Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path: <python@mrabarnett.plus.com>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.026
X-Spam-Evidence: '*H*': 0.95; '*S*': 0.00; 'beginner': 0.05; '[1,': 0.09; 'def': 0.10; 'appends': 0.16; 'from:addr:mrabarnett.plus.com': 0.16; 'from:addr:python': 0.16; 'from:name:mrab': 0.16; 'help?': 0.16; 'message- id:@mrabarnett.plus.com': 0.16; 'received:84.93': 0.16; 'received:84.93.230': 0.16; 'result:': 0.16; 'wrote:': 0.17; 'this:': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User- Agent:1': 0.26; 'received:192.168.1.3': 0.29; 'gets': 0.32; 'received:84': 0.32; 'subject:lists': 0.32; 'could': 0.32; 'to:addr:python-list': 0.33; 'code:': 0.33; 'list': 0.35; "wasn't": 0.36; 'subject:with': 0.36; 'skip:p 20': 0.36; 'why': 0.37; 'subject:: ': 0.38; 'to:addr:python.org': 0.39; 'received:192': 0.39; 'list,': 0.39; 'received:192.168': 0.40; 'skip:n 10': 0.63; 'making': 0.64; 'kindly': 0.67; 'header:Reply- To:1': 0.68; 'obtained': 0.71; 'reply-to:no real name:2**0': 0.72; 'reply-to:addr:python.org': 0.84
X-CM-Score: 0.00
X-CNFS-Analysis: v=2.0 cv=IekFqBWa c=1 sm=1 a=0nF1XD0wxitMEM03M9B4ZQ==:17 a=DKcI9XZsuF4A:10 a=ihvODaAuJD4A:10 a=OUOv7kDek9cA:10 a=8nJEP1OIZ-IA:10 a=EBOSESyhAAAA:8 a=8AHkEIZyAAAA:8 a=VQ-08raiIhACc760dGoA:9 a=wPNLvfGTeEIA:10 a=0nF1XD0wxitMEM03M9B4ZQ==:117
X-AUTH: mrabarnett:2500
Date: Mon, 06 Aug 2012 21:19:01 +0100
From: MRAB <python@mrabarnett.plus.com>
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:14.0) Gecko/20120713 Thunderbird/14.0
MIME-Version: 1.0
To: python-list@python.org
Subject: Re: A difficulty with lists
References: <jvp75j$opr$1@news.albasani.net>
In-Reply-To: <jvp75j$opr$1@news.albasani.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-BeenThere: python-list@python.org
X-Mailman-Version: 2.1.12
Precedence: list
Reply-To: python-list@python.org
List-Id: General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe: <http://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: <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups: comp.lang.python
Message-ID: <mailman.3029.1344284341.4697.python-list@python.org>
Lines: 38
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1344284341 news.xs4all.nl 6974 [2001:888:2000:d::a6]:49785
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:26655

On 06/08/2012 20:50, Mok-Kong Shen wrote:
> I ran the following code:
>
> def xx(nlist):
>     print("begin: ",nlist)
>     nlist+=[999]
>     print("middle:",nlist)
>     nlist=nlist[:-1]
>     print("final: ",nlist)
>
> u=[1,2,3,4]
> print(u)
> xx(u)
> print(u)
>
> and obtained the following result:
>
> [1, 2, 3, 4]
> begin:  [1, 2, 3, 4]
> middle: [1, 2, 3, 4, 999]
> final:  [1, 2, 3, 4]
> [1, 2, 3, 4, 999]
>
> As beginner I couldn't understand why the last line wasn't [1, 2, 3, 4].
> Could someone kindly help?
>
This:

     nlist+=[999]

appends to the list, making it [1, 2, 3, 4, 999].

This:

     nlist=nlist[:-1]

gets a slice of the list and then binds it to the local name 'nlist'.