How to get sorted list inside a dictionary with json.dumps()

I have the following problem: having a python dictionary like the following:

{"qqq": [{"bbb": "111"}, {"aaa": "333"}], "zzz": {"bbb": [5, 2, 1, 9]}}

I would like to obtain an ordered json object such as:

'{"qqq": [{"aaa": "333"}, {"bbb": "111"}], "zzz": {"bbb": [1, 2, 5, 9]}}'

At the moment I use the following:

class ListEncoder(json.JSONEncoder):
    def default(self, o):
        if isinstance(o, list):
            return sorted(o)
        return json.JSONEncoder.default(self, o)

print json.dumps(c, sort_keys=True, cls=ListEncoder)

But the two list inside my object are not sorted, and I get:

'{"qqq": [{"bbb": "111"}, {"aaa": "333"}], "zzz": {"bbb": [5, 2, 1, 9]}}'

probably because the custom JSONEncoder skips a type that already knows how to manage (list).

UPDATE

Martijn solution below works perfectly for the example above, but unfortunately I have to manage more complicated dictionaries, with a bigger depth: for example the following two

a = {
    'aaa': 'aaa',
    'op': 'ccc',
    'oppa': {
        'ggg': [{'fff': 'ev'}],
        'flt': {
            'nnn': [
                {
                'mmm': [{'a_b_d': [6]},{'a_b_c': [6,7]}]
                },
                {
                    'iii': [3, 2, 4, 5]
                }
            ]
        }
    },
    'rrr': {},
    'ttt': ['aaa-bbb-ccc']
}
b = {
    'aaa': 'aaa',
    'op': 'ccc',
    'oppa': {
        'ggg': [{'fff': 'ev'}],
        'flt': {
            'nnn': [
                {
                    'iii': [2, 3, 4, 5]
                },
                {
                'mmm': [{'a_b_c': [6,7]},{'a_b_d': [6]}]
                }
            ]
        }
    },
    'rrr': {},
    'ttt': ['aaa-bbb-ccc']
}

They would be the same if the lists inside the same would be sorted. But they aren’t with the class above, and I get 2 different json strings:

{"aaa": "aaa", "op": "ccc", "oppa": {"flt": {"nnn": [{"iii": [3, 2, 4, 1]}, {"mmm": [{"a_b_d": [6]}, {"a_b_c": [6, 7]}]}]}, "ggg": [{"fff": "ev"}]}, "rrr": {}, "ttt": ["aaa-bbb-ccc"]}
{"aaa": "aaa", "op": "ccc", "oppa": {"flt": {"nnn": [{"iii": [2, 3, 4, 5]}, {"mmm": [{"a_b_c": [6, 7]}, {"a_b_d": [6]}]}]}, "ggg": [{"fff": "ev"}]}, "rrr": {}, "ttt": ["aaa-bbb-ccc"]}

Any idea to fix this?

Answer

default isn’t called for lists; that method is only for types the encoder doesn’t know how to handle. Override the encode method instead:

class SortedListEncoder(json.JSONEncoder):
    def encode(self, obj):
        def sort_lists(item):
            if isinstance(item, list):
                return sorted(sort_lists(i) for i in item)
            elif isinstance(item, dict):
                return {k: sort_lists(v) for k, v in item.items()}
            else:
                return item
        return super(SortedListEncoder, self).encode(sort_lists(obj))

This essentially just sorts all lists (recursively) before encoding; this could have been done before passing it to json.dumps() but this way it is part of the responsibility of the encoder, just like sorting the keys is.

Demo:

>>> json.dumps(c, sort_keys=True, cls=SortedListEncoder)
'{"qqq": [{"aaa": "333"}, {"bbb": "111"}], "zzz": {"bbb": [1, 2, 5, 9]}}'
>>> json.dumps(a, sort_keys=True, cls=SortedListEncoder)
'{"aaa": "aaa", "op": "ccc", "oppa": {"flt": {"nnn": [{"iii": [2, 3, 4, 5]}, {"mmm": [{"a_b_c": [6, 7]}, {"a_b_d": [6]}]}]}, "ggg": [{"fff": "ev"}]}, "rrr": {}, "ttt": ["aaa-bbb-ccc"]}'
>>> json.dumps(b, sort_keys=True, cls=SortedListEncoder)
'{"aaa": "aaa", "op": "ccc", "oppa": {"flt": {"nnn": [{"iii": [2, 3, 4, 5]}, {"mmm": [{"a_b_c": [6, 7]}, {"a_b_d": [6]}]}]}, "ggg": [{"fff": "ev"}]}, "rrr": {}, "ttt": ["aaa-bbb-ccc"]}'