Is a Borg rebellion possible? (a metaclass question)

[Carsten Haese]

On Fri, 2007-09-07 at 12:31 +0000, André wrote:
> In my application, I make use of the Borg idiom, invented by Alex
> Martelli.
> 
> class Borg(object):
>     '''Borg Idiom, from the Python Cookbook, 2nd Edition, p:273
> 
>     Derive a class form this; all instances of that class will share
> the
>     same state, provided that they don't override __new__; otherwise,
>     remember to use Borg.__new__ within the overriden class.
>     '''
>     _shared_state = {}
>     def __new__(cls, *a, **k):
>         obj = object.__new__(cls, *a, **k)
>         obj.__dict__ = cls._shared_state
>         return obj
> 
> ----
> This has worked very well so far, but is starting to impose some
> unwanted constraints on my program design.
> 
> What I would like to do is, to put it figuratively, create a Borg
> rebellion with various splinter groups.  In concrete Python terms, I
> would like to have
> 
> class MyClass(Borg, ...):
>    ...
> 
> seven_of_nine = MyClass(...)  # part of group "BORG"
> two_of_nine = MyClass(...)
> 
> splinter1 = MyClass(..., group='splinter')
> splinter2 = MyClass(..., group='splinter')
> 
> and have splinter 1 and splinter2 share the same state, but a
> different state than the one shared by members of the BORG collective.
> 
> Any suggestions from the metaclass experts?

You don't need a metaclass. Just turn _shared_state into a dictionary of
shared states, keyed by the group name:

class SplinterBorg(object):
    _shared_states = {}
    def __new__(cls, *a, **k):
        group = k.pop("group","BORG")
        obj = object.__new__(cls, *a, **k)
        obj.__dict__ = cls._shared_states.setdefault(group,{})
        return obj

HTH,

-- 
Carsten Haese
http://informixdb.sourceforge.net