Re: How does python know?

Path	csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
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; 'algorithm': 0.04; 'cpython': 0.05; 'string': 0.09; 'literal': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'strings.': 0.09; 'subject:How': 0.10; 'python': 0.11; 'wrote': 0.14; 'dictionary.': 0.16; 'interpreter,': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'specific,': 0.16; 'subject:python': 0.16; 'weird': 0.16; 'thanks,': 0.17; 'wrote:': 0.18; '>>>': 0.22; 'appears': 0.22; 'memory': 0.22; 'print': 0.22; '(such': 0.24; 'typical': 0.24; "i've": 0.25; 'compare': 0.26; 'this:': 0.26; 'header:X-Complaints-To:1': 0.27; 'characters': 0.30; 'said,': 0.30; 'program,': 0.31; 'names.': 0.31; "skip:' 40": 0.31; 'subject:?': 0.36; 'should': 0.36; 'to:addr:python- list': 0.38; 'pm,': 0.38; 'does': 0.39; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'most': 0.60; 'new': 0.61; 'save': 0.62; 'determine': 0.67; 'introduction': 0.68; 'invalid': 0.68; 'object:': 0.84; 'subject:know': 0.84; 'defeat': 0.91
X-Injected-Via-Gmane	http://gmane.org/
To	python-list@python.org
From	Dave Angel <davea@davea.name>
Subject	Re: How does python know?
Date	Wed, 12 Feb 2014 16:59:28 -0500 (EST)
Organization	news.gmane.org
References	<lFQKu.455927$cZ.440055@fx31.iad> <pOQKu.37378$vk5.11310@fx03.iad>
X-Gmane-NNTP-Posting-Host	dpc6744192124.direcpc.com
X-Newsreader	PiaoHong Usenet NewsReaders 1.36
X-BeenThere	python-list@python.org
X-Mailman-Version	2.1.15
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>
Newsgroups	comp.lang.python
Message-ID	<mailman.6797.1392242168.18130.python-list@python.org> (permalink)
Lines	56
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1392242168 news.xs4all.nl 2923 [2001:888:2000:d::a6]:46912
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:66101

Show key headers only | View raw

 Tobiah <toby@tobiah.org> Wrote in message:
> On 02/12/2014 12:17 PM, Tobiah wrote:
>> I do this:
>>
>> a = 'lasdfjlasdjflaksdjfl;akjsdf;kljasdl;kfjasl'
>> b = 'lasdfjlasdjflaksdjfl;akjsdf;kljasdl;kfjasl'
>>
>> print
>> print id(a)
>> print id(b)
>>
>>
>> And get this:
>>
>> True
>> 140329184721376
>> 140329184721376
>>
>>
>> This works for longer strings.  Does python
>> compare a new string to every other string
>> I've made in order to determine whether it
>> needs to create a new object?
>>
>> Thanks,
>>
>> Tobiah
> 
> Weird as well, is that in the interpreter,
> the introduction of punctuation appears to
> defeat the reuse of the object:
> 
>  >>> b = 'lasdfjlasdjflaksdjflakjsdfkljasdlkfjasl'
>  >>> a = 'lasdfjlasdjflaksdjflakjsdfkljasdlkfjasl'
>  >>> a is b
> True
>  >>> a = 'la;sdfjlasdjflaksdjflakjsdfkljasdlkfjasl'
>  >>> b = 'la;sdfjlasdjflaksdjflakjsdfkljasdlkfjasl'
>  >>> a is b
> 
> 

As others have said, interning is implementation specific, so you
 should never rely on it. 

I think the current CPython algorithm is designed to save both
 memory and time in the storage and look up of symbol names.  In a
 typical program, those are the most likely to be duplicated.  So
 if your literal is of reasonable size and doesn’t invalid symbol
 characters (such as space, punctuation,  etc) then it just might
 be added to the interned dictionary. 

-- 
DaveA

Thread

How does python know? Tobiah <toby@tobiah.org> - 2014-02-12 12:17 -0800
  Re: How does python know? Tobiah <toby@tobiah.org> - 2014-02-12 12:27 -0800
    Re: How does python know? Dave Angel <davea@davea.name> - 2014-02-12 16:59 -0500
  Re: How does python know? Chris Angelico <rosuav@gmail.com> - 2014-02-13 07:33 +1100
  Re: How does python know? Gary Herron <gary.herron@islandtraining.com> - 2014-02-12 13:02 -0800
  Re: How does python know? Roy Smith <roy@panix.com> - 2014-02-12 20:54 -0500

csiph-web