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

Mathias Panzenböck grosser.meister.morti at gmx.net
Fri Jun 19 19:54:12 CEST 2009

You could write:
[x|y <- l, x <- [f(y)], x > 0]

Oh, wait. Thats Haskell. And even in haskell you would write:
[x|x <- map f l, x > 0]

In Python you can write:
[x for x in map(f,l) if x > 0]

In Python 2.x you may want to write:
from itertools import imap
[x for x in imap(f,l) if x > 0]

A more SQL like approach that would fit somewhat with pythons syntax would be 
(as you can see its exactly the same lengths as the above but needs a new name):
[f(x) as y for x in l if y > 0]

Because in SQL you can write (IIRC):
select f(x) as y from l where y > 0;

Maybe something like .Nets LINQ would be a nice idea to integrate in python?


Lie Ryan wrote:
 > In list/generator comprehension, currently we have no way to access the
 > result of the expression and have to write something like this:
 > [f(x) for x in l if f(x) > 0]
 > if f() is heavy or non-pure (i.e. have side effects), calling f() twice
 > might be undesirable.
 > Even if f() is not a function such as
 > [x + 1 for x in l if x + 1 > 0]
 > it looks ugly since we're repeating ourself.
 > We can work around it like this:
 > [y for y in (f(x) for x in l) if y > 0]
 > but then we have an unnecessary nested loop comprehension.
 > I'm suggesting about something like:
 > [f(x) for x in l if @ > 0]
 > where @ is the result of the listcomp's expression (i.e. f(x))
 > Personally, I don't like the use of symbols like @, as python is not
 > perl. I'm still thinking of a better syntax/approach and is open for
 > suggestion.
 > What do you guys think?

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