OrderedDict won't sort within a class

I have a parent class, and I want to keep a registry (in the form of a dictionary) of all instances of its sub-classes. Easy, but I want the registry to sort itself based on its keys, which are the arguments of the 2 sub-classes on initialisation. This is my code in simplified form:

from collections import OrderedDict

class Parent:
    _registry = OrderedDict()
    def __init__(self):
        # add each sub-class instance to the registry & sort the registry
        self._registry.update({self._num:self})
        self._registry = OrderedDict(sorted(self._registry.items()))

class Foo(Parent):
    def __init__(self, number):
        self._num = number
        Parent.__init__(self)
        # then do some stuff

class Bar(Parent):
    def __init__(self, number):
        self._num = number
        Parent.__init__(self)
        # then do some other stuff
...

But, although the registry updates itself with the new sub-class objects, it does not sort itself.

>>> a = Foo(3)
>>> Parent._registry # check to see if a was added to the registry
OrderedDict([(3, <Foo instance at 0x00A19C0C8>)])
>>> b = Bar(1)
>>> Parent._registry # check to see if b was inserted before a in the registry
OrderedDict([(3, <Foo instance at 0x00A19C0C8>), (1, <Bar instance at 0x00A19C1C8>)])

b comes after a in the registry!

If I do it manually in the iPython console, it works:

>>> Parent._registry = OrderedDict(sorted(Parent._registry.items()))
OrderedDict([(1, <Bar instance at 0x00A19C1C8>), (3, <Foo instance at 0x00A19C0C8>)])

Why won't it sort itself? I need it to, because later on, things have to happen to those objects in strict order of their number arguments.

That's because:

self._registry = OrderedDict(sorted(self._registry.items()))

creates a new attrbute on the instance, this doesn't affect Parent._registry.

Replace that line with:

Parent._registry = OrderedDict(sorted(self._registry.items()))

Here self._registry.items() can fetch the value of Parent._registry but that doesn't mean assignment to self._registry will affect Parent._registry.

Another way to do it using self._registry itself:

def __init__(self):
    items = sorted(self._registry.items() + [(self._num, self)]) #collect items
    self._registry.clear() #clean the dict
    self._registry.update(items) #now update it