How do you enable python's json package to encode attrdict.AttrDict objects?

Question

How do you encode AttrDict objects in json?

import sys, json, attrdict

ad = attrdict.AttrDict({'else': 1, 'inner': attrdict.AttrDict({'something': 2})})

json.dump(ad, sys.stdout)

This fails with TypeError: a{'something': 2} is not JSON serializable

Using a custom encoder like this works but I have to reference the private _mapping property:

import sys, json, attrdict

class attrDictEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, attrdict.AttrDict):
            return obj._mapping
        return json.JSONEncoder.default(self, obj)

ad = attrdict.AttrDict({'else': 1, 'inner': attrdict.AttrDict({'something': 2})})

json.dump(ad, sys.stdout, cls=attrDictEncoder)

is there a better/cleaner way? I don't want to have to rebuild the dicts, item by item into a plain dicts.

Answer 1

Your code was very close. The following solution passes a function into the JSON encoder, which convertsAttrDicts into normal dictionaries. The normal JSON machinery calls it when it finds a nonstandard type, like anAttrDict.

source

import json, attrdict

def as_attrdict(val):
    if not isinstance(val, attrdict.AttrDict):
        raise TypeError('not AttrDict')
    return dict(val)

ad = attrdict.AttrDict({'else': 1,
                        'inner': attrdict.AttrDict({'something': 2})})

print json.dumps(ad, default=as_attrdict)

output

{"inner": {"something": 2}, "else": 1}

An AttrDict is a dictionary-like object that allows its elements to be accessed both as keys and as attributes.

Thanks to user2357112 for simplifying the code.

Answer 2

It's possible to fool json into believing it is handling a real dict by subclassing dict and then monkey patching the methods that json uses to create the json object string. This way we can give json a dummy class that just calls the relevant methods of the given AttrDict.

def as_attrdict(val):
    if not isinstance(val, AttrDict):
        raise TypeError('not AttrDict')
    return AttrDictForJson(val)


class AttrDictForJson(dict):

    def __init__(self, attrdict):
        super().__init__()
        self.items = attrdict.items
        self._len = attrdict.__len__
        # key creation necessary for json.dump to work with CPython 
        # This is because optimised json bypasses __len__ on CPython
        if self._len() != 0:
            self[None] = None

    def __len__(self):
        return self._len()

Usage:

json_string = dumps(attrdict, default=as_attrdict)

I've tested this on python 3.4, if you have a different version of python then the above may require some tweaking, such as changing attrdict.items to attrdict.iteritems