Question

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'

Answer 1

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.