Python, calling multiple functions in a cascading order (not nested functions)

I am trying to do the following:

If i have:

class A():
      def a(self):
          return 1

class B():
      def b(self):
          return A()

i can do

new_b = B()
new_b.b().a() # this returns 1

My questions is:

How can i do b().a() where b and a are are given as funct parameters in a method.

I.E: the following:

def f(f1, f2):
     return f1().f2()

new_a = A()
f_a = new_a.a
new_b = B()
f_b = new_b.b

f(f_b, f_a) # error 

Traceback (most recent call last): File "", line 1, in File "", line 2, in f AttributeError: 'A' object has no attribute 'f2'

Additionally, can i provide f_b as a string "b" and still be able to call it?