You're probably doing something like:

parent.i = 3

instead of:

parent.child.field = 0.6

In the first one, you're setting an instance attribute on parent, on the second, you're modifying an attribute of the class attribute.

In any event, there's no bug here, so I'm going to close this. If you have followup questions, I suggest you use the python-list mailing list or https://discuss.python.org/c/users