How to make simplejson serializable class

Question

I have a class defined like this

class A:
    def __init__(self):
        self.item1 = None
    def __repr__(self):
        return str(self.__dict__)

when I do:

>>> import simplejson
>>> myA = A()
>>> simplejson.dumps(myA)
TypeError: {'item1': None} is not JSON serializable

I can't find the reason why.

Do I need to add any particular method to A for simplejson to serialize my class object?

Answer 1

You can't serialize arbitrary objects with simplejson. You need to pass a default and object_hook to dump and load. Here's an example:

class SerializerRegistry(object):
    def __init__(self):
        self._classes = {}
    def add(self, cls):
        self._classes[cls.__module__, cls.__name__] = cls
        return cls
    def object_hook(self, dct):
        module, cls_name = dct.pop('__type__', (None, None))
        if cls_name is not None:
            return self._classes[module, cls_name].from_dict(dct)
        else:
            return dct
    def default(self, obj):
        dct = obj.to_dict()
        dct['__type__'] = [type(obj).__module__,
                           type(obj).__name__]
        return dct

registry = SerializerRegistry()

@registry.add
class A(object):
    def __init__(self, item1):
        self.item1 = item1
    def __repr__(self):
        return str(self.__dict__)
    def to_dict(self):
        return dict(item1=self.item1)
    @classmethod
    def from_dict(cls, dct):
        return cls(**dct)

s = json.dumps(A(1), default=registry.default)
a = json.loads(s, object_hook=registry.object_hook)

This results in this:

>>> s
'{"item1": 1, "__type__": ["__main__", "A"]}'
>>> a
{'item1': 1}

But what you really need is a function default that creates dictionary from the objects that you wish to serialize, and a function object_hook that returns an object (of the correct type) when it is given a dictionary if a dictionary isn't enough. The best approach is to have methods on the serializable classes that create a dict from the object and that construct it back, and also to have a mapping that recognizes to which class the dictionaries belong.

You can also add an identifier to the classes to be used as an index for _classes. This way you wouldn't have issues if you have to move a class.

Answer 2

According to the json module docs (simplejson was adopted as json in Python 2.6), you need to extend the json.JSONEncoder class, overriding its default method to translate your object into a type that can be serialised. There doesn't appear to be a method that it looks for on your object.