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

[Andrew Dalke]

Erik Max Francis
> So you'd want conditional expressions in things like [example in C]:
>
> printf("I have %d %s%s.\n", count, noun, count == 1 ? "" : "s");
>
> Here's an example where I want to do an if...else but that is really a
> minor point the big picture.

I like minor points.  They are things I can understand.

So assuming you don't want to do internationalization, you could also
do this example with

  print "I have %d %s%s." % (count, noun, "s"[:count!=1])
or
  print "I have %d %s%s." % (count, noun, ("", "s")[count!=1])
or
  print "I have %d %s%s." % (count, noun, "s" * (count != 1)


Though I would have more likely have had something like

_irregular = {
  "person": "people",
  "mouse": "mice"
}
_self_plural = dict.from_keys("sheep fish".split())

def pluralize(noun, count):
  if count == 1 or noun in _self_plural:
    return noun
  if noun in _irregular:
    return _irregular[noun]
  if noun[-1:] in "sz":
    return noun + "es"
 if noun[-1:] == "y":
    return noun[:-1] + "ies"
 return noun + "s"

> a = c1 if e1 else \
>             c2 if e2 else ...

In most of those cases, the construct is

  a = "red" if color == 1 else "blue" if color == 2 else "green"

which is better (IMHO) written as

  a = {1: "red", 2: "blue", 3: "green"}[color]
or
  a = {1: "red", 2: "blue"}.get(color, "green")
if you don't care about error checking (note that the original
trinary expression doesn't care about error checking)


I was only somewhat joking above with the "minor points" comment.
My feeling about the trinary operator is that, yes, there are a few
places where it fits better.  However,
  1) people using it have a C background, and may ignore the possible
        solutions available in Python (like a=[false, true][bool(expr)] and
         a = x or None, or a={k1: v1, k2:v2, k3: v3}[key] )

  2) the ternary expression should not be nested, because it gets too
       confusing too quickly.  However, it's the reading which is the
       confusing part;  Writing these nested expressions is really easy.

  3) people (like me when I was younger) will write multiple lines
     of ternary code, making "cute" but hard-to-understand code

  4) it is harder to understand and debug because an if/else also
     makes it easy to add print statements or other sorts of tracing
     to see what's going on at a given step

  5) the justification on the basis of short circuiting is not that
      strong because most real-life uses of ?:  do not depend on
      short circuiting other than for slight (trivial) speedups.

                    Andrew
                    dalke at dalkescientific.com