self-registering objects?

[Gordon McMillan]

Ionel Simionescu writes:
> 
> I would like objects of a certain class to be able of registering
> themselves in a global dictionary, __register__, possibly upon
> __init__.
> 
> __register__ should contain associatins such as:
> 
> { 'ID': <__main__.certain_class instance at ####> }
> 
> Unfortunately, the naive:
> 
> def __init__(self, etc.):
>     global __register__
>     self.ID = 'instance1'
>     __register__[self.ID] = self
> 
> Does NOT work.
> 
> The object that I find in the __register__ under the given ID is
> apparently just the bare bones of the class and gets none of the
> changes incured to the instance after registration.
> 
> It's is not a problem if I register the object "from outside",
> but I would like to understand what's going on...

So would I!

--------------------------------------------
__register__ = {}

class A:
    def __init__(self, arg):
        __register__[id(self)] = self
        self.arg = arg

a1 = A(1)
a2 = A(2)

for a in __register__.values():
    print `a`, a.arg
------------------------------------------------------
When run, outputs:
<__main__.A instance at 7fb880> 2
<__main__.A instance at 7fb7f0> 1
>>>

In fact, __init__ is run after the instance construction is 
complete, so whatever is going wrong is _not_ what you think 
is going wrong!

(BTW: you don't need global on __register__, because you are 
not changing the binding, you are mutating the object).

- Gordon