import pickle
class Upgradable:
class_version = '3.0'
def __init__(self):
self.version = self.__class__.class_version
self.new_attr = 42
def pickle(self):
return pickle.dumps(self)
def new_method(self):
'''
Return the answer to life the universe and everything.
Would normally break pickles prior to the introduction of new_attr.
'''
return self.new_attr
@staticmethod
def unpickle(data):
out = pickle.loads(data)
if not hasattr(out, 'version'):
out.version = '0.0'
if out.version != out.class_version:
out.upgrade()
return out
def upgrade(self):
version = float(self.version)
print 'upgrade from version %s' % self.version
if version < 1.:
self.version = '1.0'
print 'upgrade to version 1.0'
if version < 2.:
self.version = '2.0'
print 'upgrade to version 2.0'
if version < 3.:
self.version = '3.0'
self.new_attr = 42
print 'upgrade to version 3.0'
def __test__():
ug0 = Upgradable()
# downgrade to lower version
del ug0.version
del ug0.new_attr
try:
# breaks old pickles!
ug0.new_method()
raise Exception('Should have raised AttributeError!')
except AttributeError:
pass
# check to see if automatic upgrade works
ug3 = ug0.unpickle(ug0.pickle())
assert ug3.version == '3.0'
assert ug3.new_attr == 42
assert ug3.new_method() == 42
__test__()
