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

[Andrew Dalke]

Paul Paterson
> As per a different thread where I did the reverse and searched the Python
> code in the Python standard library + assorted packages on my system.

One problem with this is that it doesn't capture the original author's
intent or practice.  Some people like to have
   if cond:
    x = expr1
  else:
    x = expr2

either because of clarity or because it's easier to keep a
smaller set of language constructs in one's head or any of
the other reasons people have suggested here.

That's why I analyzed code from a language which already has
an if/else ternary expression.  Eg, consider this example from Python's
C implementation, in cellobject.c (first one I found, but I imagine
there are plenty such examples)

           if (b->ob_ref == NULL)
                   return 0;
           return -1;

This could be written in C as

   return  b->ob_ref == NULL ? 1 : 0

but wasn't.  Why wasn't it?

To repeat, just because the language supports a ?:-like expression
doesn't mean it's used.  I believe it is used many fewer times
than it can be used.

> 769 candidates where the conditional expression could be used to replace
an
> assignment
> 381210 lines of code
>
> So as a rough estimate we get one use per (381210 - 4*769)/769 = 491
/lines
> of code.
>
> The actual usage is probably a lot more common that this as I only looked
> for the assignment pattern, but I would say the 1 in 200 lines is a fairly
> reasonable estimate.

That's a reasonable estimate of the number of times it COULD be
used.  It isn't a reasonable estimate of the number of times it WILL
be used.  I think it's much lower, closer to 1 in 1000 times or even
fewer.

                    Andrew
                    dalke at dalkescientific.com