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Re: Split a list into two parts based on a filter?

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Chris Angelico wrote:

> On Tue, Jun 11, 2013 at 6:34 AM, Roy Smith <roy@panix.com> wrote:
>> new_songs = [s for s in songs if s.is_new()]
>> old_songs = [s for s in songs if not s.is_new()]
> 
> Hmm. Would this serve?
> 
> old_songs = songs[:]
> new_songs = [songs.remove(s) or s for s in songs if s.is_new()]
> 
> Python doesn't, AFAIK, have a "destructive remove and return"
> operation, and del is a statement rather than an expression/operator,
> but maybe this basic idea could be refined into something more useful.
> It guarantees to call is_new only once per song.
> 
> The iterator version strikes my fancy. Maybe this isn't of use to you,
> but I'm going to try my hand at making one anyway.

> >>> def iterpartition(pred,it):
>         """Partition an iterable based on a predicate.
> 
>         Returns two iterables, for those with pred False and those 
True."""
>         falses,trues=[],[]
>         it=iter(it)
>         def get_false():
>                 while True:
>                         if falses: yield falses.pop(0)
>                         else:
>                                 while True:
>                                         val=next(it)
>                                         if pred(val): trues.append(val)
>                                         else: break
>                                 yield val
>         def get_true():
>                 while True:
>                         if trues: yield trues.pop(0)
>                         else:
>                                 while True:
>                                         val=next(it)
>                                         if not pred(val): 
falses.append(val)
>                                         else: break
>                                 yield val
>         return get_false(),get_true()

An alternative implementation, based on itertools.tee:

import itertools

def partition(items, predicate=bool):
    a, b = itertools.tee((predicate(item), item) for item in items)
    return ((item for pred, item in a if not pred),
            (item for pred, item in b if pred))

if __name__ == "__main__":
    false, true = partition(range(10), lambda item: item % 2)
    print(list(false))
    print(list(true))

    def echo_odd(item):
        print("checking", item)
        return item % 2

    false, true = partition(range(10), echo_odd)

    print("FALSE", [next(false) for _ in range(3)])
    print("TRUE", next(true))
    print("FALSE", list(false))
    print("TRUE", list(true))

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Split a list into two parts based on a filter? roy@panix.com (Roy Smith) - 2013-06-10 16:34 -0400
  Re: Split a list into two parts based on a filter? Chris Angelico <rosuav@gmail.com> - 2013-06-11 08:50 +1000
  Re: Split a list into two parts based on a filter? Roel Schroeven <roel@roelschroeven.net> - 2013-06-11 00:50 +0200
    Re: Split a list into two parts based on a filter? Roy Smith <roy@panix.com> - 2013-06-11 00:11 -0400
      Re: Split a list into two parts based on a filter? Serhiy Storchaka <storchaka@gmail.com> - 2013-06-11 18:27 +0300
        Re: Split a list into two parts based on a filter? Roy Smith <roy@panix.com> - 2013-06-11 20:33 -0400
          Re: Split a list into two parts based on a filter? Phil Connell <pconnell@gmail.com> - 2013-06-12 07:32 +0100
            Re: Split a list into two parts based on a filter? Roy Smith <roy@panix.com> - 2013-06-12 07:39 -0400
              Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-12 12:51 +0100
              Re: Split a list into two parts based on a filter? Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-06-12 15:06 +0300
              Re: Split a list into two parts based on a filter? Terry Reedy <tjreedy@udel.edu> - 2013-06-12 14:07 -0400
          Re: Split a list into two parts based on a filter? Serhiy Storchaka <storchaka@gmail.com> - 2013-06-12 19:28 +0300
          Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-12 17:57 +0100
          Re: Split a list into two parts based on a filter? Terry Reedy <tjreedy@udel.edu> - 2013-06-12 14:47 -0400
          Re: Split a list into two parts based on a filter? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-06-13 10:43 +0100
  Re: Split a list into two parts based on a filter? Chris Rebert <clp2@rebertia.com> - 2013-06-10 16:03 -0700
  Re: Split a list into two parts based on a filter? Tim Chase <python.list@tim.thechases.com> - 2013-06-10 18:10 -0500
  Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-11 00:08 +0100
    Re: Split a list into two parts based on a filter? alex23 <wuwei23@gmail.com> - 2013-06-11 17:44 -0700
  Re: Split a list into two parts based on a filter? Chris Angelico <rosuav@gmail.com> - 2013-06-11 09:12 +1000
  Re: Split a list into two parts based on a filter? Peter Otten <__peter__@web.de> - 2013-06-11 02:11 +0200
  Re: Split a list into two parts based on a filter? Peter Otten <__peter__@web.de> - 2013-06-11 08:43 +0200
  Re: Split a list into two parts based on a filter? Jonas Geiregat <jonas@geiregat.org> - 2013-06-11 08:47 +0200
  Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-11 14:48 +0100
    Re: Split a list into two parts based on a filter? rusi <rustompmody@gmail.com> - 2013-06-11 09:37 -0700
      Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-11 18:05 +0100
        Re: Split a list into two parts based on a filter? rusi <rustompmody@gmail.com> - 2013-06-11 10:23 -0700
          Re: Split a list into two parts based on a filter? Chris Angelico <rosuav@gmail.com> - 2013-06-12 03:37 +1000
            Re: Split a list into two parts based on a filter? rusi <rustompmody@gmail.com> - 2013-06-11 11:13 -0700
          Re: Split a list into two parts based on a filter? Fábio Santos <fabiosantosart@gmail.com> - 2013-06-11 19:05 +0100
  Re: Split a list into two parts based on a filter? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-06-11 15:22 +0100
  Re: Split a list into two parts based on a filter? Serhiy Storchaka <storchaka@gmail.com> - 2013-06-11 18:28 +0300
  Re: Split a list into two parts based on a filter? Chris Angelico <rosuav@gmail.com> - 2013-06-12 03:28 +1000
    Re: Split a list into two parts based on a filter? Roy Smith <roy@panix.com> - 2013-06-11 20:12 -0400
  Re: Split a list into two parts based on a filter? Peter Otten <__peter__@web.de> - 2013-06-11 20:13 +0200
  Re: Split a list into two parts based on a filter? Peter Otten <__peter__@web.de> - 2013-06-11 20:18 +0200
  Re: Split a list into two parts based on a filter? Chris Angelico <rosuav@gmail.com> - 2013-06-12 04:27 +1000
  Re: Split a list into two parts based on a filter? Roel Schroeven <roel@roelschroeven.net> - 2013-06-11 22:22 +0200

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