Access instance attributes in config [Enhancement]

[neighthan]

Say we have a setup like this

class A:
    def __init___(self, x, y):
        # some value computed from the parameters
        self.n = x ** 2 + y

class B: 
    def __init___(self, n):
        self.n = n

I would like to be able to do this in my config:

A.x = 3
A.y = 2
B.n = @A().n

Hard-coding B.n = 11 would be bad because we might forget to change it when we update A.x. Obviously one can just not use the config for this and create a = A() first in Python then b = B(a.n). If the instances of A and B wouldn't naturally be created in the same place, though, then this makes the code worse since we have to pass a (or at least a.n) around.