Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!weretis.net!feeder1.news.weretis.net!news.albasani.net!news2.arglkargh.de!news.wiretrip.org!newsfeed.xs4all.nl!newsfeed6.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; 'python': 0.08; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:80.91.229.12': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'received:lo.gmane.org': 0.09; 'statement.': 0.09; 'target,': 0.09; 'wrote:': 0.15; '"="': 0.16; '(remember': 0.16; '*why*': 0.16; 'and)': 0.16; 'assigns': 0.16; 'evaluates': 0.16; 'expression.': 0.16; 'from:addr:acm.org': 0.16; 'received:198.144': 0.16; 'rhs': 0.16; 'tmp': 0.16; 'tuple)': 0.16; 'yet.': 0.19; "doesn't": 0.22; 'here?': 0.23; 'wrong?': 0.23; 'thus': 0.23; 'statement': 0.25; "i'm": 0.27; '(the': 0.28; 'raise': 0.28; 'fri,': 0.28; 'guess': 0.28; '24,': 0.29; 'object': 0.30; 'subject:?': 0.31; 'expression': 0.32; 'received:198': 0.32; 'list': 0.33; 'it.': 0.33; 'done': 0.33; 'header:User-Agent:1': 0.34; 'header:X-Complaints-To:1': 0.34; 'to:addr:python-list': 0.34; 'things': 0.35; 'that,': 0.35; 'assignment': 0.35; 'latter': 0.35; 'lists,': 0.35; 'charset:us-ascii': 0.36; 'url:python': 0.36; 'but': 0.37; 'received:org': 0.38; 'url:org': 0.38; 'subject:: ': 0.38; 'should': 0.39; 'perhaps': 0.39; 'header:Mime- Version:1': 0.39; 'url:docs': 0.39; 'list,': 0.39; 'to:addr:python.org': 0.39; 'case': 0.40; 'did': 0.40; 'where': 0.40; 'would': 0.40; 'target': 0.61; 'order': 0.62; 'here.': 0.66; 'jun': 0.67; 'article': 0.76; '"an': 0.84; 'sides.': 0.84; 'url:reference': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Ned Deily Subject: Re: Interpreting Left to right? Date: Fri, 24 Jun 2011 15:10:18 -0700 References: <4E043953.5080705@stoneleaf.us> <20110624195300.GJ6075@point.cs.wisc.edu> <4E04EFD4.6050309@stoneleaf.us> <20110624200618.GK6075@point.cs.wisc.edu> <20110624210835.GL6075@point.cs.wisc.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: baybryj.net User-Agent: MT-NewsWatcher/3.5.3b3 (Intel Mac OS X) X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 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: 51 NNTP-Posting-Host: 82.94.164.166 X-Trace: 1308953433 news.xs4all.nl 512 [::ffff:82.94.164.166]:57928 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:8417 In article <20110624210835.GL6075@point.cs.wisc.edu>, Tycho Andersen wrote: > On Fri, Jun 24, 2011 at 01:24:24PM -0700, Ned Deily wrote: > > In article <20110624200618.GK6075@point.cs.wisc.edu>, > > Tycho Andersen wrote: > > > Yes, I understand that, but I guess I don't understand *why* things > > > are done that way. What is the evaluation order principle at work > > > here? I would have expected: > > > > > > tmp = {} > > > x['huh'] = tmp # NameEror! > > > > > > That is, the right hand sides of assignments are evaluated before the > > > left hand sides. That is (somehow?) not the case here. > > > > http://docs.python.org/py3k/reference/simple_stmts.html#assignment-statement > > s > > Perhaps I'm thick, but (the first thing I did was read the docs and) I > still don't get it. From the docs: > > "An assignment statement evaluates the expression list (remember that > this can be a single expression or a comma-separated list, the latter > yielding a tuple) and assigns the single resulting object to each of > the target lists, from left to right." > > For a single target, it evaluates the RHS and assigns the result to > the LHS. Thus > > x = x['foo'] = {} > > first evaluates > > x['foo'] = {} > > which should raise a NameError, since x doesn't exist yet. Where am I > going wrong? "An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right." Also, remember that in Python the "=" is not part of an expression. It's a token in the assignment statement. -- Ned Deily, nad@acm.org