2011/11/10 Jerry Zhang <jerry.scofi...@gmail.com> > > > 2011/11/10 Chris Angelico <ros...@gmail.com> > >> On Fri, Nov 11, 2011 at 12:58 AM, Jerry Zhang <jerry.scofi...@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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