Path: csiph.com!usenet.pasdenom.info!dedibox.gegeweb.org!gegeweb.eu!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Newsgroups: comp.lang.python
Date: Fri, 17 Aug 2012 10:47:16 -0700 (PDT)
In-Reply-To: <mailman.3403.1345158493.4697.python-list@python.org>
Complaints-To: groups-abuse@google.com
Injection-Info: glegroupsg2000goo.googlegroups.com; posting-host=67.184.78.189; posting-account=MQ3pigoAAACeFzUFjVAePnOjOJMNlvq9
References: <b8dd3aca-2a87-4124-ad6e-66a8720af99a@googlegroups.com> <CALwzidkR2hEOu7Y4HhUti5S4HJqAtsDf_V6h4RHt-v094T4_Ug@mail.gmail.com> <mailman.3403.1345158493.4697.python-list@python.org>
User-Agent: G2/1.0
MIME-Version: 1.0
Subject: Re: set and dict iteration
From: Aaron Brady <castironpi@gmail.com>
To: comp.lang.python@googlegroups.com
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
Cc: python-list@python.org, Aaron Brady <castironpi@gmail.com>
Precedence: list
Message-ID: <mailman.3420.1345226405.4697.python-list@python.org>
Lines: 90
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:27249

On Thursday, August 16, 2012 6:07:40 PM UTC-5, Ian wrote:
> On Thu, Aug 16, 2012 at 4:55 PM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
>=20
> > On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi@gmail.com> wr=
ote:
>=20
> >> The inconsistency is, if we remove an element from a set and add anoth=
er during iteration, the new element might appear later in the iteration, a=
nd might not, depending on the hash code; therefore comparing the size of t=
he set between iterations isn't adequate.  Example:
>=20
> >
>=20
> > It can be more than just the new element.  For example, here the
>=20
> > entire set is repeated (Python 3.2):
>=20
> >
>=20
> >>>> s =3D set(range(8, 13))
>=20
> >>>> it =3D iter(s)
>=20
> >>>> from itertools import islice
>=20
> >>>> list(islice(it, 5))  # avoid exhausting the iterator
>=20
> > [8, 9, 10, 11, 12]
>=20
> >>>> s.add(13)
>=20
> >>>> s.remove(13)
>=20
> >>>> list(it)
>=20
> > [8, 9, 10, 11, 12]
>=20
> >
>=20
> > This occurs because the addition of the sixth item triggers a resize
>=20
> > of the underlying hash table, and the existing items, which were
>=20
> > originally in slots 0-4, are now in slots 8-12.
>=20
>=20
>=20
> Another curious example:
>=20
>=20
>=20
> >>> s =3D set(range(8, 48, 8))
>=20
> >>> s
>=20
> {8, 16, 40, 24, 32}
>=20
> >>> it =3D iter(s)
>=20
> >>> from itertools import islice
>=20
> >>> list(islice(it, 4))
>=20
> [8, 16, 40, 24]
>=20
> >>> s.add(48)
>=20
> >>> s.remove(48)
>=20
> >>> list(it)
>=20
> [8, 16, 40, 24]
>=20
>=20
>=20
> Hey, what happened to 32?

Good examples.  The former occurs without the 'islice' as well.

s=3D set( range( 8, 13 ) )=20
it=3D iter( s )=20
print( [ next( it ) for _ in range( 5 ) ] )
s.add( 13 )=20
s.remove( 13 )=20
print( [ next( it ) for _ in range( 5 ) ] )

Output:

[8, 9, 10, 11, 12]
[8, 9, 10, 11, 12]