Listing attributes of namedtuple subclass

Question

I have a tiny class that extends a namedtuple, but the __dict__ property of its instances is always returning empty.

Point = namedtuple('Point', 'x y')
p1 = Point(20, 15)
print(p1, p1.__dict__)
# Point(x=20, y=15) OrderedDict([('x', 20), ('y', 15)]) <--- ok

class SubPoint(Point): pass
p2 = SubPoint(20, 15)
print(p2, p2.__dict__)
# SubPoint(x=20, y=15) {} <--- why is it empty?

p2 has the attributes, but its __dict__ is empty. They are listed correctly with dir(), though, which is strange. Note this work correctly when SubPoint extends a vanilla class.

What is happening, and how do I list the attributes in my subclass instance?

Answer 1

The problem is that __slots__ is only limited to a class it is defined in, so base classes will always have their own __dict__ attribute unless you define __slots__ there too. (And also note that the __dict__ attribute of namedtuple is not a normal dict but a @property.)

From docs:

The action of a __slots__ declaration is limited to the class where it is defined. As a result, subclasses will have a __dict__ unless they also define __slots__ (which must only contain names of any additional slots).

So, when you defined __slots__ in the subclass then it failed to look for an attribute __dict__ in that class, so moved on to base class where it found the __dict__ attribute.

A simple demo:

class A:
    __slots__=  ('a', 'b')

    @property
    def __dict__(self):
        print ('inside A')
        return self.__slots__         

class B(A):
    pass

print(B().__dict__)

print ('-'*20)

class B(A):
    __slots__ = ()
print(B().__dict__)

output:

{}
--------------------
inside A
()