When using slots, why does dir(type) have no __dict__ attribute?

Question

I'm trying to understand slots. Therefore, I have written a little script with two classes, one using slots and one not.

class A:
    def __init__(self, name):
        self.name = name

    def getName(self):
        return self.name

class C:
    __slots__ = "name"

    def __init__(self, name):
        self.name = name

    def getName(self):
        return self.name

When I use the dir() on type A and on an object of type A, the attribute __dict__ appears in the result list, as expected.

dir(A)
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'getName']

dir(A("test"))
['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__get__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'getName', 'name']

If I use type C I get

print(dir(C))
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', 'classAttributeC', 'getName', 'name']

print(dir(C("test")))
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', 'classAttributeC', 'getName', 'name']

No attribute __dict__ in the results list for dir(C("test")), as expected but also no attribute __dict__ for dir(C). Why isn't the attribute in the results list when I can call C.__dict__ and get the following output?

{'__module__': '__main__', 'classAttributeC': 9999, '__slots__': 'name', '__init__': <function C.__init__ at 0x7ff26b9ab730>, 'getName': <function C.getName at 0x7ff26b9ab7b8>, 'name': <member 'name' of 'C' objects>, '__doc__': None}

Answer 1

Since you don't override __dir__ here, in each case here it will resolve in the MRO to type.__dir__(A) or type.__dir__(C). So we look at the default implementation of __dir__ for types, here in Objects/typeobject.c

/* __dir__ for type objects: returns __dict__ and __bases__.
   We deliberately don't suck up its __class__, as methods belonging to the
   metaclass would probably be more confusing than helpful.
*/
static PyObject *
type___dir___impl(PyTypeObject *self)
{
    PyObject *result = NULL;
    PyObject *dict = PyDict_New();

    if (dict != NULL && merge_class_dict(dict, (PyObject *)self) == 0)
        result = PyDict_Keys(dict);

    Py_XDECREF(dict);
    return result;
}

The bases are the same (object,), so there is your answer in the __dict__:

>>> "__dict__" in A.__dict__
True
>>> "__dict__" in C.__dict__
False

So, types without slots implement a __dict__ descriptor, but types which implement slots don't - and you just get a __dict__ implementation from above:

>>> inspect.getattr_static(A, "__dict__")
<attribute '__dict__' of 'A' objects>
>>> inspect.getattr_static(C, "__dict__")
<attribute '__dict__' of 'type' objects>