[Python-ideas] Assignments in list/generator expressions

[Westley Martínez]

On Sun, 2011-04-10 at 20:18 -0400, Eugene Toder wrote:
> On Sun, Apr 10, 2011 at 7:06 PM, Guido van Rossum <guido at python.org> wrote:
> > Can you please be so kind to leave attributions in? (I'm glad you snip stuff
> > you don't need, but the attributions are essential.)
> >
> <snip>
> >
> > It was not clear.
> 
> Will try to improve on both accounts.
> 
> On Sun, Apr 10, 2011 at 7:06 PM, Guido van Rossum <guido at python.org> wrote:
> > (That's not to say that I am vetoing a specific proposal. But the bar
> > is high, and inventing a "Pythonic" syntax is a prerequisite. So is a
> > good story with a set of examples explaining why the new feature adds
> > something you cannot easily do today.)
> 
> This particular feature is not the top on my wish list either. I just
> don't see a good explanation for why it's missing. It's a lesser-known
> feature in Haskell (no mention in Wikipedia! :-), but it is handy at
> times. As mentioned above, people suggest it for Python from time to
> time.
> 
> As for the syntax, there are many choices. Going with <keyword> name =
> expr, <keyword> can be 'let', as in Haskell, F# and Clojure, or
> 'where' as Greg suggested above, or 'given' as it's related to PEP
> 3150 -- the scope of name partially extends to the left of it's
> definition. From the backward compatibility point, 'let' is likely too
> short for a new keyword, and PEP 3150 describes the problem with
> 'where'.
> 
> ys = [y for x in xs where y = f(x) if y]
> 
> ys = [y for x in xs given y = f(x) if y]
> 
> There are likely other words that will do. Alternatively, some
> punctuation can be used, e.g. semicolon:
> 
> ys = [y for x in xs; y = f(x) if y]
> 
> Eugene

Semi-colon is already in use and to me (perhaps because of C) tells me
they are unrelated statements. where to me seems the more Pythonic.