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Date: Thu, 10 Nov 2011 14:32:24 -0500
Subject: Re: The python implementation of the "relationships between classes".
From: Benjamin Kaplan <benjamin.kaplan@case.edu>
To: python-list@python.org
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On Thu, Nov 10, 2011 at 1:06 PM, Jerry Zhang <jerry.scofield@gmail.com> wro=
te:
>
>
> I just did an example code to describe what i am looking for.
> /*-----------------------------------------------------------------------=
-------------------------*/
> # ...
>
> class Head:
> =A0=A0=A0 def __init__(self):
> =A0=A0=A0=A0=A0=A0=A0 self.size =3D 5
>
> class Hat:
> =A0=A0=A0 def __init__(self):
> =A0=A0=A0=A0=A0=A0=A0 self.color =3D red
> =A0=A0=A0 def took_on(self, body):
> =A0=A0=A0=A0=A0=A0=A0 self.body =3D body
> =A0=A0=A0 def took_off(self, body):
> =A0=A0=A0=A0=A0=A0=A0 del self.body
>
> class Body:
> =A0=A0=A0 def __init__(self):
> =A0=A0=A0=A0=A0=A0=A0 self.head =3D Head()
> =A0=A0=A0 def take_on_hat(self, hat):
> =A0=A0=A0=A0=A0=A0=A0 self.hat =3D hat
> =A0=A0=A0=A0=A0=A0=A0 hat.take_on(self)
> =A0=A0=A0 def take_off_hat(self):
> =A0=A0=A0=A0=A0=A0=A0 hat.take_off(self)
> =A0=A0=A0=A0=A0=A0=A0 del self.hat
> =A0=A0=A0 def go_to_heaven(self):
> =A0=A0=A0=A0=A0=A0=A0 take_off_hat(self)
> =A0=A0=A0=A0=A0=A0=A0 del self.head
> /*-----------------------------------------------------------------------=
-----------------------------------*/
>
> In this example, Head and body are COMPOSITION, which means
> a. A head only live with one body, it can not live with other body. It ca=
n
> not even live without body
> b. If the body go to die, the head also go to die.
>
> Body and Hat are aggregation, which means
> a. A hat live isolate with body, its life circle is isolate
> b. A hat only live with one body at a specific time, it can not live with
> two body(association would be more than one)
>
> So when the body die, the clean dead should include take_off Hat and del
> Head, otherwise, the code definition is not prciselly describing the
> relationship, which may cause a mess system, for example, a body has dead=
,
> but one hat is still associated with a unreferenced body.
> A point on this issue, maybe python is smart that the take_off_hat(self) =
is
> not needed in go_to_heaven() method(i am not sure yet), but the del
> self.head is sure needed, otherwise, we will have a no_body_head in our
> ZODB, that is extremely horrible, right?
>
> All of these points one, the four kinds of class relationship in UML
> precisely describe the real word, if the implementation is not precisely,
> you will have unexpected results.
> Of course, python may be smart to achieve such goal with less effort, tha=
t
> is why i am asking for a code example for all of the four relationships.

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 =3D object()
>>> bar =3D 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.