The reason you are getting this error is because you are naming the setter method incorrectly. You have named the setter method as set_x which is incorrect, this is why you are getting the Attribute Error.
To fix this error, instead of naming it as set_x you need to give it the same name as the property you creating a setter for, in you case you can do it like this:
def x(self, value):
self._x = value
If you wish to learn more about OOP in python, watch this video