[Tutor] Keeping a list of attributes of a certain type

[Dave Angel]

(You top-posted, which puts your two comments out of order.  Now the 
solution comes before the problem statement)

Guilherme P. de Freitas wrote:
> Ok, I got something that seems to work for me. Any comments are welcome.
>
>
> class Member(object):
>     def __init__(self):
>         pass
>
>
> class Body(object):
>     def __init__(self):
>         self.members = []
>
>     def __setattr__(self, obj, value):
>         if isinstance(value, Member):
>             self.members.append(obj)
>             object.__setattr__(self, obj, value)
>         else:
>             object.__setattr__(self, obj, value)
>
>     def __delattr__(self, obj):
>         if isinstance(getattr(self, obj), Member):
>             self.members.remove(obj)
>             object.__delattr__(self, obj)
>         else:
>             object.__delattr__(self, obj)
>
>
>
> john = Body()
> john.arm = Member()
> print(john.members)
> del john.arm
> print(john.members)
>
>
> On Wed, Jan 13, 2010 at 6:24 PM, Guilherme P. de Freitas
> <guilherme at gpfreitas.com> wrote:
>   
>> Hi everybody,
>>
>> Here is my problem. I have two classes, 'Body' and 'Member', and some
>> attributes of 'Body' can be of type 'Member', but some may not. The
>> precise attributes that 'Body' has depend from instance to instance,
>> and they can be added or deleted. I need any instance of 'Body' to
>> keep an up-to-date list of all its attributes that belong to the class
>> 'Member'. How do I do this?
>>
>> Best,
>>
>> Guilherme
>>
>> --
>> Guilherme P. de Freitas
>> http://www.gpfreitas.com
>>
>>     
If this is a class assignment, you've probably got the desired answer.  
But if it's a real-world problem, there are tradeoffs, and until those 
are known, the simplest solution is usually the best.  Usually, what's 
desired is that the object behaves "as if" it has an up-to-date list of...

If order of the list doesn't matter, I'd consider simply writing a 
single method, called 'members' which does a list comprehension of the 
object's attributes, calculating the list when needed.  Then use a 
decorator to make this method look like a read-only data attribute  
(untested):

class Body (object):
     @property
      def members(self):
             return  [obj for .....   if .... ]

DaveA