Automatic detection of required variable to make a restart

main.py

class Variable:
    """
    Determine required variables to restaure a state:
        0: Go to state A
        1: Go from state A to state B with an operation O.
        2: Dump required variables.
        3: Go to state A, and load required variables to be at state B.
        4: Go from state B to state C with the same operation O.
    """

    instances = {}
    record_required = False

    def __init__(self,value,name):
        self._value = value
        self._erased = False
        self._used = False
        self._required = False
        self.name = name
        Variable.instances[name] = self

    @property
    def value(self):
        if Variable.record_required and not self._erased:
            self._used = True
        return self._value

    @value.setter
    def value(self, value):
        self._value = value
        if Variable.record_required:
            self._erased = True
            if self._used:
                self._required = True

    @property
    def required(self):
        return self._required

    @staticmethod
    def dump():
        return {name: var for name,var in Variable.instances.items()
                          if var.required}

class Fib():
    """
    Compute Fibonacci sequence
    """
    def __init__(self,a,b):
        self.n = Variable(5,'n')
        self.a = Variable(a,'a')
        self.b = Variable(b,'b')
        self.i = Variable(0,'i')
        self.a_backup = Variable(None,'a_backup')

    def compute(self):
        print(self.i.value, self.a.value, self.b.value, self.n.value)
        self.a_backup.value = self.a.value
        self.a.value = self.b.value
        self.b.value += self.a_backup.value
        self.i.value += 1

def main():
    f = Fib(1,1)
    Variable.record_required = True
    f.compute()
    Variable.record_required = False
    f.compute()

    backup = Variable.dump()

    g = Fib(1,1)
    for name, var in backup.items():
        # a, b and i are set. n and a_backup are not required.
        print('set %s' % name)
        setattr(g,name,var)
    while g.i.value < g.n.value-1:
        g.compute()

if __name__ == '__main__':
    main()