[Tutor] 2.5's new conditional expression syntax

[Dick Moores]

Kent,

Your examples took a lot of study, but I think I'm catching on.

Thanks very much.

Dick

At 10:53 AM 9/28/2006, Kent Johnson wrote:
>Dick Moores wrote:
> > I've been looking hard at 2.5's new conditional expression syntax
> > (<http://docs.python.org/whatsnew/pep-308.html>), and didn't
> > understand the examples there, so I tried making up my own:
> >
> >  >>> x = (1 if 2 == 2 else 3)
> >  >>> x
> > 1
> >  >>> y = (1 if 2 == 1 else 3)
> >  >>> y
> > 3
> >  >>>
> >
> > But it would help to see an example I could understand that also
> > shows the syntax's usefulness. Could someone cook up one for me?
>
>It's just a shortcut - a compact way to write a conditional. The old way
>to do this (in some cases) was to use and...or. Here are some examples
>from the project I am working on:
>
>batchTypes = types and types*len(batch) or None
>default = course and course.status or 'In Development'
>ps.setBoolean(ix, val and 1 or 0)
>
>which could be rewritten as
>batchTypes = types*len(batch) if types else None
>default = course.status if course else 'In Development'
>ps.setBoolean(ix, 1 if val else 0)
>
>Without either shortcut, these could be written out as e.g.
>if types:
>    batchTypes = types*len(batch)
>else:
>    batchTypes = None
>
>The new syntax has a few improvements over the old method:
>- it is more robust. The old syntax only works if the first value (after
>'and') is True when evaluated as a boolean; the new syntax avoids this
>trap. Workarounds that make the old method work in all conditions are
>awkward and ugly.
>- it is arguably more readable, at least for the common usage shown
>above where there is a common case and an exceptional case.
>
>When the overall expression is more complex, the simple if: else: is
>more unwieldy, e.g.
>
>expectedLen = len(expectedCourses) + (cat and 1 or 0)
>
>which would be written as
>if cat:
>    expectedLen = len(expectedCourses) + 1
>else:
>    expectedLen = len(expectedCourses)
>
>or maybe
>expectedLen = len(expectedCourses)
>if cat:
>    expectedLen += 1
>
>Kent