magical expanding hash

[braver]

I need a magical expanding hash with the following properties:

* it creates all intermediate keys

meh['foo']['bar] = 1

-- works even if meh['foo'] didn't exist before

* allows pushing new elements to leaves which are arrays

meh['foo']['list] << elem1
meh['foo']['list] << elem2

* allows incrementing numeric leaves

meh['foo']['count'] += 7

* serializable

I have such a class in ruby.  Can python do that?