我如何在子类中验证 __slots__？

Question

From the python docs 3.3.2.4.1 , it says " __slots__ declared in parents are available in child classes. However, child subclasses will get a __dict__ and __weakref__ unless they also define __slots__ (which should only contain names of any additional slots)."

However, while I was testing this as below:

class A(object):
    __slots__ = ('a')

class B(A):
    __slots__ = ('b')

b = B()
B.z = 'z'
B.a = 'a'
print(B.z) #z
print(B.a) #a
print(B.__dict__) #{'__module__': '__main__', '__slots__': ('b',), 'b': <member 'b' of 'B' objects>, '__doc__': None, 'z': 'z', 'a': 'a'}

Neither Bz nor B.__dict__ throws exception, the code manages to output.

I am confused why the __slots__ = ('b') in Class B doesn't validate. Theoretically, it should throw an exception at Bz because the keys in slots don't contain it.

Could you please help to explain this strange point? And if possible, could you please help to provide the right usage method of slots in child classes?

By the way, my python version is 3.9.7

Thank you so much.

Answer 1

The __slots__ should be a tuple there, but you don't actually have a tuple, you have a string. Try doing __slots__ = ("a",) .

Also, the slots apply to instances, not classes. You are assigning the attribute to the class object. Try the following. The instances work as expected.

a = A()
a.a = "a"
# a.b = "b"  # fails

b = B()
b.b = "b"
b.a = 'a'
# b.z = 'z'  # fails