Path: csiph.com!usenet.pasdenom.info!gegeweb.org!usenet-fr.net!nerim.net!novso.com!newsfeed.xs4all.nl!newsfeed4.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 0.027 X-Spam-Evidence: '*H*': 0.95; '*S*': 0.00; 'default.': 0.09; 'false,': 0.09; 'implements': 0.09; 'operand': 0.09; 'operator,': 0.09; 'things,': 0.09; 'defer': 0.16; 'it;': 0.16; 'message-id:@web.de': 0.16; 'unequal,': 0.16; 'wrote:': 0.18; 'obviously': 0.18; 'normally': 0.19; 'result.': 0.19; 'not,': 0.20; 'seems': 0.21; 'aug': 0.22; 'header:User-Agent:1': 0.23; 'integer': 0.24; "shouldn't": 0.24; 'equivalent': 0.26; 'second': 0.26; 'pass': 0.26; 'header:In-Reply-To:1': 0.27; 'chris': 0.29; '3.2': 0.31; 'comparison': 0.31; 'object.': 0.31; 'class': 0.32; "we're": 0.32; 'another': 0.32; 'could': 0.34; "can't": 0.35; 'knows': 0.35; 'equal': 0.35; 'but': 0.35; 'false': 0.36; 'object,': 0.36; 'returning': 0.36; 'done': 0.36; 'thanks': 0.36; 'possible': 0.36; 'behind': 0.37; 'received:192.168.2': 0.37; 'handle': 0.38; 'skip:[ 10': 0.38; 'to:addr:python-list': 0.38; 'pm,': 0.38; 'to:addr:python.org': 0.39; 'first': 0.61; 'back': 0.62; 'skip:n 10': 0.64; 'more': 0.64; 'side': 0.67; 'determine': 0.67; 'received:212.227.17': 0.68; 'received:web.de': 0.68; 'reasoning': 0.91; 'directly.': 0.95; '2013': 0.98 Date: Mon, 05 Aug 2013 16:58:03 +0200 From: Markus Rother User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130422 Thunderbird/17.0.5 MIME-Version: 1.0 To: python-list@python.org Subject: Re: Bug? ( () == [] ) != ( ().__eq__([]) ) References: <51FED74E.6080003@markusrother.de> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Provags-ID: V03:K0:Mpt/qaHS/FRaPC9v1XpOVW9KuyWMYZ6u/RRxsV9D3cj3RN3fsOh Rn7tpVEbZz6y5hgS/J5vfVZLb//S7yj1rA0RDV+VYYx3oC1laffLtuEaOQ+Z0v/H5OZJxTP KAIPsnwz0Xm/Ev/gI1b0eZVWKZ5xC4WxrkRiijSNhvHzqWMn4pNOnXjYUUW00/lb9qJHNhS B/gDTeNN9IfGjakS93UOA== 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: 42 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1375714689 news.xs4all.nl 15936 [2001:888:2000:d::a6]:41480 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:51965 Thanks for the good explanation. My intention was to pass a custom method/function as a comparator to an object. My misconception was, that __eq__ is equivalent to the '==' operator, and could be passed as a first class function. Apparently, that is not possible without wrapping the comparison into another function/method. Best regards, Markus R. On 05.08.2013 01:06, Chris Angelico wrote: > On Sun, Aug 4, 2013 at 11:35 PM, Markus Rother wrote: >> Hello, >> >> The following behaviour seen in 3.2 seems very strange to me: >> >> As expected: >>>>> () == [] >> False >> >> However: >>>>> ().__eq__([]) >> NotImplemented >>>>> [].__eq__(()) >> NotImplemented > > You don't normally want to be calling dunder methods directly. The > reasoning behind this behaviour goes back to a few things, including a > way to handle "1 == Foo()" where Foo is a custom type that implements > __eq__; obviously the integer 1 won't know whether it's equal to a Foo > instance or not, so it has to defer to the second operand to get a > result. This deferral is done by returning NotImplemented, which is an > object, and so is true by default. I don't see any particular reason > for it to be false, as you shouldn't normally be using it; it's more > like a "null" state, it means "I don't know if we're equal or not". If > neither side knows whether they're equal, then they're presumed to be > unequal, but you can't determine that from a single call to __eq__. > > ChrisA >