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

Groups > comp.lang.python > #69818

Re: Mutable objects inside tuples - good or bad?

Show all headers | View raw

On Tue, Apr 8, 2014 at 1:26 AM, Paul Kölle <pkoelle@gmail.com> wrote:
> It seems a tuple's immutability is debatable, or is this another instance of
> the small-integer-reuse-implementation-detail-artifact?
>
> Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48)
> [GCC 4.4.5] on linux2
>
> Type "help", "copyright", "credits" or "license" for more information.
>>>> a = ([1,2],[3,4])
>>>> b = a
>>>> a is b
> True
>>>> a == b
> True
>>>> c = (1,2,3)
>>>> d = (1,2,3)
>>>> c is d
> False
>>>> c == d
> True

That's nothing to do with mutability or reuse. With a and b, you
assigned one to be the same as the other, so they are by definition
identical (and equal; tuples assume that identity implies equality,
even though that may not be true of their elements). With c and d, you
assigned separate tuples, so they're allowed to be separate objects.
I'm not sure if they're allowed to be constant-folded, but CPython
apparently isn't doing so. They are still equal, though; they contain
equal elements, ergo they are equal. (Note that (1, 2, 3) and (1.0,
2.0, 3.0) are equal, but they obviously can't be identical any more
than "1 is 1.0" can ever be True.)

ChrisA

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

Thread

Mutable objects inside tuples - good or bad? John Ladasky <john_ladasky@sbcglobal.net> - 2014-04-05 23:53 -0700
  Re: Mutable objects inside tuples - good or bad? Gary Herron <gary.herron@islandtraining.com> - 2014-04-06 00:25 -0700
  Re: Mutable objects inside tuples - good or bad? Devin Jeanpierre <jeanpierreda@gmail.com> - 2014-04-06 00:55 -0700
    Re: Mutable objects inside tuples - good or bad? Rustom Mody <rustompmody@gmail.com> - 2014-04-06 08:07 -0700
  Re: Mutable objects inside tuples - good or bad? Chris Angelico <rosuav@gmail.com> - 2014-04-06 18:25 +1000
  Re: Mutable objects inside tuples - good or bad? Devin Jeanpierre <jeanpierreda@gmail.com> - 2014-04-06 04:01 -0700
  Re: Mutable objects inside tuples - good or bad? Paul Kölle <paul@subsignal.org> - 2014-04-07 17:26 +0200
  Re: Mutable objects inside tuples - good or bad? Chris Angelico <rosuav@gmail.com> - 2014-04-08 01:44 +1000
  Re: Mutable objects inside tuples - good or bad? Paul Kölle <paul@subsignal.org> - 2014-04-07 21:46 +0200
  Re: Mutable objects inside tuples - good or bad? Chris Angelico <rosuav@gmail.com> - 2014-04-08 08:47 +1000
  Re: Mutable objects inside tuples - good or bad? Terry Reedy <tjreedy@udel.edu> - 2014-04-07 20:16 -0400
    Re: Mutable objects inside tuples - good or bad? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-04-08 02:53 +0000

csiph-web