pickle_cheat.py

#!/usr/bin/env python
import pickle

if 'slots error':

    # Use -1 pickling version.
    # http://stackoverflow.com/questions/2204155/why-am-i-getting-an-error-about-my-class-defining-slots-when-trying-to-pickl/2204702#2204702

    class C(object):
        __slots__ = 'a'
        def __init__(self, a):
            self.a = a
    s = pickle.dumps(C(1), -1)
    assert pickle.loads(s).a == 1

if 'instancemethod error':

    # http://stackoverflow.com/questions/27318290/why-can-i-pass-an-instance-method-to-multiprocessing-process-but-not-a-multipro

    # Basic example

    class A(object):
        def z(self):
            return  1
    try:
        pickle.dumps(A.z, -1)
    except pickle.PicklingError:
        pass
    else:
        raise

    # Less obvious via inner member:

    class A(object):
        def __init__(self):
            self.f = self.f
        def f():
            return 1
    try:
        pickle.dumps(A(), -1)
    except pickle.PicklingError:
        pass
    else:
        raise

    # Fine with regular functions:

    def f():
        return 1
    class A(object):
        def __init__(self):
            self.f = f
    s = pickle.dumps(A(), -1)
    pickle.loads(s).f() == 1

if '# getstate # setstate':

    if 'Minimal example':

        """
        - __getstate__ spits a dict
        - __setstate__ reads the dict and uses it to setup an object
        """

        class C(object):

            def __init__(self, i):
                self.i = i

            def f(self):
                return self.i + 1

            def __getstate__(self):
                return {'a': self.i}

            def __setstate__(self, d):
                self.i = d['a']

        assert pickle.loads(pickle.dumps(C(1), -1)).i == 1
        assert pickle.loads(pickle.dumps(C(1), -1)).f() == 2

    if 'Mischief example':

        x = 0

        class C(object):

            def __init__(self, i):
                self.i = i

            def __getstate__(self):
                global x
                x = 1
                self.i *= 2
                return self.__dict__

            def __setstate__(self, d):
                global x
                x = 2
                self.__dict__ = d
                self.i *= 3

        c = C(1)
        assert c.i == 1
        assert x == 0

        s = pickle.dumps(C(1))
        # __getstate__ was called.
        assert x == 1
        # But it is not called on the original object, which was not changed.
        assert c.i == 1

        c2 = pickle.loads(s)
        # __setstate__ was called.
        assert x == 2
        assert c.i == 1
        assert c2.i == 6