The python implementation of the "relationships between classes".

[Jerry Zhang]

2011/11/10 Jerry Zhang <jerry.scofield at gmail.com>

>
>
> 2011/11/10 Chris Angelico <rosuav at gmail.com>
>
>> On Fri, Nov 11, 2011 at 12:58 AM, Jerry Zhang <jerry.scofield at gmail.com>
>> wrote:
>> > Cls_a:
>> >     def __init__(self):
>> >         self.at1 = 1
>> > Cls_b:
>> >     def __init__(self):
>> >         self.com1 = Cls_a()
>> >     def __del__(self):
>> >         del self.com1
>> > Is it a right implementation for composition?
>>
>> Yes, except that you don't need to explicitly del it. Python (at
>> least, CPython) is reference-counted; your Cls_b object owns a
>> reference to the Cls_a object, so (assuming nothing else has a
>> reference) that Cls_a will be happily cleaned up when the Cls_b is.
>>
>> Python doesn't really talk about "composition" etc. It's much simpler:
>> everything's an object, and you have references to that object. A
>> named variable is a reference to some object. A member on an object
>> is, too. Whenever an object is expired, all objects that it references
>> lose one reference, and if that was the sole reference, those objects
>> get expired too. It's a change of thinking, perhaps, but not a
>> difficult one in my opinion; and it's so easy to work with when you
>> grok it.
>>
>
> Unfortunately there is a difference between composition and aggregation in
> my real word, and my application really care this since it is trying to
> simulate this real world model, so my system should track this difference accurately,
> otherwise the system may not work well.
>
> For example,
> a. the Cls_arm and Cls_body may be composition, but not aggregation. My
> app must ensure that " one arm instance only live with one body instance,
> if the body instance die, the arm instance must die.
>  b. the Cls_auto and the Cls_tyre may be aggregation. "One tyre still can
> live even the auto is dead."
>
> Meanwhile, I have a ZODB running, which stores all the living objects.
>
So lifecycle should be ensured by class definition. I originally thought
this topic would be talked somewhere since many python OOP designer should
have met this, or am i wrong on some point?


>
>
>> ChrisA
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
>
>
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