change an object variable from other object(delegation)

I am using python to implement something like following.

I have an Object A, which has a member variable a. I have an Object C, which has an function c()

Basically, I want to be able to set A.a from c(). The restriction is that I can not declare and init object A in C. So I heard that delegation may work, but I have no clue.

It would be great if you can provide some hints or suggestions.

Thanks