Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!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 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'argument': 0.04; 'context': 0.05; 'none:': 0.05; 'default.': 0.07; 'referring': 0.07; 'subject:skip:c 10': 0.07; 'mutable': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'subject:method': 0.09; 'subject:using': 0.09; 'def': 0.10; 'contextlib': 0.16; 'defined;': 0.16; 'message- id:@dough.gmane.org': 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; 'subject:class': 0.16; 'wrote:': 0.17; 'fix': 0.17; 'example.': 0.17; 'yield': 0.17; 'changes': 0.20; 'import': 0.21; 'object.': 0.22; 'class.': 0.23; 'seems': 0.23; 'tried': 0.25; 'header:User-Agent:1': 0.26; 'skip:@ 10': 0.27; 'header:X-Complaints-To:1': 0.28; 'email name:': 0.29; 'once,': 0.29; 'skip:_ 10': 0.29; 'class': 0.29; 'expect': 0.31; 'instances': 0.33; 'values.': 0.33; 'problem': 0.33; 'to:addr :python-list': 0.33; 'list': 0.35; 'nature': 0.35; 'expected': 0.35; 'received:org': 0.36; 'method': 0.36; 'should': 0.36; 'why': 0.37; 'data': 0.37; 'subject:: ': 0.38; 'nothing': 0.38; 'to:addr:python.org': 0.39; 'header:Received:5': 0.40; 'your': 0.60; 'thomas': 0.62; 'circuit': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Peter Otten <__peter__@web.de> Subject: Re: Unexpected behavior using contextmanager on a class method Date: Tue, 07 Aug 2012 18:04:29 +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: p508497e6.dip.t-dialin.net User-Agent: KNode/4.7.3 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: 42 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1344355474 news.xs4all.nl 6869 [2001:888:2000:d::a6]:38205 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:26726 Thomas Draper wrote: > I want to use with..as in a "reversible circuit generator". However, it > seems that @contextmanager changes the expected nature of the class. I > tried to distill the problem down to a simple example. > > import contextlib > > class SymList: The problem you experience has nothing to do with context managers, you have a mutable default argument in your __init__(). > def __init__(self, L=[]): L is initialised with an empty list exactly once, when the method is defined; any changes you make to the list will be seen by all instances that use the default. The fix is def __init__(self, L=None): if L is None: L = [] > self.L = L > > @contextlib.contextmanager > def SymAdd(self, a): > self.L.append(a) > yield > self.L.append(a) > > SL = SymList() > with SL.SymAdd(3): > SL.L.append(5) > print(SL.L) # Expect and see [3, 5, 3] > SL2 = SymList() # New object. Should have default values. > print(SL2.L) # Expect [] and see [3, 5, 3] > > Why is the data member SL2.L referring to the data member SL.L? Has the > @contextmanager somehow made all instantions of the class related?