How to change json encoding behaviour for serializable python object?

It seems that to achieve the behavior you want, with the given restrictions, you’ll have to delve into the JSONEncoder class a little. Below I’ve written out a custom JSONEncoder that overrides the iterencode method to pass a custom isinstance method to _make_iterencode. It isn’t the cleanest thing in the world, but seems to be the best given the options and it keeps customization to a minimum.

# customencoder.py
from json.encoder import (_make_iterencode, JSONEncoder,
                          encode_basestring_ascii, FLOAT_REPR, INFINITY,
                          c_make_encoder, encode_basestring)


class CustomObjectEncoder(JSONEncoder):

    def iterencode(self, o, _one_shot=False):
        """
        Most of the original method has been left untouched.

        _one_shot is forced to False to prevent c_make_encoder from
        being used. c_make_encoder is a funcion defined in C, so it's easier
        to avoid using it than overriding/redefining it.

        The keyword argument isinstance for _make_iterencode has been set
        to self.isinstance. This allows for a custom isinstance function
        to be defined, which can be used to defer the serialization of custom
        objects to the default method.
        """
        # Force the use of _make_iterencode instead of c_make_encoder
        _one_shot = False

        if self.check_circular:
            markers = {}
        else:
            markers = None
        if self.ensure_ascii:
            _encoder = encode_basestring_ascii
        else:
            _encoder = encode_basestring
        if self.encoding != 'utf-8':
            def _encoder(o, _orig_encoder=_encoder, _encoding=self.encoding):
                if isinstance(o, str):
                    o = o.decode(_encoding)
                return _orig_encoder(o)

        def floatstr(o, allow_nan=self.allow_nan,
                     _repr=FLOAT_REPR, _inf=INFINITY, _neginf=-INFINITY):
            if o != o:
                text="NaN"
            elif o == _inf:
                text="Infinity"
            elif o == _neginf:
                text="-Infinity"
            else:
                return _repr(o)

            if not allow_nan:
                raise ValueError(
                    "Out of range float values are not JSON compliant: " +
                    repr(o))

            return text

        # Instead of forcing _one_shot to False, you can also just
        # remove the first part of this conditional statement and only
        # call _make_iterencode
        if (_one_shot and c_make_encoder is not None
                and self.indent is None and not self.sort_keys):
            _iterencode = c_make_encoder(
                markers, self.default, _encoder, self.indent,
                self.key_separator, self.item_separator, self.sort_keys,
                self.skipkeys, self.allow_nan)
        else:
            _iterencode = _make_iterencode(
                markers, self.default, _encoder, self.indent, floatstr,
                self.key_separator, self.item_separator, self.sort_keys,
                self.skipkeys, _one_shot, isinstance=self.isinstance)
        return _iterencode(o, 0)

You can now subclass the CustomObjectEncoder so it correctly serializes your custom objects. The CustomObjectEncoder can also do cool stuff like handle nested objects.

# test.py
import json
import datetime
from customencoder import CustomObjectEncoder


class MyEncoder(CustomObjectEncoder):

    def isinstance(self, obj, cls):
        if isinstance(obj, (mList, mDict)):
            return False
        return isinstance(obj, cls)

    def default(self, obj):
        """
        Defines custom serialization.

        To avoid circular references, any object that will always fail
        self.isinstance must be converted to something that is
        deserializable here.
        """
        if isinstance(obj, datetime.datetime):
            return obj.isoformat()
        elif isinstance(obj, mDict):
            return {"orig": dict(obj), "attrs": vars(obj)}
        elif isinstance(obj, mList):
            return {"orig": list(obj), "attrs": vars(obj)}
        else:
            return None


class mList(list):
    pass


class mDict(dict):
    pass


def main():
    zelda = mList(['zelda'])
    zelda.src = "https://stackoverflow.com/questions/16405969/oldschool"
    games = mList(['mario', 'contra', 'tetris', zelda])
    games.src="https://stackoverflow.com/questions/16405969/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=MyEncoder))

if __name__ == '__main__':
    main()