In python, objects are referred as references

Python uses shallow copy for object copying. If you see this code, The value b.a.val is modified externally.

class A(object):
    def __init__(self):
        self.val = 20
    
class B(object):
    def __init__(self, a):
        self.a = a
    
if __name__ == "__main__":
    a = A()

    b = B(a)
    print b.a.val # 20
    a.val = 40
    print b.a.val # 40

You need to use copy.deepcopy to avoid this pitfall.

from copy import *

class A(object):
    def __init__(self):
        self.val = 20
    
class B(object):
    def __init__(self, a):
        self.a = deepcopy(a)
        
    
if __name__ == "__main__":
    a = A()

    b = B(a)
    
    print b.a.val
    a.val = 40
    print b.a.val