type evolution with jsonpickle (python)

Is there any support of this in jsonpickle?

E.g. I store and object, them modify its schema, then try to load it back.

The following change, for instance, (attribute addition)

import jsonpickle

class Stam(object):

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

   def __str__(self):
     return '%s with a=%s' % (self.__class__.__name__, str(self.a))


js = jsonpickle.encode(Stam(123))
print 'encoded:', js

class Stam(object):

   def __init__(self, a, b):
     self.a = a
     self.b = b

   def __str__(self):
     return '%s with a=%s, b=%s' % (self.__class__.__name__, str(self.a), str(self.b))

s=jsonpickle.decode(js)
print 'decoded:', s

produces an error:

encoded: {"py/object": "__main__.Stam", "a": 123}
decoded: Traceback (most recent call last):
  File "C:\gae\google\appengine\ext\admin\__init__.py", line 317, in post
    exec(compiled_code, globals())
  File "<string>", line 25, in <module>
  File "<string>", line 22, in __str__
AttributeError: 'Stam' object has no attribute 'b'

Solution

There is no support for type evolution or type migrations within jsonpickle.

Your best course of action would be to load (via json.loads) the JSON representation of your data into a basic Python structure of lists / dicts / strings / numbers. Traverse this Python representation, adding in empty/default b keys. Then re-save the JSON via json.dumps.

You can then use jsonpickle to load the modified version of the data.

temp = json.loads(js)
temp['b'] = None
js = json.dumps(temp)
jsonpickle.decode(js)

This obviously gets more complicated if your object model is more complex, but you can check the py/object key to see if you need to modify the object.