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Groups > comp.lang.python > #27211

Re: set and dict iteration

References <b8dd3aca-2a87-4124-ad6e-66a8720af99a@googlegroups.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date 2012-08-16 16:55 -0600
Subject Re: set and dict iteration
Newsgroups comp.lang.python
Message-ID <mailman.3402.1345157765.4697.python-list@python.org> (permalink)

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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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Thread

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 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> - 2012-08-18 19:28 -0700
      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 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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