Question

I’m trying to load JSON back into an object. The “loads” method seems to work without error, but the object doesn’t seem to have the properties I expect.

How can I go about examining/inspecting the object that I have (this is web-based code).

  results = {"Subscriber": {"firstname": "Neal", "lastname": "Walters"}}
  subscriber = json.loads(results)


  for item in inspect.getmembers(subscriber): 
     self.response.out.write("<BR>Item")
     for subitem in item: 
         self.response.out.write("<BR>&nbsp;SubItem=" + subitem)

The attempt above returned this:

   Item
     SubItem=__class__

I don’t think it matters, but for context:
The JSON is actually coming from a urlfetch in Google App Engine to
a rest web service created using this utility:
http://code.google.com/p/appengine-rest-server.
The data is being retrieved from a datastore with this definition:

class Subscriber(db.Model):
    firstname    = db.StringProperty()
    lastname     = db.StringProperty()

Thanks,
Neal

Update #1: Basically I’m trying to deserialize JSON back into an object.
In theory it was serialized from an object, and I want to now get it back into an object.
Maybe the better question is how to do that?

Update #2: I was trying to abstract a complex program down to a few lines of code, so I made a few mistakes in “pseudo-coding” it for purposes of posting here.

Here’s a better code sample, now take out of website where I can run on PC.

results = '{"Subscriber": {"firstname": "Neal", "lastname": "Walters"}}'
subscriber = json.loads(results)
for key, value in subscriber.items():
    print " %s: %s" %(key, value)

The above runs, what it displays doesn’t look any more structured than the JSON string itself. It displays this:
Subscriber: {u’lastname’: u’Walters’, u’firstname’: u’Neal’}

I have more of a Microsoft background, so when I hear serialize/deserialize, I think going from an object to a string, and from a string back to an object. So if I serialize to JSON, and then deserialize, what do I get, a dictionary, a list, or an object? Actually, I’m getting the JSON from a REST webmethod, that is on my behalf serializing my object for me.

Ideally I want a subscriber object that matches my Subscriber class above, and ideally, I don’t want to write one-off custom code (i.e. code that would be specific to “Subscriber”), because I would like to do the same thing with dozens of other classes. If I have to write some custom code, I will need to do it generically so it will work with any class.

Update #3: This is to explain more of why I think this is a needed tool. I’m writing a huge app, probably on Google App Engine (GAE). We are leaning toward a REST architecture for several reasons, but one is that our web GUI should access the data store via a REST web layer. (I’m a lot more used to SOAP, so switching to REST is a small challenge in itself). So one of the classic ways of getting and update data is through a business or data tier. By using the REST utility mention above, I have the choice of XML or JSON. I’m hoping to do a small working prototype of both before we develop the huge app). Then, suppose we have a successful app, and GAE doubles it prices. Then we can rewrite just the data tier, and take our Python/Django user tier (web code), and run it on Amazon or somewhere else.

If I’m going to do all that, why would I want everything to be dictionary objects. Wouldn’t I want the power of full-blown class structure? One of the next tricks is sort of an object relational mapping (ORM) so that we don’t necessarily expose our exact data tables, but more of a logical layer.

We also want to expose a RESTful API to paying users, who might be using any language. For them, they can use XML or JSON, and they wouldn’t use the serialize routine discussed here.

Answer 1

json only encodes strings, floats, integers, javascript objects (python dicts) and lists.

You have to create a function to turn the returned dictionary into a class and then pass it to a json.loads using the object_hook keyword argument along with the json string. Heres some code that fleshes it out:

import json

class Subscriber(object):
    firstname = None
    lastname = None


class Post(object):
    author = None
    title = None


def decode_from_dict(cls,vals):
    obj = cls()
    for key, val in vals.items():
        setattr(obj, key, val)
    return obj


SERIALIZABLE_CLASSES = {'Subscriber': Subscriber,
                        'Post': Post}

def decode_object(d):
    for field in d:
        if field in SERIALIZABLE_CLASSES:
            cls = SERIALIZABLE_CLASSES[field]
            return decode_from_dict(cls, d[field])
    return d


results = '''[{"Subscriber": {"firstname": "Neal", "lastname": "Walters"}},
              {"Post": {"author": {"Subscriber": {"firstname": "Neal",
                                                  "lastname": "Walters"}}},
                        "title": "Decoding JSON Objects"}]'''
result = json.loads(results, object_hook=decode_object)
print result
print result[1].author

This will handle any class that can be instantiated without arguments to the constructor and for which setattr will work.

Also, this uses json. I have no experience with simplejson so YMMV but I hear that they are identical.

Note that although the values for the two subscriber objects are identical, the resulting objects are not. This could be fixed by memoizing the decode_from_dict class.