if/elif chain with assignment expressions

[John Roth]

"Paul Rubin" <http://phr.cx@NOSPAM.invalid> wrote in message
news:7x4qoe6vae.fsf at ruckus.brouhaha.com...
> steve holden <sholden at holdenweb.com> writes:
> > Indeed, you  might think so. But this topic has been discussed
> > exhaustively (which isn't to say you aren't about to hear a lot more
> > about the subject) in this group, and the assignment operation is
> > *not* an operator for specific reasons the FAQ attempts to make clear
> > in
> >
http://www.python.org/doc/faq/general.html#why-can-t-i-use-an-assignment-in-an-expression
>
> Yeah, I've seen those discussions before.  The if/elif/elif example was
> something I hadn't seen in those discussions, and it came up in some
> code I was writing yesterday, so I posted about it.

I thoroughly agree. I've come up with that any number
of times, and allowing an assignment in an expression
is one of the things that I think should be done.

Note that I don't really care about the syntax, and
I care even less if it's really intuitive to novices. It's
one of those things that leads to simpler, more expressive
code when you need it, but it's not something that's
absolutely necessary to hack together a working
program while you're learning the language.

The use case is an expression in an if statement, not
an expression in a while statement. The latter can
be handled with an iterator and a for statement,
which might be a good idea, and it might not depending
on the specific situation.

There is an alternative, which is to find all of the
cases where this is likely to be a problem, and fix
the calls so that they naturally return an iterator.

To make this specific, let's take a look at the
<string>.find and <string>.rfind. This method
pair is probably the poster child of how not to
do it in a pythonic manner.

Let's assume we had a <string>.findall()
method, defined to return a list of all
indexes of the first character of each
substring that matches the given string.

Then we could say something like:

for firstMatch in myString.findall("fubar")[:1]:
   do something with it

While this is not exactly obvious to the novice,
it not only gets the job done, but the findall()
method also has a number of other very
nice properties. It behaves properly in a boolean
context, and it never returns an incorrect index
(that is, -1.)

If you really did want to process all matches,
it's even easier.

John Roth