need help on need help on generator...

[Francis Girard]

Hi,

I recently read David Mertz (IBM DeveloperWorks) about generators and got 
excited about using lazy constructs in my Python programming.

But besides the fact that generators are either produced with the new "yield" 
reserved word or by defining the __new__ method in a class definition, I 
don't know much about them.

In particular, I don't know what Python constructs does generate a generator. 
I know this is now the case for reading lines in a file or with the new 
"iterator" package. But what else ? Does Craig Ringer answer mean that list 
comprehensions are lazy ? Where can I find a comprehensive list of all the 
lazy constructions built in Python ? (I think that to easily distinguish lazy 
from strict constructs is an absolute programmer need -- otherwise you always 
end up wondering when is it that code is actually executed like in Haskell).

Thank you

Francis Girard
FRANCE

Le vendredi 21 Janvier 2005 15:38, Craig Ringer a écrit :
> On Fri, 2005-01-21 at 17:14 +0300, Denis S. Otkidach wrote:
> > On 21 Jan 2005 05:58:03 -0800
> >
> > joh12005 at yahoo.fr (Joh) wrote:
> > > i'm trying to understand how i could build following consecutive sets
> > > from a root one using generator :
> > >
> > > l = [1,2,3,4]
> > >
> > > would like to produce :
> > >
> > > [1,2], [2,3], [3,4], [1,2,3], [2,3,4]
> > >
> > >>> def consecutive_sets(l):
> >
> > ...     for i in xrange(2, len(l)):
> > ...         for j in xrange(0, len(l)-i+1):
> > ...             yield l[j:j+i]
>
> Since you have a much faster brain than I (though I ended up with
> exactly the same thing barring variable names) and beat me to posting
> the answer, I'll post the inevitable awful generator expression version
> instead:
>
> consecutive_sets = ( x[offset:offset+subset_size]
>                      for subset_size in xrange(2, len(x))
>                      for offset in xrange(0, len(x) + 1 - subset_size) )
>
> --
> Craig Ringer