class V:
  def __init__(self, x): 
    self._x = x
  
  @property
  def x(self):
    return self._x 

>v = V(5)
>v.x
5
>v._x
5
>v.x = 4
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: cant set attribute
>v._x = 4
>v.x
4

class V:
  def __init__(self,x):
    self.__x = x

  @property
  def x(self):
    return self.__x

>v = V(5)
>v.__x = 4
>v.__x
4
>v.x
5

>v = V(5)
>v.__dict__
{'_V__x': 5}
>v.__x = 4
>v.__dict__
{'_V__x': 5, '__x': 4}