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Re: 'is not' or '!='

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Skip Montanaro <skip@pobox.com>:

> 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.

Object identity stirs a lot of passions on this forum. I'm guessing the
reason is that it is not defined very clearly (<URL:
https://docs.python.org/3/library/functions.html#id>):

   id(object)

       Return the “identity” of an object. This is an integer which is
       guaranteed to be unique and constant for this object during its
       lifetime. Two objects with non-overlapping lifetimes may have the
       same id() value.

       CPython implementation detail: This is the address of the object
       in memory.


The "is" relation can be defined trivially through the id() function:

   X is Y iff id(X) == id(Y)

What remains is the characterization of the (total) id() function. For
example, we can stipulate that:

   X = Y
   assert(id(X) == id(Y))
   # assignment preserves identity

(assuming X and Y are not modified in other threads or signal handlers).

We know further that:

   i = id(X)
   time.sleep(T)
   assert(i == id(X))
   # the identity does not change over time

   def f(x, y):
       return id(x) == id(y)
   assert(f(X, X))
   # parameter passing preserves the identity

   def f(x):
       return x
   assert(id(f(X) == id(X)))
   # the return statement preserves the identity

   assert(id((X0, X1, ..., X)[k]) == id(Xk))
   # tuple composition and projection preserve the identity

   i = id(X)
   X.Y
   assert(i == id(X))
   # reference does not change the identity

and so on. The nice thing about these kinds of formal definitions is
that they make no metaphysical reference to "objects" or "lifetime" (or
the CPython implementation). They can also be converted into
implementation conformance statements and test cases.

A much tougher task is to define when id(X) != id(Y). After all, all of
the above would be satisfied by this implementation of id():

   def id(x):
       return 0

The nonidentity will probably have to be defined separately for each
builtin datatype. For example, for integers and strings we know only
that:

   assert(X == Y or id(X) != id(Y))
   # inequality implies nonidentity


Marko

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Thread

'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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