python - How to convert string int JSON into real int with json.loads

Question

I'm trying to convert a string which represents a JSON object to a real JSON object using json.loads but it doesn't convert the integers:

(in the initial string, integers are always strings)

$> python
Python 2.7.9 (default, Aug 29 2016, 16:00:38)
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> c = '{"value": "42"}'
>>> json_object = json.loads(c, parse_int=int)
>>> json_object
{u'value': u'42'}
>>> json_object['value']
u'42'
>>>

Instead of {u'value': u'42'} I'd like it becomes {u'value': 42}. I know I can run through the whole object, but I don't want to do that, it's not really efficient to do it manually, since this parse_int argument exists (https://docs.python.org/2/library/json.html#json.loads).

Thanks to Pierce's proposition:

Python 2.7.9 (default, Aug 29 2016, 16:00:38)
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>>
>>> class Decoder(json.JSONDecoder):
...     def decode(self, s):
...         result = super(Decoder, self).decode(s)
...         return self._decode(result)
...     def _decode(self, o):
...         if isinstance(o, str) or isinstance(o, unicode):
...             try:
...                 return int(o)
...             except ValueError:
...                 try:
...                     return float(o)
...                 except ValueError:
...                     return o
...         elif isinstance(o, dict):
...             return {k: self._decode(v) for k, v in o.items()}
...         elif isinstance(o, list):
...             return [self._decode(v) for v in o]
...         else:
...             return o
...
>>>
>>> c = '{"value": "42", "test": "lolol", "abc": "43.4",  "dcf": 12, "xdf": 12.4}'
>>> json.loads(c, cls=Decoder)
{u'test': u'lolol', u'dcf': 12, u'abc': 43.4, u'value': 42, u'xdf': 12.4}

Answer 1

In addition to the Pierce response, I think you can use the json.loads object_hook parameter instead of cls one, so you don't need to walk the json object twice.

For example:

def _decode(o):
    # Note the "unicode" part is only for python2
    if isinstance(o, str) or isinstance(o, unicode):
        try:
            return int(o)
        except ValueError:
            return o
    elif isinstance(o, dict):
        return {k: _decode(v) for k, v in o.items()}
    elif isinstance(o, list):
        return [_decode(v) for v in o]
    else:
        return o

# Then you can do:
json.loads(c, object_hook=_decode)

As @ZhanwenChen pointed out in a comment, the code above is for python2. For python3 you'll need to remove the or isinstance(o, unicode) part in the first if condition.