3.14.5.1 Pickling and unpickling normal class instances

When a pickled class instance is unpickled, its __init__() method is normally not invoked. If it is desirable that the __init__() method be called on unpickling, a class can define a method __getinitargs__(), which should return a tuple containing the arguments to be passed to the class constructor (i.e. __init__()). The __getinitargs__() method is called at pickle time; the tuple it returns is incorporated in the pickle for the instance.

Classes can further influence how their instances are pickled; if the class defines the method __getstate__(), it is called and the return state is pickled as the contents for the instance, instead of the contents of the instance's dictionary. If there is no __getstate__() method, the instance's __dict__ is pickled.

Upon unpickling, if the class also defines the method __setstate__(), it is called with the unpickled state^3.7. If there is no __setstate__() method, the pickled state must be a dictionary and its items are assigned to the new instance's dictionary. If a class defines both __getstate__() and __setstate__(), the state object needn't be a dictionary and these methods can do what they want.^3.8

Warning: For new-style classes, if __getstate__() returns a false value, the __setstate__() method will not be called.

Footnotes

... state ^3.7: These methods can also be used to implement copying class instances.
... want.^3.8: This protocol is also used by the shallow and deep copying operations defined in the copy module.

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