Python, __slots__, inheritance, and class variables ==> attribute is read-only bug

Question

I have a big tree with hundreds of thousands of nodes, and I'm using __slots__ to reduce the memory consumption. I just found a very strange bug and fixed it, but I don't understand the behavior that I saw.

Here's a simplified code sample:

class NodeBase(object):
    __slots__ = ["name"]
    def __init__(self, name):
        self.name = name

class NodeTypeA(NodeBase):
    name = "Brian"
    __slots__ = ["foo"]

I then execute the following:

>>> node = NodeTypeA("Monty")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
AttributeError: 'NodeTypeA' object attribute 'name' is read-only

There is no error if NodeTypeA.name is not defined (side note: that attribute was there by mistake, and had no reason for being there). There is also no error if NodeTypeA.__slots__ is never defined, and it therefore has a __dict__.

The thing I don't understand is: why does the existence of a class variable in a superclass interfere with setting an instance variable in a slot in the child class?

Can anybody explain why this combination results in the object attribute is read-only error? I know my example is contrived, and is unlikely to be intentional in a real program, but that doesn't make this behavior any less strange.

Thanks,
Jonathan

Answer 1

A smaller example:

class C(object):
    __slots__ = ('x',)
    x = 0

C().x = 1

The documentation on slots states at one point:

__slots__ are implemented at the class level by creating descriptors (Implementing Descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by __slots__; otherwise, the class attribute would overwrite the descriptor assignment.

When __slots__ is in use, attribute assignment to slot attributes needs to go through the descriptors created for the slot attributes. Shadowing the descriptors in a subclass causes Python to be unable to find the routine needed to set the attribute. Python can still see that an attribute is there, though (because it finds the object that's shadowing the descriptor), so it reports that the attribute is read-only.