Path: csiph.com!goblin2!goblin.stu.neva.ru!newsfeed1.swip.net!uio.no!news.tele.dk!news.tele.dk!small.news.tele.dk!newsgate.cistron.nl!newsgate.news.xs4all.nl!nzpost1.xs4all.net!not-for-mail
Return-Path: <python-python-list@m.gmane.org>
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; '(python': 0.05; 'method.': 0.05; 'ugly': 0.07; '-------': 0.09; 'derived': 0.09; 'methods,': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'subclass': 0.09; 'subject:ctypes': 0.09; 'jan': 0.11; '(),': 0.16; 'customizing': 0.16; 'linux)': 0.16; 'metaclass': 0.16; 'metaclasses': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'reedy': 0.16; 'stuff.': 0.16; 'wrote:': 0.16; 'why.': 0.18; '(on': 0.22; 'ctypes': 0.22; 'trying': 0.22; 'implemented': 0.24; 'header:In-Reply-To:1': 0.24; 'module': 0.25; 'header:User-Agent:1': 0.26; 'header:X-Complaints- To:1': 0.26; 'figure': 0.27; 'idea': 0.28; 'structure,': 0.29; '-----': 0.29; 'array': 0.29; "i'm": 0.30; 'subject:/': 0.30; 'classes': 0.30; 'checks': 0.30; 'skip:_ 10': 0.32; 'running': 0.34; 'skip:- 50': 0.35; 'false': 0.35; 'instance': 0.35; 'but': 0.36; 'there': 0.36; 'to:addr:python-list': 0.36; 'pm,': 0.36; 'subject:: ': 0.37; 'really': 0.37; 'method': 0.37; 'received:org': 0.37; 'things': 0.38; 'to:addr:python.org': 0.40; 'some': 0.40; 'more': 0.63; '3.4': 0.84; 'indirect': 0.84; 'lying': 0.84; 'manual,': 0.84; 'union,': 0.84; 'received:fios.verizon.net': 0.91
X-Injected-Via-Gmane: http://gmane.org/
To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: isinstance() and multiple inheritance/ctypes
Date: Wed, 26 Aug 2015 17:46:17 -0400
References: <mrlalj$nlt$1@dont-email.me>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
X-Gmane-NNTP-Posting-Host: pool-98-114-97-173.phlapa.fios.verizon.net
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0
In-Reply-To: <mrlalj$nlt$1@dont-email.me>
X-BeenThere: python-list@python.org
X-Mailman-Version: 2.1.20+
Precedence: list
List-Id: General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe: <https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive: <http://mail.python.org/pipermail/python-list/>
List-Post: <mailto:python-list@python.org>
List-Help: <mailto:python-list-request@python.org?subject=help>
List-Subscribe: <https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups: comp.lang.python
Message-ID: <mailman.66.1440625616.11709.python-list@python.org>
Lines: 47
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1440625616 news.xs4all.nl 23752 [2001:888:2000:d::a6]:37310
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:95678

On 8/26/2015 5:21 PM, Rob Gaddi wrote:
> I'm running into some strangeness trying to work with the bitfield module
> from my ctypes-bitfield package (on PyPi).  I'm trying to use isinstance
> (), and it's kinda sorta lying to me.

isinstace(inst, klass) is implemented as
klass.__instancecheck__(inst) ==
type(klass).__instancecheck__(klass, inst)
In other words, __instancecheck__ is a metaclass method.
Reference Manual, 3.3.4. Customizing instance and subclass checks

> ----- IPython session (Python 3.4 under Linux) -------
> In [649]: bf.__mro__
> Out[649]: (bitfield._TD, _ctypes.Union, _ctypes._CData,
> bitfield.Bitfield, builtins.object)

Find the metaclasses with "for cl in bf.__mro__: print(type(bf))" and 
then take a look at the __instancecheck__ method if not 'type'.

> In [650]: isinstance(bf, bitfield.Bitfield)
> Out[650]: False
>
> In [651]: bf.__bases__
> Out[651]: (_ctypes.Union, bitfield.Bitfield)
>
> In [652]: bf.__bases__[1]
> Out[652]: bitfield.Bitfield
>
> In [653]: bf.__bases__[1] is bitfield.Bitfield
> Out[653]: True
> -------------------------------------------------------
>
> Is there an issue with isinstance and multiple inheritance?  Conversely
> is there an issue with isinstance and ctypes derived classes (Bitfield
> isn't, but Bitfield is a mixin that always works with Unions)  I know
> that ctypes classes can get really wonky on this stuff.
>
> More generally, is there any good way to introspect ctypes derived
> classes?  I have to figure out whether things are derived from Structure,
> Union, Array etc. through some ugly indirect methods, and have no idea
> why.
>


-- 
Terry Jan Reedy