Are there disadvantages of using __slots__?

I’m using Python 3.7 and Django. I was reading about __slots__ . Evidently, __slots__ can be used to optimize memory allocation for a large number of those objects by listing all the object properties ahead of time.

class MyClass(object):
    __slots__ = ['name', 'identifier']
    def __init__(self, name, identifier):
        self.name = name
        self.identifier = identifier
        self.set_up()

My perhaps obvious question is why wouldn’t we want to do this for all objects? Are there disadvantages for using __slots__?

Answer

Fluent Python by Luciano Ramalho lists the following caveats

• You must remember to redeclare __slots__ in each subclass, since the inherited attribute is ignored by the interpreter.

• Instances will only be able to have the attributes listed in __slots__, unless you include __dict__ in __slots__ — but doing so may negate the memory savings.

• Instances cannot be targets of weak references unless you remember to include __weakref__ in __slots__.