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Groups > comp.lang.python > #27194 > unrolled thread

set and dict iteration

Started byAaron Brady <castironpi@gmail.com>
First post2012-08-16 11:00 -0700
Last post2012-08-17 10:47 -0700
Articles 5 on this page of 45 — 13 participants

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  set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-16 11:00 -0700
    Re: set and dict iteration Dave Angel <d@davea.name> - 2012-08-16 15:49 -0400
      Re: set and dict iteration Paul Rubin <no.email@nospam.invalid> - 2012-08-16 14:26 -0700
        Re: set and dict iteration Dave Angel <davea@dejaviewphoto.com> - 2012-08-16 19:11 -0400
          Re: set and dict iteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-17 02:24 +0000
            Re: set and dict iteration Paul Rubin <no.email@nospam.invalid> - 2012-08-16 19:30 -0700
              Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-17 11:11 -0700
            Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-17 11:37 -0700
              Re: set and dict iteration Chris Angelico <rosuav@gmail.com> - 2012-08-18 07:57 +1000
                Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-18 13:29 -0700
                Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-18 13:29 -0700
                  Re: set and dict iteration MRAB <python@mrabarnett.plus.com> - 2012-08-18 23:14 +0100
                    Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-18 19:28 -0700
                      Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-23 09:49 -0700
                        Re: set and dict iteration Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-27 13:17 -0600
                          Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-02 10:43 -0700
                            Re: set and dict iteration Ian Kelly <ian.g.kelly@gmail.com> - 2012-09-03 13:29 -0600
                              Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-03 13:04 -0700
                                Re: set and dict iteration Dave Angel <d@davea.name> - 2012-09-03 16:27 -0400
                                  Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-03 17:24 -0700
                                  Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-03 17:24 -0700
                              Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-03 13:04 -0700
                                Re: set and dict iteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-04 01:26 +0000
                                  Re: set and dict iteration Dave Angel <d@davea.name> - 2012-09-03 21:50 -0400
                                    Re: set and dict iteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-04 01:59 +0000
                                      Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-08 08:42 -0700
                          Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-02 10:43 -0700
                        Re: set and dict iteration Serhiy Storchaka <storchaka@gmail.com> - 2012-09-04 00:46 +0300
                      Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-23 09:49 -0700
                        Re: set and dict iteration Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-08-23 18:11 +0000
                          Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-27 11:10 -0700
                          Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-09-08 15:07 -0700
                    Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-18 19:28 -0700
                    Re: set and dict iteration Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-09-08 22:06 +0200
                      Re: set and dict iteration Hans Mulder <hansmu@xs4all.nl> - 2012-09-08 23:18 +0200
                      Re: set and dict iteration MRAB <python@mrabarnett.plus.com> - 2012-09-08 22:22 +0100
        Re: set and dict iteration Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 17:43 -0600
          Re: set and dict iteration Paul Rubin <no.email@nospam.invalid> - 2012-08-16 18:01 -0700
            Re: set and dict iteration Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-08-17 13:16 +0200
            Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-17 11:03 -0700
            Re: set and dict iteration 88888 Dihedral <dihedral88888@googlemail.com> - 2012-09-10 13:14 -0700
    Re: set and dict iteration Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 16:55 -0600
    Re: set and dict iteration Ian Kelly <ian.g.kelly@gmail.com> - 2012-08-16 17:07 -0600
      Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-17 10:47 -0700
      Re: set and dict iteration Aaron Brady <castironpi@gmail.com> - 2012-08-17 10:47 -0700

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#28850

From88888 Dihedral <dihedral88888@googlemail.com>
Date2012-09-10 13:14 -0700
Message-ID<753e4641-0c61-42c0-a87d-7826be736aa6@googlegroups.com>
In reply to#27216
Paul Rubin於 2012年8月17日星期五UTC+8上午9時01分39秒寫道：
> Ian Kelly <ian.g.kelly@gmail.com> writes:
> 
> > With regard to key insertion and deletion while iterating over a dict
> 
> > or set, though, there is just no good reason to be doing that
> 
> > (especially as the result is very implementation-specific), and I
> 
> > wouldn't mind a more complete low-level check against it as long as
> 
> > it's not too expensive (which is not clearly the case with the current
> 
> > suggestion at all).
> 
> 
> 
> One possible approach is to freeze the dictionary against modification
> 
> while any iterator is open on it.  You could keep a count of active
> 
> iterators in the dict structure, adjusting it whenever an iterator is
> 
> created or closed/destroyed.

If there is only one iterator of a frozen dictionary,
then nothing is saved.

But if there are manny iterators based on the same frozen dictionary, 
this approach saves a lot.

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#27211

FromIan Kelly <ian.g.kelly@gmail.com>
Date2012-08-16 16:55 -0600
Message-ID<mailman.3402.1345157765.4697.python-list@python.org>
In reply to#27194
On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi@gmail.com> wrote:
> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:

It can be more than just the new element.  For example, here the
entire set is repeated (Python 3.2):

>>> s = set(range(8, 13))
>>> it = iter(s)
>>> from itertools import islice
>>> list(islice(it, 5))  # avoid exhausting the iterator
[8, 9, 10, 11, 12]
>>> s.add(13)
>>> s.remove(13)
>>> list(it)
[8, 9, 10, 11, 12]

This occurs because the addition of the sixth item triggers a resize
of the underlying hash table, and the existing items, which were
originally in slots 0-4, are now in slots 8-12.

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#27212

FromIan Kelly <ian.g.kelly@gmail.com>
Date2012-08-16 17:07 -0600
Message-ID<mailman.3403.1345158493.4697.python-list@python.org>
In reply to#27194
On Thu, Aug 16, 2012 at 4:55 PM, Ian Kelly <ian.g.kelly@gmail.com> wrote:
> On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi@gmail.com> wrote:
>> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:
>
> It can be more than just the new element.  For example, here the
> entire set is repeated (Python 3.2):
>
>>>> s = set(range(8, 13))
>>>> it = iter(s)
>>>> from itertools import islice
>>>> list(islice(it, 5))  # avoid exhausting the iterator
> [8, 9, 10, 11, 12]
>>>> s.add(13)
>>>> s.remove(13)
>>>> list(it)
> [8, 9, 10, 11, 12]
>
> This occurs because the addition of the sixth item triggers a resize
> of the underlying hash table, and the existing items, which were
> originally in slots 0-4, are now in slots 8-12.

Another curious example:

>>> s = set(range(8, 48, 8))
>>> s
{8, 16, 40, 24, 32}
>>> it = iter(s)
>>> from itertools import islice
>>> list(islice(it, 4))
[8, 16, 40, 24]
>>> s.add(48)
>>> s.remove(48)
>>> list(it)
[8, 16, 40, 24]

Hey, what happened to 32?

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#27249

FromAaron Brady <castironpi@gmail.com>
Date2012-08-17 10:47 -0700
Message-ID<mailman.3420.1345226405.4697.python-list@python.org>
In reply to#27212
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:
> 
> > On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi@gmail.com> wrote:
> 
> >> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:
> 
> >
> 
> > It can be more than just the new element.  For example, here the
> 
> > entire set is repeated (Python 3.2):
> 
> >
> 
> >>>> s = set(range(8, 13))
> 
> >>>> it = iter(s)
> 
> >>>> from itertools import islice
> 
> >>>> list(islice(it, 5))  # avoid exhausting the iterator
> 
> > [8, 9, 10, 11, 12]
> 
> >>>> s.add(13)
> 
> >>>> s.remove(13)
> 
> >>>> list(it)
> 
> > [8, 9, 10, 11, 12]
> 
> >
> 
> > This occurs because the addition of the sixth item triggers a resize
> 
> > of the underlying hash table, and the existing items, which were
> 
> > originally in slots 0-4, are now in slots 8-12.
> 
> 
> 
> Another curious example:
> 
> 
> 
> >>> s = set(range(8, 48, 8))
> 
> >>> s
> 
> {8, 16, 40, 24, 32}
> 
> >>> it = iter(s)
> 
> >>> from itertools import islice
> 
> >>> list(islice(it, 4))
> 
> [8, 16, 40, 24]
> 
> >>> s.add(48)
> 
> >>> s.remove(48)
> 
> >>> list(it)
> 
> [8, 16, 40, 24]
> 
> 
> 
> Hey, what happened to 32?

Good examples.  The former occurs without the 'islice' as well.

s= set( range( 8, 13 ) ) 
it= iter( s ) 
print( [ next( it ) for _ in range( 5 ) ] )
s.add( 13 ) 
s.remove( 13 ) 
print( [ next( it ) for _ in range( 5 ) ] )

Output:

[8, 9, 10, 11, 12]
[8, 9, 10, 11, 12]

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#27251

FromAaron Brady <castironpi@gmail.com>
Date2012-08-17 10:47 -0700
Message-ID<ee99dd41-94c5-4a88-9b2f-933f4e578739@googlegroups.com>
In reply to#27212
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:
> 
> > On Thu, Aug 16, 2012 at 12:00 PM, Aaron Brady <castironpi@gmail.com> wrote:
> 
> >> The inconsistency is, if we remove an element from a set and add another during iteration, the new element might appear later in the iteration, and might not, depending on the hash code; therefore comparing the size of the set between iterations isn't adequate.  Example:
> 
> >
> 
> > It can be more than just the new element.  For example, here the
> 
> > entire set is repeated (Python 3.2):
> 
> >
> 
> >>>> s = set(range(8, 13))
> 
> >>>> it = iter(s)
> 
> >>>> from itertools import islice
> 
> >>>> list(islice(it, 5))  # avoid exhausting the iterator
> 
> > [8, 9, 10, 11, 12]
> 
> >>>> s.add(13)
> 
> >>>> s.remove(13)
> 
> >>>> list(it)
> 
> > [8, 9, 10, 11, 12]
> 
> >
> 
> > This occurs because the addition of the sixth item triggers a resize
> 
> > of the underlying hash table, and the existing items, which were
> 
> > originally in slots 0-4, are now in slots 8-12.
> 
> 
> 
> Another curious example:
> 
> 
> 
> >>> s = set(range(8, 48, 8))
> 
> >>> s
> 
> {8, 16, 40, 24, 32}
> 
> >>> it = iter(s)
> 
> >>> from itertools import islice
> 
> >>> list(islice(it, 4))
> 
> [8, 16, 40, 24]
> 
> >>> s.add(48)
> 
> >>> s.remove(48)
> 
> >>> list(it)
> 
> [8, 16, 40, 24]
> 
> 
> 
> Hey, what happened to 32?

Good examples.  The former occurs without the 'islice' as well.

s= set( range( 8, 13 ) ) 
it= iter( s ) 
print( [ next( it ) for _ in range( 5 ) ] )
s.add( 13 ) 
s.remove( 13 ) 
print( [ next( it ) for _ in range( 5 ) ] )

Output:

[8, 9, 10, 11, 12]
[8, 9, 10, 11, 12]

[toc] | [prev] | [standalone]

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