[Python-ideas] if expensive_computation() as x:

[Nathan Schneider]

On Thu, Feb 13, 2014 at 9:58 PM, Chris Angelico <rosuav at gmail.com> wrote:

> Responding to your post in different order to the original.
>
> On Fri, Feb 14, 2014 at 8:59 AM, Ram Rachum <ram.rachum at gmail.com> wrote:
> > If you'd like to bind to a variable only a part of the condition, this
> would
> > work too:
> >
> >     if x<5 with expensive_computation_0() as x:
> >         # Do something with x
>
> Definitely don't like this syntax - while it might be useful to
> snapshot part of a condition (I've done it in C plenty of times), this
> notation feels clumsy. However...
>
>
>
I agree that a non-clunky way to extract variables from conditions with an
operator would be nice. Maybe a better syntax would be:

    if (expensive_computation_0() as x)<5:
         # Do something with x

And likewise for `while` loops,

    while (expensive_computation_0() as x)<5:
         # Do something with x

> My suggestion:
> >
> >     if expensive_computation_0() as x:
> >         # Do something with x...
> >     elif expensive_computation_1() as x:
> >         # Do something with x...
> >     elif expensive_computation_2() as x:
> >         # Do something with x...
>
> ... this simpler form does look reasonable. The "as" part will *only*
> come at the end of the expression, and it *always* applies to the
> whole expression, so it's fairly clear.
>

Agreed, this looks reasonable to me.

These are special cases of PEP 379, "Adding an Assignment Expression" (
http://www.python.org/dev/peps/pep-0379/) from 2009, which has been
withdrawn. Perhaps it would be better received if restricted to if/while
conditions.

Nathan
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20140213/0d824a8d/attachment-0001.html>