Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!tudelft.nl!txtfeed1.tudelft.nl!multikabel.net!newsfeed20.multikabel.net!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Date: Thu, 10 Nov 2011 12:31:23 -0800
From: Ethan Furman <ethan@stoneleaf.us>
User-Agent: Thunderbird 1.5.0.10 (Windows/20070221)
MIME-Version: 1.0
To: python-list@python.org
Subject: Re: The python implementation of the "relationships between classes".
References: <CANVwdDr-6x8bt4iNL+Q=jZ7+edk7XbR2K_LeJC3Z60qy=vVFQA@mail.gmail.com>	<CAPTjJmoOBTiRE8vUNz_xQcwDZA6gazG1fAMkFfhPMomtAKtp6w@mail.gmail.com>	<CANVwdDr6qBueBGXykLrhH+MLiwyozD_U1+YRBnrWNfge0=uvDQ@mail.gmail.com>	<CAPTjJmqkH6WzbEXWLf6T9yyJyfkH-yCFk1HWd_Vrqwi7iz3FrQ@mail.gmail.com>	<CANVwdDqYt+YiJ-hz1F7r+HWnQvFRaJzHvESGoiJu5TKdKE5W7Q@mail.gmail.com>	<1320941348.7601.57.camel@tim-laptop>	<CANVwdDrZD0Ay7y93uP1r9u+nYS6p==kXq9XZ3m5VHP6w5g1Y+Q@mail.gmail.com> <CAMuTYXj+FOy8tgGAkt-iOUq4O8Ph5-W2OHd=4QSE1OWstu0U0Q@mail.gmail.com>
In-Reply-To: <CAMuTYXj+FOy8tgGAkt-iOUq4O8Ph5-W2OHd=4QSE1OWstu0U0Q@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.2623.1320958355.27778.python-list@python.org>
Lines: 58
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:15561

Benjamin Kaplan wrote:
> You're still misunderstanding Python's object model. del does NOT
> delete an object. It deletes a name. The only way for an object to be
> deleted is for it to be inaccessible (there are no references to it,
> or there are no reachable references to it).
>>>> foo = object()
>>>> bar = foo
>>>> foo
> <object object at 0x01CE14C0>
>>>> bar
> <object object at 0x01CE14C0>
>>>> del foo
>>>> bar
> <object object at 0x01CE14C0>
>>>> foo
> 
> Traceback (most recent call last):
>   File "<pyshell#7>", line 1, in <module>
>     foo
> NameError: name 'foo' is not defined
> 
> 
> There is no way to force the go_to_heaven method to delete the head
> unless you can make sure that all other references to the head are
> weak references. If you need strictly-enforced relationships, Python
> is probably not the right language for the job- you'll be better off
> with C++ or Java.

Having said all that, you could do something like:

class BodyPart(object):
     _alive = True
     def __nonzero__(self):
         return self._alive
     def die(self):
         self._alive = False

class Head(BodyPart):
     "will contain things like Brain, Eyes, etc"
     size = 5

class Body(BodyPart):
     def __init__(self):
         self._head = Head()
     def go_to_heaven(self):
         self._head.die()
         self.die()

John_Doe = Body()

if John_Doe:
     print "John Doe is alive!"
John_Doe.go_to_heaven()
if John_Doe:
     print "uh oh - something wrong"
else:
     print "John Doe is no longer with us"
     print "and his head is %s" % ('alive' if John_Doe._head else 'dead')