custom JSON sort_keys order in Python

Question

Is there any way in Python 2.6 to supply a custom key or cmp function to JSON's sort_keys?

I've got a list of dicts coming from JSON like so:

[
  {
    "key": "numberpuzzles1",
    "url": "number-puzzle-i.html",
    "title": "Number Puzzle I",
    "category": "nestedloops",
    "points": "60",
    "n": "087"
  },
  {
     "key": "gettingindividualdigits",
     "url": "getting-individual-digits.html",
     "title": "Getting Individual Digits",
     "category": "nestedloops",
     "points": "80",
     "n": "088"
  }
]

...which I've stored into the list variable assigndb. I'd like to be able to load in the JSON, modify it, and serialized it back out with dumps (or whatever), keeping the orders of the keys intact.

So far, I've tried something like this:

ordering = {'key': 0, 'url': 1, 'title': 2, 'category': 3,
             'flags': 4, 'points': 5, 'n': 6}

def key_func(k):
    return ordering[k]

# renumber assignments sequentially
for (i, a) in enumerate(assigndb):
    a["n"] = "%03d" % (i+1)

s = json.dumps(assigndb, indent=2, sort_keys=True, key=key_func)

...but of course dumps doesn't support a custom key like list.sort() does. Something with a custom JSONEncoder maybe? I can't seem to get it going.

Answer 1

An idea (tested with 2.7):

import json
import collections
json.encoder.c_make_encoder = None
d = collections.OrderedDict([("b", 2), ("a", 1)])
json.dumps(d)
# '{"b": 2, "a": 1}'

See: OrderedDict + issue6105. The c_make_encoder hack seems only to be needed for Python 2.x. Not a direct solution because you have to change dicts for OrderedDicts, but it may be still usable. I checked the json library (encode.py) and the ordered is hardcoded:

if _sort_keys:
    items = sorted(dct.items(), key=lambda kv: kv[0])