Mutable class attribute shared state

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Accepted Answer

The list `members` is a class attribute shared by every instance. Calling `self.members.append(...)` mutates the same list in place - it does not create an instance attribute, because append modifies the existing object rather than rebinding the name. Both `a` and `b` refer to the same list, so after both calls the list has two entries. To give each instance its own list, initialize it in `__init__` with `self.members = []`.