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

Mathias Panzenböck grosser.meister.morti at gmx.net
Fri Jun 19 21:56:44 CEST 2009

Chris Rebert wrote:
 > On Fri, Jun 19, 2009 at 10:54 AM, Mathias
 > Panzenböck<grosser.meister.morti at gmx.net> wrote:
 >> 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?
 > Comprehensions and generator expressions already give us most of the
 > LINQ functionality. Add in `list()` and the ability to `.sort()` lists
 > with a `key` argument and you have the entire thing, except for the
 > one corner case being discussed. Unless I've overlooked something...

Yes: With LINQ its possible to build a query object out of an LINQ expression 
instead of evaluating it eagerly. This is used primarily to generate SQL code 
while still using syntax native to the host language (C#) and preserving type 
safety (ok the later cannot be done in python).


More information about the Python-ideas mailing list