Path: csiph.com!usenet.pasdenom.info!news.albasani.net!newsfeed.freenet.ag!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!194.109.133.84.MISMATCH!newsfeed.xs4all.nl!newsfeed5.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 0.001 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'args': 0.04; 'intermediate': 0.05; 'append': 0.07; 'function,': 0.07; 'repeated': 0.07; 'indeed,': 0.09; 'oh,': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'def': 0.10; 'cleanly': 0.16; 'commandline': 0.16; 'concatenate': 0.16; 'eckhardt': 0.16; 'generator.': 0.16; 'preserving': 0.16; 'received:80.91.229.3': 0.16; 'received:dip.t-dialin.net': 0.16; 'received:plane.gmane.org': 0.16; 'received:t-dialin.net': 0.16; 'tcp': 0.16; 'tempted': 0.16; 'string': 0.17; 'wrote:': 0.17; 'helper': 0.17; 'yield': 0.17; 'versions': 0.20; 'explicit': 0.22; 'implicit': 0.22; 'this:': 0.23; 'least': 0.25; 'header:User- Agent:1': 0.26; 'creating': 0.26; 'right.': 0.27; 'header:X -Complaints-To:1': 0.28; 'lines': 0.28; 'loop,': 0.29; 'overhead': 0.29; 'though.': 0.29; 'no,': 0.29; "i'm": 0.29; "skip:' 10": 0.30; 'function': 0.30; 'channel': 0.32; 'not.': 0.32; 'could': 0.32; 'message.': 0.33; 'avoiding': 0.33; 'to:addr:python-list': 0.33; 'another': 0.33; 'version': 0.34; 'text': 0.34; 'done': 0.34; 'sequence': 0.35; 'so,': 0.35; 'doing': 0.35; 'list.': 0.35; 'received:org': 0.36; 'but': 0.36; 'test': 0.36; 'two': 0.37; 'why': 0.37; 'rather': 0.37; 'far': 0.37; 'subject:: ': 0.38; 'instead': 0.39; 'to:addr:python.org': 0.39; 'takes': 0.39; 'called': 0.39; 'where': 0.40; 'header:Received:5': 0.40; 'your': 0.60; 'save': 0.61; 'between': 0.63; 'times': 0.63; 'unnecessary': 0.65; 'dear': 0.66; 'counts': 0.81; 'alternative.': 0.84; 'expressive': 0.84; 'original.': 0.84; 'subject:better': 0.84; 'doubling': 0.91 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Peter Otten <__peter__@web.de> Subject: Re: better way for ' '.join(args) + '\n'? Date: Fri, 26 Oct 2012 10:58:56 +0200 Organization: None References: Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Gmane-NNTP-Posting-Host: p50849ef2.dip.t-dialin.net User-Agent: KNode/4.7.3 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: 54 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1351241912 news.xs4all.nl 6959 [2001:888:2000:d::a6]:55982 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:32202 Ulrich Eckhardt wrote: > Hi! > > General advise when assembling strings is to not concatenate them > repeatedly but instead use string's join() function, because it avoids > repeated reallocations and is at least as expressive as any alternative. > > What I have now is a case where I'm assembling lines of text for driving > a program with a commandline interface. In this scenario, I'm currently > doing this: > > args = ['foo', 'bar', 'baz'] > line = ' '.join(args) + '\n' > > So, in other words, I'm avoiding all the unnecessary copying, just to > make another copy to append the final newline. > > The only way around this that I found involves creating an intermediate > sequence like ['foo', ' ', 'bar', ' ', 'baz', '\n']. This can be done > rather cleanly with a generator: > > def helper(s): > for i in s[:-1]: > yield i > yield ' ' > yield s[-1] > yield '\n' > line = ''.join(tmp(args)) > > Efficiency-wise, this is satisfactory. No, it is not. In a quick timeit test it takes 5 to 10 times as long as the original. Remember that function calls are costly, and that with s[:-1] you are trading the extra string for an extra list. Also, you are doubling the loop implicit in str.join() with the explicit one in your oh-so-efficient generator. > However, readability counts and > that is where this version fails and that is the reason why I'm writing > this message. So, dear fellow Pythonistas, any ideas to improve the > original versions efficiency while preserving its expressiveness? > > Oh, for all those that are tempted to tell me that this is not my > bottleneck unless it's called in a very tight loop, you're right. > Indeed, the overhead of the communication channel TCP between the two > programs is by far dwarving the few microseconds I could save here. I'm > still interested in learning new and better solutions though. Even if it were the bottleneck the helper generator approach would still be unhelpful.