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

Groups > comp.lang.python > #103849

Effects of caching frequently used objects, was Re: Explaining names vs variables in Python

Path csiph.com!fu-berlin.de!uni-berlin.de!not-for-mail
From Peter Otten <__peter__@web.de>
Newsgroups comp.lang.python
Subject Effects of caching frequently used objects, was Re: Explaining names vs variables in Python
Date Wed, 02 Mar 2016 10:12:48 +0100
Organization None
Lines 70
Message-ID <mailman.98.1456909984.20602.python-list@python.org> (permalink)
References <a894d5ed-d906-4ff7-a537-32bf0187e062@googlegroups.com>
Mime-Version 1.0
Content-Type text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding 7Bit
X-Trace news.uni-berlin.de 49IUsOjP7XePj5q+Wb5DMgNIBA1aHTVgxks9Pj2vzxPQ==
Return-Path <python-python-list@m.gmane.org>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.001
X-Spam-Evidence '*H*': 1.00; '*S*': 0.00; 'subject:Python': 0.05; 'cache': 0.05; 'sys': 0.05; 'instance.': 0.09; 'integers': 0.09; 'none.': 0.09; 'objects.': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'variables,': 0.09; 'python': 0.10; 'argument': 0.15; 'interpreter': 0.15; 'properly': 0.15; 'dio': 0.16; 'received:80.91.229.3': 0.16; 'received:dip0.t-ipconnect.de': 0.16; 'received:io': 0.16; 'received:plane.gmane.org': 0.16; 'received:psf.io': 0.16; 'received:t-ipconnect.de': 0.16; 'wrote:': 0.16; 'attribute': 0.18; '>>>': 0.20; 'names.': 0.22; 'import': 0.24; 'written': 0.24; 'header:User-Agent:1': 0.26; 'developers': 0.26; 'header:X -Complaints-To:1': 0.26; 'linux': 0.26; 'behaviour': 0.29; 'consistency': 0.29; 'expose': 0.29; 'objects': 0.29; 'similar': 0.33; 'done': 0.35; 'false': 0.35; 'identity': 0.35; 'something': 0.35; 'according': 0.36; 'but': 0.36; 'should': 0.36; 'to:addr :python-list': 0.36; 'subject:: ': 0.37; 'received:org': 0.37; 'names': 0.38; 'someone': 0.38; 'does': 0.39; 'rather': 0.39; 'build': 0.40; 'to:addr:python.org': 0.40; 'where': 0.40; 'received:de': 0.40; 'hello,': 0.40; 'some': 0.40; 'save': 0.60; 'behavior': 0.61; 'subject: ': 0.61; 'more': 0.63; 'reuse': 0.66; 'decided': 0.66; 'therefore': 0.67; 'targeted': 0.70; 'salvatore': 0.84; 'versions)': 0.84; 'lot,': 0.95
X-Injected-Via-Gmane http://gmane.org/
X-Gmane-NNTP-Posting-Host p57bd9cac.dip0.t-ipconnect.de
User-Agent KNode/4.13.3
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.21
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>
Xref csiph.com comp.lang.python:103849

Show key headers only | View raw

Salvatore DI DIO wrote:

> Hello,
> 
> I know Python does not have variables, but names.
> Multiple names cant then be bound to the same objects.
> 
> So this behavior
> 
>>>> b = 234
>>>> v = 234
>>>> b is v
> True
> 
> according to the above that is ok
> 
> 
> 
> But where is the consistency ? if I try :
> 
>>>> v = 890
>>>> w = 890
>>>> v is w
> False
> 
> It is a little difficult to explain this behavior to a newcommer in Python
> 
> Can someone give me the right argument to expose ?

You should not bother with object identity for objects other than None.
Some small integers are used a lot, e. g.

Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.getrefcount(0)
606
>>> sys.getrefcount(1)
918
>>> sys.getrefcount(256)
31
>>> sys.getrefcount(-1)
51

therefore as an optimization the Python developers decided to put -5...256 
(actual range may vary across interpreter versions) into a cache and reuse 
them rather than build a new object for every instance. This may save both 
time and memory, but is otherwise irrelevant.

Something similar is done for strings:

>>> a = "hello"
>>> b = "hello"
>>> a is b
True
>>> a = "hello, world"
>>> b = "hello, world"
>>> a is b
False

But:

>>> a = "hello, world"; b = "hello, world"
>>> a is b
True

Again this is an optimization (mostly targeted at attribute names) which 
should not affect the behaviour of a properly written Python program.

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

Thread

Explaining  names  vs variables  in Python Salvatore DI DIO <salvatore.didio@gmail.com> - 2016-03-02 00:32 -0800
  Re: Explaining names vs variables in Python Jesper K Brogaard <jesper@brogAAaard.eu> - 2016-03-02 10:03 +0100
    Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-02 21:32 +1100
      Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 14:34 +0200
        Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-02 23:50 +1100
          Re: Explaining names vs variables in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 15:11 +0200
          Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 15:39 +0200
            Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-03 00:48 +1100
              Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 16:11 +0200
                Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 07:08 -0800
                Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 04:23 +1100
                Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 09:28 -0800
                Re: Explaining names vs variables in Python Marko Rauhamaa <marko@pacujo.net> - 2016-03-02 20:12 +0200
                Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 12:52 +1100
                Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-03 09:03 -0800
                Re: Explaining names vs variables in Python Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-03 12:53 -0700
                Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-02 21:49 +0000
                Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 13:05 +1100
                Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-03 16:09 +0000
                Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-03 08:52 +1100
                Re: Explaining names vs variables in Python Rustom Mody <rustompmody@gmail.com> - 2016-03-02 17:23 -0800
                Re: Explaining names vs variables in Python Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-02 22:51 +0000
              Re: Explaining names vs variables in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-03 04:10 +1100
  Re: Explaining names vs variables in Python Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-03-02 10:08 +0100
  Effects of caching frequently used objects, was Re: Explaining  names  vs variables  in Python Peter Otten <__peter__@web.de> - 2016-03-02 10:12 +0100
  Re: Explaining  names  vs variables  in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 11:35 +0200
    Re: Explaining names vs variables in Python Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-02 08:13 -0700
      Re: Explaining names vs variables in Python Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-03-02 17:37 +0200
  Re: Explaining  names  vs variables  in Python Steven D'Aprano <steve@pearwood.info> - 2016-03-02 21:16 +1100
  Re: Explaining  names  vs variables  in Python "ast" <nomail@invalid.com> - 2016-03-02 11:52 +0100
    Re: Explaining  names  vs variables  in Python Salvatore DI DIO <salvatore.didio@gmail.com> - 2016-03-02 02:58 -0800
  Re: Effects of caching frequently used objects, was Re: Explaining  names  vs variables  in Python Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2016-03-02 09:16 -0500
  Re: Explaining  names  vs variables  in Python Ben Finney <ben+python@benfinney.id.au> - 2016-03-03 04:53 +1100
  RE: Effects of caching frequently used objects, was Re: Explaining names  vs variables  in Python Albert-Jan Roskam <sjeik_appie@hotmail.com> - 2016-03-25 13:03 +0000
  Re: Effects of caching frequently used objects, was Re: Explaining names vs variables in Python Chris Angelico <rosuav@gmail.com> - 2016-03-26 00:22 +1100
  Re: Effects of caching frequently used objects, was Re: Explaining names  vs variables  in Python Ethan Furman <ethan@stoneleaf.us> - 2016-03-25 09:45 -0700

csiph-web