because you directly modified the entire reference of self.x . The operation of self.x + = 1 is to change the pointing to of self.x from the original value to another value, rather than changing the value it points to to change to another value, while self.z still stores the old value pointed to by self.x , of course it will not be updated. To achieve the similar effect you want, try

a = []
b = []
c = [ a, b ]
print(c)
a.append(1)
b.append(2)
print(c)

has nothing to do with instantiation.

the above code modifies the values that a and b point to, rather than modifying the entire reference to point to the new value. That is to say, the directions of the modified a and b have not changed (while self.x + = 1 modified the self.x direction), so the two elements of c can still find the values consistent with the current a and b through their references

wrap the function as a property with @ property

python3

>>> class A():
    def __init__(self):
        self.x = 0
        self.y = 0
        
    @property -sharp 
    def z(self):
        return [self.x, self.y]

    def add_one(self):
        self.x += 1

        
>>> a=A()
>>> a.add_one()
>>> a.x
1
>>> a.z
[1, 0]
>>>