Q:

Is it a good practice to make nested class in python?

I manage to define a class A, use a list of instance of another class B as class A's instance variable. class B has a function to change class A's another instance variable a1. Class A also has a function to change class B's instance variable bb. So class A can access class B and class B can access class A.
The two classed are nested together. I know we can make things easier to change all instance variable and function of class B to class A. But in my project, this nested structure is what real things are. 
class A:
  class B:
      count = 0
      def __init__(self,b1=None,b2=None):
          self.b1 = b1
          self.b2 = b2
      def funcb(self,bb):
          A.a1 = pyfunc1(bb)
  def __init__(self,a1,a2):
      self.a1 = a1
      self.a2 = a2
      self.listb = [A.B()]
  def funca(self,aa):
      A.B.count += 1
      b = A.B(self.a1,self.a2)
      listb.append(b)
      listb[A.B.count].b1 = listb[A.B.count-1].b1.pyfunc2(aa)
      listb[A.B.count].b2 = pyfunc3(aa,self.a2)
      listb[A.B.count].funcb(self.a2)

What I want to know is that if such kind of nested class will reduce the python efficiency? Any better solution?

A:

Nesting a class doesn't reduce nor increase execution efficiency. It may alter maintenance and understanding efficiency.
The nested class becomes just another attribute on the parent class. You'll have to reference it as A.B rather than B. That's it, you deferred the lookup to a different namespace. In other words, your __init__ method will fail, because there is no global name B, only A.B and self.B exist (both referencing the same class object).
There is otherwise no special relationship between a nested class and their parent, as there is in Java.
Most Python developers do not nest classes, so when you do so you break convention and increase maintenance cost.

