How to change json encoding behaviour for serializable python object?

Question

It is easy to change the format of an object which is not JSON serializable eg datetime.datetime.

My requirement, for debugging purposes, is to alter the way some cu

Answer 1

Try the below. It produces the output you want and looks relatively simple. The only real difference from your encoder class is that we should override both decode and encode methods (since the latter is still called for types the encoder knows how to handle).

import json
import datetime

class JSONDebugEncoder(json.JSONEncoder):
    # transform objects known to JSONEncoder here
    def encode(self, o, *args, **kw):
        for_json = o
        if isinstance(o, mDict):
            for_json = { 'orig' : o, 'attrs' : vars(o) }
        elif isinstance(o, mList):
            for_json = { 'orig' : o, 'attrs' : vars(o) }
        return super(JSONDebugEncoder, self).encode(for_json, *args, **kw)

    # handle objects not known to JSONEncoder here
    def default(self, o, *args, **kw):
        if isinstance(o, datetime.datetime):
            return o.isoformat()
        else:
            return super(JSONDebugEncoder, self).default(o, *args, **kw)


class mDict(dict):
    pass

class mList(list):
    pass

def test_debug_json():
    games = mList(['mario','contra','tetris'])
    games.src = 'console'
    scores = mDict({'dp':10,'pk':45})
    scores.processed = "unprocessed"
    test_json = { 'games' : games , 'scores' : scores , 'date': datetime.datetime.now() }
    print(json.dumps(test_json,cls=JSONDebugEncoder))

if __name__ == '__main__':
    test_debug_json()