How to unpickle a Python instance that was saved before the class changed a member variable to a property?

Question

I have a class that used to have a field data, but later the class was changed and now data is a property.

I would like to be able to unpickle instances that were pickled before the change, to preserve backward compatibility. A minimal example for illustration (in Python 2, though it should be the same in Python 3):

import pickle

class X(object):
    def __init__(self):
        self.data = 100

pickle.dump(X(), open("x-file",'w'))

# Redefine the class
class X(object):
    def __init__(self):
        self._data = 101
    @property
    def data(self):
        return self._data

y = pickle.load(open("x-file")) # cannot access the original data through y
print(y.data)

What I would like is to define a function load that unpickles the object, detects it is old style (e.g. by seeing that it doesn't have the _data field), and return a new style instance with its data instead. However, since the field data now is a property, the old data field was overwritten by the class definition.

Is there any easy way I can access the old data (i.e. other than parsing the pickle file myself, for example)?

EDIT After Peter Wood's answer, I got to this solution:

import pickle

class X(object):
    def __init__(self):
        self.data = 100

pickle.dump(X(), open("x-file",'w'))

# Redefine the class
class X(object):
    def __init__(self):
        self._data = 101
    @property
    def data(self):
        return self._data
    def __setstate__(self, state):
        if not '_data' in state:
            self._data = state['data']
            del state['data']
        self.__dict__.update(state)

y = pickle.load(open("x-file")) # cannot access the original data through y
print(y.data)

Answer 1

The documentation says in What can be pickled and unpickled?:

If you plan to have long-lived objects that will see many versions of a class, it may be worthwhile to put a version number in the objects so that suitable conversions can be made by the class’s __setstate__() method.

This is one of four "magic" methods you can define on your classes to facilitate serialisation. See Pickling Class Instances.