Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > comp.lang.python > #67522
| From | Ben Finney <ben+python@benfinney.id.au> |
|---|---|
| Subject | Object identity (was: Reference) |
| Date | 2014-03-03 21:00 +1100 |
| References | <53144e8d$0$2149$426a74cc@news.free.fr> |
| Newsgroups | comp.lang.python |
| Message-ID | <mailman.7632.1393840833.18130.python-list@python.org> (permalink) |
"ast" <nomail@invalid.com> writes: > >>> A=7 > >>> B=7 > >>> A is B > True > > I understand that there is a single object 7 somewhere in memory and > both variables A and B point toward this object 7 Try not to think in terms of “point to”. Rather, the names “A” and “B” are bound to that object. The distinction is subtle; but it's important to realise that *all* references in Python do this, and there's no way to talk about an object in Python without using a reference. The “pointer” model from other languages doesn't exist in Python. > now do the same with a list: > > >>> l1 = [1, 2] > >>> l2 = [1, 2] > >>> l1 is l2 > False > > It seems this time that there are 2 distincts objects [1, 2] in > memory. That's correct. > I dont really understand why the behavior is different. Both integer 7 > and list [1, 2] are objects. Why is it different ? Short answer: object identity is an implementation detail. It's up to the Python implementation to decide when to re-use an object when a new one is requested. No guarantee is made, when you ask to create an object, that you won't get an existing one if that would work just as well. Since the integer object 7 is immutable, it will behave the same no matter how many times you ask for a new one, the Python implementation can choose to give you the same object. But it might not — don't depend on this! Since two separate lists are mutable, each one can have a distinct history after creation, so it would be less helpful to return an existing list when you ask for a new one. But again, there's no guarantee here either! A Python implementation might decide to give you an existing list, if existing guarantees can be kept. The moral is: Don't depend on differences in object identity. You can be guaranteed that an object will retain its own identity, and its identity will always be different from all other co-existing objects that have different values. Beyond that, don't make any assumptions. -- \ “Programs must be written for people to read, and only | `\ incidentally for machines to execute.” —Abelson & Sussman, | _o__) _Structure and Interpretation of Computer Programs_ | Ben Finney
Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Reference "ast" <nomail@invalid.com> - 2014-03-03 10:42 +0100
Object identity (was: Reference) Ben Finney <ben+python@benfinney.id.au> - 2014-03-03 21:00 +1100
Re: Object identity (was: Reference) "ast" <nomail@invalid.com> - 2014-03-03 11:21 +0100
Re: Reference "Mark H. Harris" <harrismh777@gmail.com> - 2014-03-03 05:09 -0800
Re: Reference Grant Edwards <invalid@invalid.invalid> - 2014-03-03 14:29 +0000
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-03 07:52 -0800
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 08:10 +1100
Re: Reference Tim Chase <python.list@tim.thechases.com> - 2014-03-03 15:24 -0600
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 08:31 +1100
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-03 21:35 +0000
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 00:07 +0200
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 09:18 +1100
Re: Reference Alister <alister.ware@ntlworld.com> - 2014-03-04 11:10 +0000
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 11:48 +0000
Re: Reference "Rhodri James" <rhodri@wildebst.org.uk> - 2014-03-05 00:25 +0000
Re: Reference Tim Chase <python.list@tim.thechases.com> - 2014-03-03 15:51 -0600
Re: Reference Jerry Hill <malaclypse2@gmail.com> - 2014-03-03 17:02 -0500
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-04 00:22 +0200
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-04 09:27 +1100
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 09:33 +1100
Re: Reference Steven D'Aprano <steve@pearwood.info> - 2014-03-04 04:52 +0000
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-04 16:24 +1100
Re: Reference "Rhodri James" <rhodri@wildebst.org.uk> - 2014-03-05 01:08 +0000
Re: Reference Roy Smith <roy@panix.com> - 2014-03-04 21:09 -0500
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 19:36 -0800
Re: Reference Ian Kelly <ian.g.kelly@gmail.com> - 2014-03-04 21:08 -0700
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 20:31 -0800
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-05 15:32 +1100
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 20:47 -0800
Re: Reference Steven D'Aprano <steve@pearwood.info> - 2014-03-05 05:06 +0000
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 21:47 -0800
Re: Reference alex23 <wuwei23@gmail.com> - 2014-03-05 16:01 +1000
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 22:10 -0800
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-05 17:22 +1100
Re: Reference alex23 <wuwei23@gmail.com> - 2014-03-05 16:28 +1000
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-05 12:24 +0000
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-05 12:21 +0000
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-05 17:20 +1100
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-05 09:40 -0800
Re: Reference Tim Chase <python.list@tim.thechases.com> - 2014-03-05 12:12 -0600
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 05:33 +1100
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-05 18:19 +0000
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 22:23 +0200
Re: Reference Grant Edwards <invalid@invalid.invalid> - 2014-03-05 20:31 +0000
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 22:46 +0200
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 08:07 +1100
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 08:10 +1100
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-05 21:34 +0000
Re: Reference Terry Reedy <tjreedy@udel.edu> - 2014-03-05 18:00 -0500
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-06 03:01 +0000
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 22:03 -0800
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-05 17:26 +1100
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 17:32 +1100
Re: Reference Tim Chase <python.list@tim.thechases.com> - 2014-03-05 08:24 -0600
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-05 18:29 +0000
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 22:34 +0200
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 08:01 +1100
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 23:14 +0200
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-06 08:26 +1100
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 23:50 +0200
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-06 00:35 +0000
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-06 11:50 +1100
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-06 17:46 +0000
Re: Reference Tim Chase <python.list@tim.thechases.com> - 2014-03-05 15:33 -0600
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 08:37 +1100
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-06 02:52 +0200
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-06 12:05 +1100
Re: Reference alex23 <wuwei23@gmail.com> - 2014-03-06 12:12 +1000
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-05 21:46 +0000
Re: Reference Marko Rauhamaa <marko@pacujo.net> - 2014-03-05 08:23 +0200
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 22:33 -0800
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-05 17:40 +1100
Re: Reference Mark Lawrence <breamoreboy@yahoo.co.uk> - 2014-03-05 12:35 +0000
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 23:45 +1100
Re: Reference Jerry Hill <malaclypse2@gmail.com> - 2014-03-04 10:19 -0500
Re: Reference Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-03-04 15:42 +0000
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 03:02 +1100
Re: Reference Roy Smith <roy@panix.com> - 2014-03-04 11:14 -0500
Re: Reference MRAB <python@mrabarnett.plus.com> - 2014-03-04 17:12 +0000
Re: Reference Rustom Mody <rustompmody@gmail.com> - 2014-03-04 08:24 -0800
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 02:25 +1100
Re: Reference Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2014-03-05 14:37 +0100
Re: Reference Steven D'Aprano <steve@pearwood.info> - 2014-03-04 03:59 +0000
Re: Reference Ben Finney <ben+python@benfinney.id.au> - 2014-03-04 09:17 +1100
Re: Reference Roy Smith <roy@panix.com> - 2014-03-03 18:02 -0500
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-04 10:09 +1100
Re: Reference Steven D'Aprano <steve@pearwood.info> - 2014-03-04 04:38 +0000
Re: Reference Terry Reedy <tjreedy@udel.edu> - 2014-03-03 13:48 -0500
Re: Reference Steven D'Aprano <steve@pearwood.info> - 2014-03-04 03:45 +0000
Re: Reference Alexander Blinne <news@blinne.net> - 2014-03-04 13:55 +0100
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 01:06 +1100
Re: Reference Alexander Blinne <news@blinne.net> - 2014-03-04 22:53 +0100
Re: Reference Chris Angelico <rosuav@gmail.com> - 2014-03-05 09:01 +1100
csiph-web