
                    dalke at dalkescientific.com






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