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}
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>