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Re: 'is not' or '!='

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On Tue, Aug 19, 2014 at 7:42 AM, Martin S <shieldfire@gmail.com> wrote:
>> For example, in CPython 3.4.1:
>>>>> (254 + 3) is 257
>> False
>>>>> (254 + 3) == 257
>> True
>>>>> ('asd' + '@sd') is 'asd@sd'
>> False
>>>>> ('asd' + '@sd') == 'asd@sd'
>> True
>
> Now you have managed to confuse this newbie: What would a valid
> "is-example" look like?

Perhaps adding to the confusion:

>>> (4 + 3) is 7
True

The use of "is" or "is not" is the right thing to do when the object
of the comparison is known to be a singleton. That is true for None.
(I suspect it's true for True and False as well, though for historical
and idiomatic reasons "x is True" is never used.) It would also be
true if you created a sentinel object like this:

SENTINEL = []

then used it to represent "no valid value" in situations where you
can't otherwise assume some value like None is outside the domain of
values:

if mything is not SENTINEL:
    # do whatever is necessary to initialize mything
    ...

The case of "(4 + 3) is 7" simply exposes an optimization in the
implementation of integers in CPython (the small integer cache), and
*must not be relied on*.

Note that

SENTINEL = ()

would be a mistake if () is in the domain of your data, since the
empty tuple is itself a singleton. That is a CPython implementation
detail you shouldn't rely on:

>>> x = ()
>>>
>>> y = (1,2,3)
>>>
>>> z = y[0:0]
>>> z
()
>>> x
()
>>> x == z
True
>>> x is z
True

That's why the first definition of SENTINEL uses the empty list. I can
guarantee that there is no other object in the system which "is" that
particular object.

(Strings which look like identifiers are also "interned" so there is
only one copy in memory. I suspect that's why Chris's string example
included an "@" - to defeat that optimization.)

The CPython runtime is under no obligation to retain these
optimizations. Relying on them is a bug. OTOH, CPython guarantees
there is only one None object in the system.

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'is not' or '!=' "ElChino" <elchino@cnn.cn> - 2014-08-18 22:35 +0200
  Re: 'is not' or '!=' MRAB <python@mrabarnett.plus.com> - 2014-08-18 21:53 +0100
  Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-18 23:53 +0300
    Re: 'is not' or '!=' "ElChino" <elchino@cnn.cn> - 2014-08-18 22:58 +0200
      Re: 'is not' or '!=' Ethan Furman <ethan@stoneleaf.us> - 2014-08-18 14:42 -0700
      Re: 'is not' or '!=' Ethan Furman <ethan@stoneleaf.us> - 2014-08-18 15:19 -0700
      Re: 'is not' or '!=' Chris Kaynor <ckaynor@zindagigames.com> - 2014-08-18 15:04 -0700
        Re: 'is not' or '!=' Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2014-08-19 11:49 +0200
      Re: 'is not' or '!=' Martin S <shieldfire@gmail.com> - 2014-08-19 14:42 +0200
      Re: 'is not' or '!=' Skip Montanaro <skip@pobox.com> - 2014-08-19 08:12 -0500
        Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-19 20:29 +0300
          Re: 'is not' or '!=' Tim Chase <python.list@tim.thechases.com> - 2014-08-19 12:34 -0500
            Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-19 21:18 +0300
          Re: 'is not' or '!=' Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-20 04:26 +1000
            Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-19 21:52 +0300
              Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-20 00:00 +0300
                Re: 'is not' or '!=' Ian Kelly <ian.g.kelly@gmail.com> - 2014-08-19 16:21 -0600
                Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-20 02:02 +0300
                Re: 'is not' or '!=' Rustom Mody <rustompmody@gmail.com> - 2014-08-20 22:03 -0700
                Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-21 08:56 +0300
            Re: 'is not' or '!=' Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-08-19 20:05 +0100
            Re: 'is not' or '!=' Terry Reedy <tjreedy@udel.edu> - 2014-08-19 18:23 -0400
      Re: 'is not' or '!=' Ian Kelly <ian.g.kelly@gmail.com> - 2014-08-19 10:42 -0600
        Re: 'is not' or '!=' Marko Rauhamaa <marko@pacujo.net> - 2014-08-19 20:36 +0300
      Re: 'is not' or '!=' Skip Montanaro <skip@pobox.com> - 2014-08-19 13:44 -0500
    Re: 'is not' or '!=' alister <alister.nospam.ware@ntlworld.com> - 2014-08-19 09:35 +0000
    Re: 'is not' or '!=' Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2014-08-19 11:53 +0200
  Re: 'is not' or '!=' Rustom Mody <rustompmody@gmail.com> - 2014-08-18 19:29 -0700
    Re: 'is not' or '!=' Steven D'Aprano <steve@pearwood.info> - 2014-08-19 08:36 +0000
      Re: 'is not' or '!=' Chris Angelico <rosuav@gmail.com> - 2014-08-19 18:47 +1000
      Re: 'is not' or '!=' Tim Chase <python.list@tim.thechases.com> - 2014-08-19 05:36 -0500
      Re: 'is not' or '!=' Ben Finney <ben+python@benfinney.id.au> - 2014-08-20 09:24 +1000
        Re: 'is not' or '!=' Rustom Mody <rustompmody@gmail.com> - 2014-08-19 21:01 -0700
          Re: 'is not' or '!=' Steven D'Aprano <steve@pearwood.info> - 2014-08-20 04:59 +0000
            Re: 'is not' or '!=' Rustom Mody <rustompmody@gmail.com> - 2014-08-19 22:25 -0700
              Re: 'is not' or '!=' Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-20 21:05 +1000
                Re: 'is not' or '!=' Chris Angelico <rosuav@gmail.com> - 2014-08-20 21:17 +1000
                Re: 'is not' or '!=' Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2014-08-21 20:31 +0200
                Re: 'is not' or '!=' Tim Chase <python.list@tim.thechases.com> - 2014-08-20 06:26 -0500
                Re: 'is not' or '!=' Chris Angelico <rosuav@gmail.com> - 2014-08-20 21:33 +1000
                Re: 'is not' or '!=' Robert Kern <robert.kern@gmail.com> - 2014-08-20 12:40 +0100
                Re: 'is not' or '!=' Rustom Mody <rustompmody@gmail.com> - 2014-08-20 05:01 -0700
  Re: 'is not' or '!=' Dan Stromberg <drsalists@gmail.com> - 2014-08-21 12:24 -0700
    Re: 'is not' or '!=' "ElChino" <elchino@cnn.cn> - 2014-08-21 21:42 +0200

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