How to dynamically change __slots__ attribute?

Question

Suppose I have a class with __slots__

class A:
    __slots__ = [\'x\']

a = A()
a.x = 1   # works fine
a.y = 1   # AttributeError (as expected)
         


        

Answer 1

It appears to me a type turns __slots__ into a tuple as one of it's first orders of action. It then stores the tuple on the extended type object. Since beneath it all, the python is looking at a tuple, there is no way to mutate it. Indeed, I'm not even sure you can access it unless you pass a tuple in to the instance in the first place.

The fact that the original object that you set still remains as an attribute on the type is (perhaps) just a convenience for introspection.

You can't modify __slots__ and expect to have that show up somewhere (and really -- from a readability perspective, You probably don't really want to do that anyway, right?)...

Of course, you can always subclass to extend the slots:

>>> class C(A):
...   __slots__ = ['z']
... 
>>> c = C()
>>> c.x = 1
>>> c.z = 1