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

[David Eppstein]

In article <mailman.1044675560.14943.python-list at python.org>,
 Carel Fellinger <carel.fellinger at chello.nl> wrote:

> > Carel> The proposal as is looks reasonable to me, it would clean up
> > Carel> those few uses of (a and b or c) that I have in my code.
> > 
> > Would you mind showing us some concrete examples?  I think that
> > some real code examples would be quite useful to the discussion.
> 
> Bumbers, it has been quit some time since I used that construct, and
> it's not as easily spotted as C's ternary operator either.  I've found
> one example, others are made up.

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]

Here cols[i] is a string, and I didn't want to assume that it is always 
nonempty, so the and-or construct doesn't work.  I think this would be a 
good candidate for a one-liner:

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

I also just grepped through the same small project (1000 lines), and 
found two examples where I did use the construct.  In both cases I used 
"x and 1 or 0" to normalize a boolean. Of course that could have been 
"not not x" or better (in 2.3) "bool(x)"...

To me, the fact that people are treating "a and b or c" as a standard 
idiom is a sign that we should replace it with something less likely to 
be buggy and more understandable.

-- 
David Eppstein       UC Irvine Dept. of Information & Computer Science
eppstein at ics.uci.edu http://www.ics.uci.edu/~eppstein/