Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.003 X-Spam-Evidence: '*H*': 0.99; '*S*': 0.00; 'argument': 0.05; '(so': 0.07; 'referring': 0.07; '[1,': 0.09; 'explanation': 0.09; 'here?': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'python': 0.11; '#python': 0.16; '2.7.3': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'underlying': 0.16; 'wrote:': 0.18; 'starts': 0.20; '>>>': 0.22; 'memory': 0.22; 'print': 0.22; 'header:User-Agent:1': 0.23; 'second': 0.26; 'values': 0.27; 'header:X-Complaints-To:1': 0.27; 'header:In-Reply-To:1': 0.27; 'forgot': 0.30; 'list:': 0.30; "i'm": 0.30; 'that.': 0.31; '>>>>': 0.31; 'object.': 0.31; 'sep': 0.31; 'thanks!': 0.32; 'could': 0.34; 'there': 0.35; 'list.': 0.37; 'to:addr:python-list': 0.38; 'list,': 0.38; 'pm,': 0.38; 'sure': 0.39; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'how': 0.40; 'increased': 0.61; 'new': 0.61; 'simply': 0.61; 'first': 0.61; 'name': 0.63; 'refer': 0.63; 'more': 0.64; 'below.': 0.71; 'idiom': 0.84; '2013,': 0.91 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Ned Batchelder Subject: Re: L[:] Date: Fri, 10 Jan 2014 17:03:49 -0500 References: <1389375507.21198.YahooMailBasic@web163801.mail.gq1.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Gmane-NNTP-Posting-Host: 18.189.30.229 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 In-Reply-To: <1389375507.21198.YahooMailBasic@web163801.mail.gq1.yahoo.com> X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 45 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1389391443 news.xs4all.nl 2953 [2001:888:2000:d::a6]:36276 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:63669 On 1/10/14 12:38 PM, Albert-Jan Roskam wrote: > In Python Cookbook, one of the authors (I forgot who) consistently used the "L[:]" idiom like below. If the second line simply starts with "L =" (so no "[:]") only the name "L" would be rebound, not the underlying object. That was the authorś explanation as far as I can remember. I do not get that. Why is the "L[:]" idiom more memory-efficient here? How could the increased efficiency be demonstrated? > > #Python 2.7.3 (default, Sep 26 2013, 16:38:10) [GCC 4.7.2] on linux2 >>>> L = [x ** 2 for x in range(10)] >>>> L[:] = ["foo_" + str(x) for x in L] > I'm not sure there is a memory efficiency argument to make here. The big difference is that the first line make L refer to a completely new list, while the second line replaces the contents of an existing list. This makes a big difference if there are other names referring to the list: >>> L = [1, 2, 3, 4] >>> L2 = L >>> L[:] = [] >>> print L2 [] >>> L = [1, 2, 3, 4] >>> L2 = L >>> L = [] >>> print L2 [1, 2, 3, 4] Names and values in Python can be confusing. Here's an explanation of the mechanics: http://nedbatchelder.com/text/names.html HTH, --Ned. > > Thanks! > > > Regards, > > Albert-Jan -- Ned Batchelder, http://nedbatchelder.com