restrict attr names

>>> class Foo:
...     def __init__(self, x):
...             self.x = x
...     def __setattr__(self, name, value):
...             if name not in {'x'}:
...                     raise AttributeError('only x is allowed')
...             super().__setattr__(name, value)
... 
>>> f = Foo(2)
>>> f.x
2
>>> f.y = 30
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in __setattr__
AttributeError: only x is allowed