Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #86980

Re: Is nan in (nan,) correct?

Path csiph.com!usenet.pasdenom.info!goblin2!goblin.stu.neva.ru!newsfeed.xs4all.nl!newsfeed2a.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Return-Path <ethan@stoneleaf.us>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.004
X-Spam-Evidence '*H*': 0.99; '*S*': 0.00; 'operator': 0.03; 'interpreter': 0.05; '"""': 0.07; 'assuming': 0.09; 'correct,': 0.09; 'from:addr:ethan': 0.09; 'from:addr:stoneleaf.us': 0.09; 'from:name:ethan furman': 0.09; 'here?': 0.09; 'message- id:@stoneleaf.us': 0.09; '~ethan~': 0.09; 'python': 0.11; 'behave': 0.16; 'bonus.': 0.16; 'container,': 0.16; 'container.': 0.16; 'filename:fname piece:signature': 0.16; 'finney': 0.16; 'implies': 0.16; 'nan': 0.16; "type's": 0.16; 'wrote:': 0.18; '>>>': 0.22; 'header:User-Agent:1': 0.23; '(or': 0.24; 'specially': 0.26; 'values': 0.27; 'header:In-Reply-To:1': 0.27; 'point': 0.28; 'correct': 0.29; '[1]': 0.29; 'along': 0.30; 'membership': 0.31; 'directly,': 0.31; 'steven': 0.31; 'writes:': 0.31; 'class': 0.32; 'skip:_ 10': 0.34; 'problem': 0.35; 'subject: (': 0.35; 'no,': 0.35; 'objects': 0.35; 'but': 0.35; 'false': 0.36; 'subject:?': 0.36; 'example,': 0.37; 'being': 0.38; 'ben': 0.38; 'needed': 0.38; 'to:addr:python-list': 0.38; 'issue': 0.38; 'pm,': 0.38; 'does': 0.39; 'to:addr:python.org': 0.39; 'how': 0.40; 'is.': 0.60; 'received:173': 0.61; 'skip:n 10': 0.64; 'skip:\xe2 10': 0.65; 'equals': 0.68; 'optimized': 0.68; '8bit%:43': 0.74; 'other.': 0.75; 'behavior': 0.77; 'as:': 0.81; '"spam"': 0.84; 'ethan': 0.84; 'furman': 0.84; 'optimisation': 0.84
Date Thu, 05 Mar 2015 19:44:56 -0800
From Ethan Furman <ethan@stoneleaf.us>
User-Agent Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0
MIME-Version 1.0
To python-list@python.org
Subject Re: Is nan in (nan,) correct?
References <mailman.67.1425594415.21433.python-list@python.org> <54f90c53$0$12994$c3e8da3$5496439d@news.astraweb.com> <858ufazvff.fsf@benfinney.id.au> <54F91C70.6020303@stoneleaf.us> <85wq2uyffs.fsf@benfinney.id.au>
In-Reply-To <85wq2uyffs.fsf@benfinney.id.au>
Content-Type multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="xEMNxtaUcJUSV79HcQbtLMs4tcatvMbKA"
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.19
Precedence list
List-Id General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe <https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive <http://mail.python.org/pipermail/python-list/>
List-Post <mailto:python-list@python.org>
List-Help <mailto:python-list-request@python.org?subject=help>
List-Subscribe <https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups comp.lang.python
Message-ID <mailman.82.1425613536.21433.python-list@python.org> (permalink)
Lines 91
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1425613536 news.xs4all.nl 2958 [2001:888:2000:d::a6]:56219
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:86980

Show key headers only | View raw

[Multipart message — attachments visible in raw view] - view raw

On 03/05/2015 07:26 PM, Ben Finney wrote:
> Ethan Furman <ethan@stoneleaf.us> writes:
> 
>> On 03/05/2015 06:55 PM, Ben Finney wrote:
>>
>>>     class NullType(object):
>>>         """ A type whose value never equals any other.
>>>
>>>             This type's values will behave correctly when tested for
>>>             membership in a collection::
>>>
>>>                 >>> foo = NullType()
>>>                 >>> bar = NullType()
>>>                 >>> foo is foo
>>>                 True
>>>                 >>> foo is bar
>>>                 False
>>>                 >>> foo == foo
>>>                 False
>>>                 >>> foo == bar
>>>                 False
>>>                 >>> quux = [foo, "spam"]
>>>                 >>> "spam" in quux
>>>                 True
>>>                 >>> foo in quux
>>>                 True
>>
>> Did you mean False here?  Because True is current behavior.
> 
> Isn't the point at issue that the Python interpreter *may* optimise by
> assuming ‘is implies equality’, so the ‘in’ operator can fail if that
> assumption is false?

No, it's not a /may/, it's a /does/, and that it can be optimized is a bonus.

> I thought the problem was that types with custom behaviour, as with the
> ‘NullType’ example, needed to deal specially with the ‘is implies
> equality’ optimisation Steven explained.

The NaN-type objects cannot deal with it directly, as that behavior is in the container.

> If that's the correct behaviour, and we can *depend* on it being
> correct, then I don't see what the problem is.

Well, we can depend on it for native Python types -- but if you write your own container, along with your own
__contains__, then you might unwittingly do  `for item in self.container: if item == target:  return True` and then NaN
(or NullType, or what-have-you) would not work "correctly" [1] for your container.

--
~Ethan~

[1] Otherwise known as: how Python does it.

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 17:26 -0500
  Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:11 -0800
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:20 +1100
      Re: Is nan in (nan,) correct? sohcahtoa82@gmail.com - 2015-03-05 15:27 -0800
        Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 10:39 +1100
        Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:40 +1100
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 10:25 +1100
  Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-06 13:09 +1100
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 13:55 +1100
    Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:18 -0800
    Re: Is nan in (nan,) correct? Ben Finney <ben+python@benfinney.id.au> - 2015-03-06 14:26 +1100
    Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-05 19:44 -0800
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 14:49 +1100
    Re: Is nan in (nan,) correct? random832@fastmail.us - 2015-03-05 23:37 -0500
      Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 04:07 +1100
    Re: Is nan in (nan,) correct? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-03-06 04:46 +0000
  Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 01:50 -0800
    Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-06 21:01 +1100
      Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 02:22 -0800
        Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:59 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 10:04 -0800
            Re: Is nan in (nan,) correct? Ethan Furman <ethan@stoneleaf.us> - 2015-03-06 10:16 -0800
      Re: Is nan in (nan,) correct? Grant Edwards <invalid@invalid.invalid> - 2015-03-06 15:34 +0000
    Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 03:43 +1100
      Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:04 -0800
        Re: Is nan in (nan,) correct? Chris Angelico <rosuav@gmail.com> - 2015-03-07 04:16 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 09:36 -0800
        Re: Is nan in (nan,) correct? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-07 10:33 +1100
          Re: Is nan in (nan,) correct? Rustom Mody <rustompmody@gmail.com> - 2015-03-06 18:37 -0800

csiph-web