Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!news.chainon-marquant.org!news-transit.tcx.org.uk!rt.uk.eu.org!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; 'python,': 0.01; 'interpreter': 0.05; 'attribute': 0.07; 'python': 0.08; 'dynamically': 0.09; 'foo': 0.09; 'foo,': 0.09; 'statement.': 0.09; 'underlying': 0.09; 'def': 0.13; 'received:209.85.210.174': 0.13; 'received:mail-iy0-f174.google.com': 0.13; 'outputs': 0.16; '\xa0def': 0.16; '\xa0here': 0.16; 'cc:addr:python-list': 0.16; 'wrote:': 0.18; 'instance': 0.18; 'cc:no real name:2**0': 0.20; '(most': 0.21; 'mechanism': 0.21; 'dec': 0.22; 'header:In-Reply- To:1': 0.22; 'wrong?': 0.23; 'steve': 0.24; 'cc:2**0': 0.24; 'fine': 0.24; 'traceback': 0.24; "i'm": 0.26; 'function': 0.27; 'variable': 0.28; 'subject:" ': 0.28; 'message- id:@mail.gmail.com': 0.28; 'cc:addr:python.org': 0.29; 'error': 0.29; 'pm,': 0.29; 'class': 0.29; 'object.': 0.30; 'python3': 0.30; 'does': 0.32; 'tue,': 0.32; 'object': 0.33; '17,': 0.34; 'last):': 0.34; 'url:python': 0.36; 'subject:with': 0.36; 'skip:" 10': 0.37; 'but': 0.37; 'received:google.com': 0.37; 'doing': 0.38; 'using': 0.38; 'received:209.85': 0.38; 'skip:\xa0 10': 0.39; 'url:docs': 0.39; 'url:org': 0.39; 'subject: (': 0.40; 'received:209': 0.40; '8bit%:8': 0.40; '2011': 0.61; 'full': 0.62; 'special': 0.68; 'to:addr:yahoo.com': 0.83; 'howell': 0.84; 'tricky': 0.84; 'url:datamodel': 0.84; 'url:reference': 0.84 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; bh=0VOOl5HrvXQedFmp9u7xEov0yKifQX7YUOijPt0n1DE=; b=qIS2tlRA6+MVjO0NwMm4/zOMoNedRnCpzUWghavytxxigmwcr/Lix2eprv1tI9eila 0e8jQP8xNV2TwPF95LjC+hqK5T9bpeUP2GBZjZRCtSSxlNWIsf35MDMnQFKbGQidzYEf VCbKGtKR9Osp5OGFC604qzPXRf3L6JdsUYQPs= MIME-Version: 1.0 In-Reply-To: References: Date: Tue, 13 Dec 2011 23:05:53 -0700 Subject: Re: AttributeError in "with" statement (3.2.2) From: Eric Snow To: Steve Howell Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Cc: python-list@python.org 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: 48 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1323842756 news.xs4all.nl 6986 [2001:888:2000:d::a6]:50949 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:17181 On Tue, Dec 13, 2011 at 10:42 PM, Steve Howell wrote: > I'm using Python 3.2.2, and the following program gives me an error > that I don't understand: > > class Foo: > =A0pass > > foo =3D Foo() > foo.name =3D "Steve" > > def add_goodbye_function(obj): > =A0def goodbye(): > =A0 =A0print("goodbye " + obj.name) > =A0obj.goodbye =3D goodbye > > add_goodbye_function(foo) > foo.goodbye() # outputs goodbye Steve > foo.__exit__ =3D foo.goodbye > foo.__exit__() # outputs goodbye Steve > > with foo: # fails with AttributeError: =A0__exit__ > =A0print("doing stuff") > > I am dynamically adding an attribute __exit__ to the variable foo, > which works fine when I call it directly, but it fails when I try to > use foo as the expression in the with statement. =A0Here is the full > output: > >> python3 with.coffee > goodbye Steve > goodbye Steve > Traceback (most recent call last): > =A0File "with.coffee", line 17, in > =A0 =A0with foo: # fails with AttributeError: > AttributeError: __exit__ > > What am I doing wrong? That is a tricky one. As with many of the special methods (start and end with __) in Python, the underlying mechanism in the interpreter is directly pulling the function from the class object. It does not look to the instance object for the function at any time. See http://docs.python.org/reference/datamodel.html#special-method-lookup-for-n= ew-style-classes. -eric