更改不可 JSON 序列化的对象的格式很容易，例如 datetime.datetime。

出于调试目的，我的要求是改变一些自定义对象从基本对象扩展的方式，例如dict and list，以json格式序列化。代码 ：

import datetime
import json

def json_debug_handler(obj):
    print("object received:")
    print type(obj)
    print("\n\n")
    if  isinstance(obj, datetime.datetime):
        return obj.isoformat()
    elif isinstance(obj,mDict):
        return {'orig':obj , 'attrs': vars(obj)}
    elif isinstance(obj,mList):
        return {'orig':obj, 'attrs': vars(obj)}
    else:
        return None


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,default=json_debug_handler))

if __name__ == '__main__':
    test_debug_json()

DEMO : http://ideone.com/hQJnLy http://ideone.com/hQJnLy

Output:

{"date": "2013-05-07T01:03:13.098727", "games": ["mario", "contra", "tetris"], "scores": {"pk": 45, "dp": 10}}

期望的输出：

{"date": "2013-05-07T01:03:13.098727", "games": { "orig": ["mario", "contra", "tetris"] ,"attrs" : { "src":"console"}} , "scores": { "orig": {"pk": 45, "dp": 10},"attrs": "processed":"unprocessed }}

是否default处理程序不适用于可序列化对象？ 如果没有，我如何覆盖它，而不将 toJSON 方法添加到扩展类？

另外，这个版本的 JSON 编码器不起作用：

class JsonDebugEncoder(json.JSONEncoder):
    def default(self,obj):
        if  isinstance(obj, datetime.datetime):
            return obj.isoformat()
        elif isinstance(obj,mDict):
            return {'orig':obj , 'attrs': vars(obj)}
        elif isinstance(obj,mList):
            return {'orig':obj, 'attrs': vars(obj)}
        else:
            return json.JSONEncoder.default(self, obj)

如果有黑客攻击pickle,__getstate__,__setstate__,然后在 pickle.loads 对象上使用 json.dumps ，我也对此持开放态度，我尝试过，但这不起作用。

似乎要在给定的限制下实现您想要的行为，您必须深入研究JSONEncoder类一点。下面我写了一个自定义JSONEncoder覆盖iterencode传递自定义的方法isinstance方法_make_iterencode。它不是世界上最干净的东西，但考虑到选项似乎是最好的，并且它将定制保持在最低限度。

# 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)

您现在可以对CustomObjectEncoder因此它可以正确序列化您的自定义对象。这CustomObjectEncoder还可以做一些很酷的事情，比如处理嵌套对象。

# 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 = "oldschool"
    games = mList(['mario', 'contra', 'tetris', zelda])
    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=MyEncoder))

if __name__ == '__main__':
    main()