The python implementation of the "relationships between classes".

Thu Nov 10 14:38:58 EST 2011

On 11/10/2011 9:31 AM, Jerry Zhang wrote:
> 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.

Create the arm as a private member '_arm' of body and make sure that no 
method of body passes out a reference to the arm. (In Python, outside 
code can still grab a reference to the private attribute, but that is a 
coding bug.)
I will point out that in the real world, dead donor transplants are 
based on the fact the parts of the body do NOT have to die when the 
composition does. I will not be surprised if we someday see arm transplants.
-- 
Terry Jan Reedy