Json in Python: Receive/Check duplicate key error

Question

The json module of python acts a little of the specification when having duplicate keys in a map:

import json
>>> json.loads('{"a": "First", "a": "Second"}')
{u'a': u'Second'}

I know that this behaviour is specified in the documentation:

The RFC specifies that the names within a JSON object should be unique, but does not specify how repeated names in JSON objects should be handled. By default, this module does not raise an exception; instead, it ignores all but the last name-value pair for a given name:

For my current project, I absolutely need to make sure that no duplicate keys are present in the file and receive an error/exception if this is the case? How can this be accomplished?

I'm still stuck on Python 2.7, so a solution which also works with older versions would help me most.

Answer 1

Well, you could try using the JSONDecoder class and specifying a custom object_pairs_hook, which will receive the duplicates before they would get deduped.

import json

def dupe_checking_hook(pairs):
    result = dict()
    for key,val in pairs:
        if key in result:
            raise KeyError("Duplicate key specified: %s" % key)
        result[key] = val
    return result

decoder = json.JSONDecoder(object_pairs_hook=dupe_checking_hook)

# Raises a KeyError
some_json = decoder.decode('''{"a":"hi","a":"bye"}''')

# works
some_json = decoder.decode('''{"a":"hi","b":"bye"}''')