what is the __dict__ object?

[Gribouillis]

Obviously the call id(A.__dict__) allocates a new object of type mappingproxy, that is to say a smart pointer. You are printing the id of these smart pointers. In the examples above, when B.__dict__ is accessed, the mappingproxy created for A.__dict__ is already destroyed, so the same address is reused. If you keep a reference to the proxy, two different addresses are used

>>> x = A.__dict__
>>> y = B.__dict__
>>> id(x), id(y)
(140355706316360, 140355683451672)