[Python-ideas] Accessing the result of comprehension's expression from the conditional

[8mayday at gmail.com]

On Jun 21, 2009 1:06pm, Chris Rebert <pyideas at rebertia.com> wrote:
> > On Sun, Jun 21, 2009 at 10:36 AM, Lie Ryanlie.1296 at gmail.com> wrote:

> >> Ben Finney wrote:

> >>> Lie Ryan lie.1296 at gmail.com> writes:

> >>>

> >>>> Steven D'Aprano wrote:

> >>>>

> >>>>> The only case it doesn't cover is where the second filter depends on

> >>>>> the value of x, and even that can be covered with a *tiny* bit more

> >>>>> work:

> >>>>>

> >>>>> gen = ((x, 3*x**2-5*x+4) for x in seq if x % 3 != 2)

> >>>>> result = [y[1] for y in gen if -y[0]
> >>>>>

> >>>>> It requires no new syntax, no changes to the behaviour of list

> >>>>> comps, no new meaning on "as", it's understandable and readable.

> >>>

> >>> I think it would be more readable without index references, but  
> instead

> >>> using tuple unpacking::

> >>>

> >>> gen = ((x, 3*x**2-5*x+4) for x in seq if x % 3 != 2)

> >>> result = [b for (a, b) in gen if -a
> >>>

> >>> It can even be done as a single expression without (IMO) significantly

> >>> affecting readability::

> >>>

> >>> result = [

> >>> b for (a, b) in

> >>> ((x, 3*x**2-5*x+4) for x in seq if x % 3 != 2)

> >>> if -a
> >>

> >> IMHO, when a comprehension requires more than a single line, it should

> >> turn into explicit loop.

> >>

> >>

> >>

> On Sun, Jun 21, 2009 at 1:54 AM, Andrey Popp8mayday at gmail.com> wrote:

> > What about where/let expression?

> > [(y, y) for x in some_list if y
> > --

> > С уважением, Андрей Попп.

> > +7 911 740 24 91



> (A) Please don't top-post. (http://en.wikipedia.org/wiki/Top-post)

> (B) That has the distinct disadvantage of adding a new keyword. I

> instead prefer the "as" version of the proposal for this reason.



> Cheers,

> Chris

> --

> http://blog.rebertia.com

(A) Sorry
(B) There is no difference, except "where" is widely used in othe  
languages. Anyway, I think that "where" like functionality would be useful  
in list comprehensions, lambdas...
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