Split a list into two parts based on a filter?

[Fábio Santos]

On 12 Jun 2013 12:43, "Roy Smith" <roy at panix.com> wrote:
>
> In article <mailman.3050.1371018754.3114.python-list at python.org>,
>  Phil Connell <pconnell at gmail.com> wrote:
>
> > > Well, continuing down this somewhat bizarre path:
> > >
> > > new_songs, old_songs = [], []
> > > itertools.takewhile(
> > >     lambda x: True,
> > >     (new_songs if s.is_new() else old_songs).append(s) for s in songs)
> > >     )
> > >
> > > I'm not sure I got the syntax exactly right, but the idea is anything
> > > that will iterate over a generator expression.  That at least gets rid
> > > of the memory requirement to hold the throw-away list :-)
> >
> > You could equivalently pass the generator to deque() with maxlen=0 -
this
> > consumes the iterator with constant memory usage.
> >
> > We are of course firmly in the twilight zone at this point (although
this
> > can be a useful technique in general).
>
> We've been in the twilight zone for a while.  That's when the fun
> starts.  But, somewhat more seriously, I wonder what, exactly, it is
> that freaks people out about:
>
> >>>> [(new_songs if s.is_new() else old_songs).append(s) for s in songs]
>
> Clearly, it's not the fact that it build and immediately discards a
> list, because that concern is addressed with the generator hack, and I
> think everybody (myself included) agrees that's just horrible.

I think someone has already said that the problem is that you're creating
the list comprehension just for the side effects.

> Or, is it the use of the conditional to create the target for append()?

I particularly liked that part.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-list/attachments/20130612/9ef42384/attachment.html>