__getstate__ method not being called when pickling a subclass of set

Question

From what I understand, when calling pickle.dumps on an object, it will call the object's __getstate__ method (if it has one) to determine what to pickle.

If I create a class such as:

class DictClass(dict):
    def __getstate__(self):
        print "pickling"
        return self

I get this result:

>>> pickle.dumps(DictClass())
pickling
'ccopy_reg\n_reconstructor\np0...'

I can do the same thing, replacing 'dict' with 'list':

class ListClass(list):
    def __getstate__(self):
        print "pickling"
        return self

>>> pickle.dumps(ListClass())
pickling
'ccopy_reg\n_reconstructor\np0...'

But if I use 'set', something different happens:

class SetClass(set):
    def __getstate__(self):
        print "pickling"
        return self

>>> pickle.dumps(SetClass())
'c__main__\nSetClass...'

The __getstate__ method doesn't get called. Why is this, and is it possible to specify what part of a subclass of a set to pickle?

Answer 1

list does not implement __reduce__(), whereas set does:

>>> list().__reduce__()
...
TypeError: can't pickle list objects
>>> set().__reduce__()
(<type 'set'>, ([],), None)

It's the last tuple in the above example that gets pickled, so SetClass.__getstate__() never enters the picture.