Question

I’m implementing my own JSONEncoder for handling different classes exactly as I need them. Unfortunately, my custom encoder returns somewhat malformed strings. They are surrounded by quotes and certain characters (quotes) are escaped.

Please use the following code to reproduce the behaviour:

import json

class CustomEncoder(json.JSONEncoder):
    def default(self, givenObject):
        #for testing purposes this always returns the same string
        str = '{"id":0,"name":"Peter"}'
        return str;

class AnyClass(object):
    pass

encoder = CustomEncoder()
dummyClass = AnyClass()

#expected output: {"id":0,"name":"Peter"}
print encoder.encode(dummyClass)

#output: "{\"id\":0,\"name\":\"Peter\"}"

I’m using Python 2.7.

How can I prevent this behaviour? What do I do wrong?

Answer 1

default should return an object, not a chunk of json:

import json

class CustomEncoder(json.JSONEncoder):
    def default(self, givenObject):
        obj = {"id":0,"name":"Peter"}
        return obj

class AnyClass(object):
    pass

encoder = CustomEncoder()
dummyClass = AnyClass()

print encoder.encode(dummyClass) # {"id": 0, "name": "Peter"}

http://docs.python.org/library/json.html#json.JSONEncoder.default:

Implement this method in a subclass such that it returns a serializable object for o, or calls the base implementation (to raise a TypeError).