Python Language Pickle data serialisation Customize Pickled Data


Example

Some data cannot be pickled. Other data should not be pickled for other reasons.

What will be pickled can be defined in __getstate__ method. This method must return something that is picklable.

On the oposite side is __setstate__: it will receive what __getstate__ created and has to initialize the object.

class A(object):
    def __init__(self, important_data):
        self.important_data = important_data
        
        # Add data which cannot be pickled:
        self.func = lambda: 7
        
        # Add data which should never be pickled, because it expires quickly:
        self.is_up_to_date = False
    
    def __getstate__(self):
        return [self.important_data] # only this is needed
    
    def __setstate__(self, state):
        self.important_data = state[0]
        
        self.func = lambda: 7  # just some hard-coded unpicklable function
        
        self.is_up_to_date = False  # even if it was before pickling

Now, this can be done:

>>> a1 = A('very important')
>>>
>>> s = pickle.dumps(a1)  # calls a1.__getstate__()
>>>
>>> a2 = pickle.loads(s)  # calls a1.__setstate__(['very important'])
>>> a2
<__main__.A object at 0x0000000002742470>
>>> a2.important_data
'very important'
>>> a2.func()
7

The implementation here pikles a list with one value: [self.important_data]. That was just an example, __getstate__ could have returned anything that is picklable, as long as __setstate__ knows how to do the oppoisite. A good alternative is a dictionary of all values: {'important_data': self.important_data}.

Constructor is not called! Note that in the previous example instance a2 was created in pickle.loads without ever calling A.__init__, so A.__setstate__ had to initialize everything that __init__ would have initialized if it were called.