Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder4.news.weretis.net!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed4a.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!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 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'python.': 0.02; 'python,': 0.02; 'example:': 0.03; 'cpython': 0.05; 'args': 0.07; 'postgresql': 0.07; 'rename': 0.07; 'arguments': 0.09; 'happens.': 0.09; 'immutable': 0.09; 'implements': 0.09; 'meaningful': 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; 'jan': 0.12; 'mechanism.': 0.16; 'mutable': 0.16; 'mutated': 0.16; 'pointers.': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'reedy': 0.16; 'sense,': 0.16; 'subject:variable': 0.16; 'wrote:': 0.18; 'variable': 0.18; 'trying': 0.19; '(where': 0.19; 'passing': 0.19; 'memory': 0.22; 'print': 0.22; 'header:User-Agent:1': 0.23; 'passes': 0.24; 'pointer': 0.24; 'question': 0.24; 'pass': 0.26; 'header:X -Complaints-To:1': 0.27; 'header:In-Reply-To:1': 0.27; 'extract': 0.31; 'file': 0.32; 'skip:- 30': 0.32; 'running': 0.33; 'copying': 0.34; 'objects': 0.35; 'version': 0.36; 'functions.': 0.36; 'var': 0.36; 'example,': 0.37; 'clear': 0.37; 'to:addr:python-list': 0.38; 'issue': 0.38; 'pm,': 0.38; 'delete': 0.39; 'to:addr:python.org': 0.39; 'either': 0.39; 'skip:p 20': 0.39; 'received:org': 0.40; 'how': 0.40; 'easy': 0.60; 'received:173': 0.61; 'skip:\xe2 10': 0.65; '8bit%:40': 0.68; 'as:': 0.81; 'burning': 0.84; 'received:fios.verizon.net': 0.84; 'skip:/ 30': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Pass variable by reference Date: Tue, 06 May 2014 15:16:54 -0400 References: <235C4BFA-9770-481A-9FCF-21C3F036769C@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Gmane-NNTP-Posting-Host: pool-173-75-254-207.phlapa.fios.verizon.net User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0 In-Reply-To: <235C4BFA-9770-481A-9FCF-21C3F036769C@gmail.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: 44 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1399406113 news.xs4all.nl 2869 [2001:888:2000:d::a6]:53224 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:70980 On 5/5/2014 8:39 PM, Satish Muthali wrote: > Hello experts, > > I have a burning question on how to pass variable by reference in > Python. Python passes objects to functions. CPython implements this by passing=20 object pointers. In one sense, your request is impossible. In another,=20 it already happens. > I understand that the data type has to be mutable. In C, all data are mutable, so copying a block of memory versus passing=20 a pointer is the meaningful distinction. In Python, mutable arguments=20 can be mutated and immutable args cannot, so *that* is the only=20 meaningful distinction. Strings are not mutable. > For example, here=E2=80=99s the issue I am running in to: > > I am trying to extract the PostgreSQL DB version for example: > > /pgVer =3D [s.split() for s in os.popen("psql > --version").read().splitlines()]/ > / print pgVer[0]/ > / for i, var in enumerate(pgVer[0]):/ > / if i =3D=3D len(pgVer[0]) - 1:/ > / pgversion =3D var/ > / > / > I would now like to pass =E2=80=98pgversion=E2=80=99 (where the value o= f pgversion is > 9.3.4) by reference, for example: > > I want to nuke /var/lib/postgresql/9.3.4/main/data , however > programatically I want it to be as: /var/lib/postgresql/*/ pgversion>/*/main/data It is not clear whether you want to delete a file or rename it. Either=20 is easy and neither depend on passing mechanism. --=20 Terry Jan Reedy