[Tutor] Ok, take one step back and let you look

[K P]

Hmmm, I can see the possiblities in other areas. Thanks for the 
'snippet'. Maybe a small example would refine what I am seeking:

Quick example: 3 books: red, blue, green. All instances of class 
Book. 3 glasses: yellow, purple, aqua, all instances of class 
Glassware. In my inventory, I would like them to appear thus:

Item          Qty
Book         3
Glass        3
Not like this, which is currently the case:

Item         Qty
Blue         1
Green       1
Red          1
yellow       1
purple       1
aqua         1

I'm not too concerned with having tons of instances of a particular 
class/object. Just how they would be tracked in the inventory, and 
also displayed(on screen). If I am still not being very clear with this, 
I shall try harder to clarify myself :)

Ken
[big snip]
> 
> It sounds like you want some form of "semaphore" when creating class
> instances: a certain number of instances of that class can be
> created, but beyond that the semaphore "blocks" (raises an exception
> in this case).  Here is a mix-in you can look at and see how to work
> into your code.
> 
> class InstanceLimiter:
>   class InstanceLimiterError(Exception):
>     pass
>   _Instance_Limiter_count = 10
>   def new_instance(self):
>     klass = self.__class__
>     if klass._Instance_Limiter_count > 0:
>       klass._Instance_Limiter_count = klass._Instance_Limiter_count - 1
>     else:
>       raise self.InstanceLimiterError, \
>           'too many instances of %s' % klass.__name__
>   def remove_instance(self):
>     klass = self.__class__
>     klass._Instance_Limiter_count = klass._Instance_Limiter_count + 1
> 
> Then you can work it into a class like:
> 
> class Person(InstanceLimiter):
>   """A person under population control."""
>   _Instance_Limiter_count = 1000
>   def __init__(self, name, location, birthdate):
>     self.new_instance()
>     ...
>   def __del__(self):
>     self.remove_instance()
>     ...
> 
> Now only one thousand instances of Person can exist.
> 
> I'm not sure if this is what you want, if not maybe you can use the
> techniques in other places.
> 
>   -Arcege
> 
> 
> -- 
> ------------------------------------------------------------------------
> | Michael P. Reilly, Release Engineer | Email: arcege@shore.net        |
> | Salem, Mass. USA  01970             |                                |
> ------------------------------------------------------------------------
> 
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