Serializing a Python namedtuple to json

What is the recommended way of serializing a namedtuple to json with the field names retained?

Serializing a namedtuple to json results in only the values being serialized and the field names being lost in translation. I would like the fields also to be retained when json-ized and hence did the following:

class foobar(namedtuple('f', 'foo, bar')):
    __slots__ = ()
    def __iter__(self):
        yield self._asdict()

The above serializes to json as I expect and behaves as namedtuple in other places I use (attribute access etc.,) except with a non-tuple like results while iterating it (which fine for my use case).

What is the "correct way" of converting to json with the field names retained?

If it's just one namedtuple you're looking to serialize, using its _asdict() method will work (with Python >= 2.7)

>>> from collections import namedtuple
>>> import json
>>> FB = namedtuple("FB", ("foo", "bar"))
>>> fb = FB(123, 456)
>>> json.dumps(fb._asdict())
'{"foo": 123, "bar": 456}'

This is pretty tricky, since namedtuple() is a factory which returns a new type derived from tuple. One approach would be to have your class also inherit from UserDict.DictMixin, but tuple.__getitem__ is already defined and expects an integer denoting the position of the element, not the name of its attribute:

>>> f = foobar('a', 1)
>>> f[0]
'a'

At its heart the namedtuple is an odd fit for JSON, since it is really a custom-built type whose key names are fixed as part of the type definition, unlike a dictionary where key names are stored inside the instance. This prevents you from "round-tripping" a namedtuple, e.g. you cannot decode a dictionary back into a namedtuple without some other a piece of information, like an app-specific type marker in the dict {'a': 1, '#_type': 'foobar'}, which is a bit hacky.

This is not ideal, but if you only need to encode namedtuples into dictionaries, another approach is to extend or modify your JSON encoder to special-case these types. Here is an example of subclassing the Python json.JSONEncoder. This tackles the problem of ensuring that nested namedtuples are properly converted to dictionaries:

from collections import namedtuple
from json import JSONEncoder

class MyEncoder(JSONEncoder):

    def _iterencode(self, obj, markers=None):
        if isinstance(obj, tuple) and hasattr(obj, '_asdict'):
            gen = self._iterencode_dict(obj._asdict(), markers)
        else:
            gen = JSONEncoder._iterencode(self, obj, markers)
        for chunk in gen:
            yield chunk

class foobar(namedtuple('f', 'foo, bar')):
    pass

enc = MyEncoder()
for obj in (foobar('a', 1), ('a', 1), {'outer': foobar('x', 'y')}):
    print enc.encode(obj)

{"foo": "a", "bar": 1}
["a", 1]
{"outer": {"foo": "x", "bar": "y"}}

It looks like you used to be able to subclass simplejson.JSONEncoder to make this work, but with the latest simplejson code, that is no longer the case: you have to actually modify the project code. I see no reason why simplejson should not support namedtuples, so I forked the project, added namedtuple support, and I'm currently waiting for my branch to be pulled back into the main project. If you need the fixes now, just pull from my fork.

EDIT: Looks like the latest versions of simplejson now natively support this with the namedtuple_as_object option, which defaults to True.