Keeping track of subclasses and instances?

[Erik Jones]

On Oct 11, 2007, at 12:49 AM, Andreas Kraemer wrote:

> On Oct 10, 6:19 pm, Karlo Lozovina <_kar... at mosor.net> wrote:
>> Larry Bates wrote:
>>> I'm not completely sure I understand the question but here goes.
>>> Instances of
>>> classes are classes can be stored in lists or dictionaries.  In  
>>> lists you
>>> reference them via their index (or iterate over them) and in  
>>> dictionaries
>>> you can give them a name that is used as a key.
>>
>> I wish if it were that simple :).
>>
>> Here is a longer description - I have a function that given input  
>> creates a
>> custom class and returns it back. The user is free to subclass  
>> that (even
>> more, he should do that), and of course he will make instances of  
>> those
>> subclasses. Now, my question is how to keep track of subclasses  
>> and their
>> instances, without the need for user interaction (appending them  
>> to a list,
>> or adding to dictionary)?
>>
>> Thanks,
>>
>> --
>> Karlo Lozovina - Mosorclass Meta(type):
>
> What about the following solution?
>
> class Meta(type):
>   def __new__(mcl,*args,**kw):
>     class_ = super(Meta,mcl).__new__(mcl,*args,**kw)
>     mcl._classes.append(class_)
>     class_._instances = []
>     return class_
>   _classes = []
>
> def factory():
>   class Class(object):
>     __metaclass__ = Meta
>     def __new__(cls,*args,**kw):
>       instance = super(Class,cls).__new__(cls,*args,**kw)
>       cls._instances.append(instance)
>       return instance
>   return Class
>
>>>> A = factory()
>>>> class B(A): pass
> ...
>>>> a = A()
>>>> b = B()
>>>> Meta._classes
> [<class 'meta.Class'>, <class '__main__.B'>]
>>>> A._instances
> [<meta.Class object at 0xb7dbb08c>]
>>>> B._instances
> [<__main__.B object at 0xb7dbb0ec>]
>
> So, you see that you can access all classes, their subclasses, and
> instances from Meta.
>
> Of course in a more elaborate version _classes and _instances should
> store weak references, so that classes and instances can actually be
> deleted. I also haven't explored under which circumstances this can
> break ...
>
> I can imagine different use cases for this, (though they certainly are
> non-standard :-)). I once contemplated the (toy-) implementation of a
> frame-based knowledge representation system using Python's object
> model, where one definitely needs classes to keep track of their
> instances ...

Another use case that I've run across is when defining model classes  
using an ORM against a database that uses table inheritance  
extensively with the result that the database has way more tables  
than you'd actually want to manually maintain model classes/files for.

Erik Jones

Software Developer | Emma®
erik at myemma.com
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