>
>
>     def __init__(self):
>         vars(self).setdefault('default', self.set_default())
>         vars(self).setdefault('default', self.set_default())
>

This code is equivalent to

    def __init__(self):
        x = self.set_default()
        vars(self).setdefault('default', x)

        x = self.set_default()
        vars(self).setdefault('default', x)

>
because the argument is evaluated before the call to setdefault() so you
can't optimise anything here.