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Tue Jan 13 05:33:52 EST 2004 · https://mail.python.org/pipermail/python-dev/2004-January/041977.html

ok. I was confused, sorry. :)

(I thought range() is evaluated somewhat like xrange() if it's in "for ...
in range()" )

I'll fix the patch so that expr is evaluated when generator expression is
created.

Regards,
Jiwon

----- Original Message ----- 
From: "Jeremy Hylton" <jeremy at alum.mit.edu>
To: "jiwon" <jiwon at softwise.co.kr>
Cc: "Michael Hudson" <mwh at python.net>; <python-dev at python.org>
Sent: Tuesday, January 13, 2004 12:01 PM
Subject: Re: [Python-Dev] [872326] generator expression implementation

> On Mon, 2004-01-12 at 21:53, jiwon wrote:
> > I'm the originator of the patch (SF patch 872326 - generator
expression).
> > If "immediate evaluation of expr in generator expression" means that
> > generator expression:
> >
> > (x for x in range())
> >
> > should be equivalent to:
> >
> > def __foo(_range=range()):
> >     for x in _range:
> >         yield x
> >
> > it seems to me that generator expression has almost no merit over list
> > comprehension.
> > I think the merit of generator is that it does not need to allocate all
the
> > memory that it needs in the initial time, right?
> > Or, am I confusing something? :)
>
> The range() case isn't all the interesting, because you wouldn't delay
> the memory allocation very long.  The first time an element was consumed
> from the g.expr it would materialize the whole list.  So you would only
> delay for the likely brief time between creating the generator and
> consuming its first element.
>
> There are two more common cases.  One is that the expression is already
> a fully materialized sequence.  Most of the examples in PEP 289 fall
> into this category:  You've got a list of objects and you want to sum
> their spam_score attributes.  The other case is that the expression is
> itself a generator, e.g.
>
>     dotproduct = sum(x*y for x,y in itertools.izip(x_vector, y_vector))
>
> For all these cases, the benefit is that the result list is never
> materialized, not that the input list is never materialized.
>
> Jeremy
>
>
>
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[Python-Dev] [872326] generator expression implementation