versioned_attr.py

class VersionedAttribute:

    def __init__(self, status_sequence):
        self.value_status_mapping = {state: None for state in status_sequence}

    def __get__(self, instance, owner):
        return self.value_for(instance.status)

    def __set__(self, instance, value):
        self.value_status_mapping[instance.status] = value

    def value_for(self, status):
        return self.value_status_mapping[status]


PLANNING = 1
FILLING_IN = 2
CORRECTING = 3

class Assignment(object):

    status = PLANNING
    start_time = VersionedAttribute([PLANNING, FILLING_IN, CORRECTING])


a = Assignment()

a.start_time = "around nine-ish"
print(a.start_time)

a.status = FILLING_IN
a.start_time = "the fuck he only showed up after lunch and didn't do shit"
print(a.start_time)

a.status = PLANNING
print(a.start_time)