Sequence splitting

[Paul Rubin]

Brad <schickb at gmail.com> writes:

> On Jul 2, 8:14 pm, Paul Rubin <http://phr...@NOSPAM.invalid> wrote:
> > schickb <schi... at gmail.com> writes:
> > > def split(seq, func=None):
> > >     if func is None:
> > >         func = bool
> > >     t, f = [], []
> > >     for item in seq:
> > >         if func(item):
> > >             t.append(item)
> > >         else:
> > >             f.append(item)
> > >     return (t, f)
> >
> > untested:
> >
> >    def split(seq, func=bool):
> >       xs = zip(seq, itertools.imap(func, seq))
> >       t = list(x for (x,y) in xs if y)
> >       f = list(x for (x,y) in xs if not y)
> >       return (t, f)
> 
> In my testing that is 3.5x slower than the original solution (and less
> clear imo). I fixed my version to take a bool default. Either way, I'm
> not really looking for additional ways to do this in Python unless
> I've totally missed something. What I am considering is writing it in
> C, much like filter.

I'm a little skeptical that the C version will help much, if it's
evaluating a python function at every list element.  Here's a variant
of your version:

 def split(seq, func=bool):
     t, f = [], []
     ta, fa = t.append, f.append
     for item in seq:
         (ta if func(item) else fa)(item)
     return (t, f)

This avoids some dict lookups and copying.  I wonder if that helps
significantly.