Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!weretis.net!feeder4.news.weretis.net!feeder.news-service.com!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; 'passes': 0.05; 'figures': 0.07; 'terry': 0.07; 'python': 0.08; 'assert': 0.09; 'dict': 0.09; 'loop.': 0.09; '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; 'pm,': 0.10; 'subject:error': 0.11; 'def': 0.12; 'broken': 0.14; 'wrote:': 0.14; 'bases,': 0.16; 'carefully.': 0.16; 'discarded': 0.16; 'docstring': 0.16; 'forwards': 0.16; 'metaclass': 0.16; 'metaclasses': 0.16; 'mro': 0.16; 'reedy': 0.16; 'spam"': 0.16; 'endless': 0.19; 'jan': 0.20; 'header:In-Reply-To:1': 0.21; 'loop': 0.22; '(or': 0.24; 'pass': 0.27; 'class': 0.29; 'it.': 0.31; 'steven': 0.32; 'header:X-Complaints-To:1': 0.32; 'to:addr :python-list': 0.33; '...': 0.34; 'header:User-Agent:1': 0.35; "d'aprano": 0.35; 'fails': 0.35; 'skip:@ 10': 0.35; 'skip:{ 10': 0.35; 'source,': 0.35; 'test': 0.35; 'none': 0.37; 'skip:o 20': 0.37; 'received:org': 0.38; 'docs': 0.38; 'subject:: ': 0.38; 'skip:s 20': 0.39; 'empty': 0.39; 'header:Mime-Version:1': 0.39; 'to:addr:python.org': 0.39; 'getting': 0.40; 'want,': 0.74; '11:34': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Recursion error in metaclass Date: Sat, 11 Jun 2011 01:33:25 -0400 References: <4df2e251$0$30002$c3e8da3$5496439d@news.astraweb.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: rain.gmane.org User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.17) Gecko/20110414 Lightning/1.0b2 Thunderbird/3.1.10 In-Reply-To: <4df2e251$0$30002$c3e8da3$5496439d@news.astraweb.com> 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: 39 NNTP-Posting-Host: 82.94.164.166 X-Trace: 1307770420 news.xs4all.nl 49181 [::ffff:82.94.164.166]:60791 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:7435 On 6/10/2011 11:34 PM, Steven D'Aprano wrote: > I have a metaclass in Python 3.1: > > class MC1(type): > @staticmethod > def get_mro(bases): > print('get_mro called') > return type('K', bases, {}).__mro__[1:] The call to type figures out the proper metaclass from bases and forwards the call to that (or to its __new__ method). See Objects/typeobject.c in the source, or read the docs on metaclasses carefully. If the proper metaclass is MC1, ... > def __new__(cls, name, bases, dict): > mro = None > docstring = dict.get('__doc__') > if docstring == 'ham': > mro = cls.get_mro(bases) and you unconditionally call get_mro again, to call this again... > dict['__doc__'] = "spam spam spam" > # Create the class we want, and return it. > K = super().__new__(cls, name, bases, dict) > if mro: > assert K.__mro__ == (K,) + mro > return K you are in an endless loop. Since uou do not pass dict to get_mro. it passes {} to type and MC1 and the test for docstring fails and the loop is broken and the empty class is discarded after getting its mro. -- Terry Jan Reedy