JSON.Loads Allows Duplicate Keys in a Dictionary, Overwriting the First Value

json.loads allows duplicate keys in a dictionary, overwriting the first value

The rfc 4627 for application/json media type recommends unique keys but it doesn't forbid them explicitly:

The names within an object SHOULD be unique.

From rfc 2119:

SHOULD This word, or the adjective "RECOMMENDED", mean that there

may exist valid reasons in particular circumstances to ignore a

particular item, but the full implications must be understood and

carefully weighed before choosing a different course.

import json

def dict_raise_on_duplicates(ordered_pairs):
    """Reject duplicate keys."""
    d = {}
    for k, v in ordered_pairs:
        if k in d:
           raise ValueError("duplicate key: %r" % (k,))
        else:
           d[k] = v
    return d

json.loads(raw_post_data, object_pairs_hook=dict_raise_on_duplicates)
# -> ValueError: duplicate key: u'1'

Dealing with JSON with duplicate keys

You can't have duplicate keys. You can change the object to array instead.

[
    {
        'content': 'stuff',
        'timestamp': '123456789'
    },
    {
        'content': 'weird stuff',
        'timestamp': '93828492'
    }
]

make a dict/json from string with duplicate keys Python

something like the following can be done.

import json

def join_duplicate_keys(ordered_pairs):
    d = {}
    for k, v in ordered_pairs:
        if k in d:
           if type(d[k]) == list:
               d[k].append(v)
           else:
               newlist = []
               newlist.append(d[k])
               newlist.append(v)
               d[k] = newlist
        else:
           d[k] = v
    return d

raw_post_data = '{"a":1, "b":{"b1":1,"b2":2}, "b": { "b1":3, "b2":2,"b4":8} }'
newdict = json.loads(raw_post_data, object_pairs_hook=join_duplicate_keys)
print (newdict)

Please note that above code depends on value type, if type(d[k]) == list. So if original string itself gives a list then there could be some error handling required to make the code robust.

Python json parser allow duplicate keys

You can use JSONDecoder.object_pairs_hook to customize how JSONDecoder decodes objects. This hook function will be passed a list of (key, value) pairs that you usually do some processing on, and then turn into a dict.

However, since Python dictionaries don't allow for duplicate keys (and you simply can't change that), you can return the pairs unchanged in the hook and get a nested list of (key, value) pairs when you decode your JSON:

from json import JSONDecoder

def parse_object_pairs(pairs):
    return pairs

data = """
{"foo": {"baz": 42}, "foo": 7}
"""

decoder = JSONDecoder(object_pairs_hook=parse_object_pairs)
obj = decoder.decode(data)
print obj

Output:

[(u'foo', [(u'baz', 42)]), (u'foo', 7)]

How you use this data structure is up to you. As stated above, Python dictionaries won't allow for duplicate keys, and there's no way around that. How would you even do a lookup based on a key? dct[key] would be ambiguous.

So you can either implement your own logic to handle a lookup the way you expect it to work, or implement some sort of collision avoidance to make keys unique if they're not, and then create a dictionary from your nested list.

---

Edit: Since you said you would like to modify the duplicate key to make it unique, here's how you'd do that:

from collections import OrderedDict
from json import JSONDecoder

def make_unique(key, dct):
    counter = 0
    unique_key = key

    while unique_key in dct:
        counter += 1
        unique_key = '{}_{}'.format(key, counter)
    return unique_key

def parse_object_pairs(pairs):
    dct = OrderedDict()
    for key, value in pairs:
        if key in dct:
            key = make_unique(key, dct)
        dct[key] = value

    return dct

data = """
{"foo": {"baz": 42, "baz": 77}, "foo": 7, "foo": 23}
"""

decoder = JSONDecoder(object_pairs_hook=parse_object_pairs)
obj = decoder.decode(data)
print obj

Output:

OrderedDict([(u'foo', OrderedDict([(u'baz', 42), ('baz_1', 77)])), ('foo_1', 7), ('foo_2', 23)])

The make_unique function is responsible for returning a collision-free key. In this example it just suffixes the key with _n where n is an incremental counter - just adapt it to your needs.

Because the object_pairs_hook receives the pairs exactly in the order they appear in the JSON document, it's also possible to preserve that order by using an OrderedDict, I included that as well.

JSON convert duplicate entries into array, but recover original order

One workaround I thought of was to generate duplicated keys for each entry, anna_1, anna_2, etc as suggested here: https://stackoverflow.com/a/29323197/7471760, so that one can have unique entries, and then hook the pair to an OrderedDict.

Other option would be to return in the hook the key-value tuples directly and process it later https://stackoverflow.com/a/29322077/7471760.

However, it was quite useful for me to keep the array structure, and what suited me most was to use this workaround that keeps the order explicitly in an extra key:

def array_on_duplicates_keep_order(ordered_pairs):
    """Convert duplicate keys to arrays and store order on an extra key."""
#    https://www.semicolonworld.com/question/56998/python-json-parser-allow-duplicate-keys
#    https://stackoverflow.com/questions/14902299/json-loads-allows-duplicate-keys-in-a-dictionary-overwriting-the-first-value

    d = {}
    order = 0
    for k, v in ordered_pairs:
        if type(v) is dict:
            v['o'] = order
        if k in d:
            if type(d[k]) is list:
                d[k].append(v)
            else:
                d[k] = [d[k],v]
        else:
           d[k] = v
        order += 1
    return d

which produces:

jobj = json.loads(jstring, object_pairs_hook=array_on_duplicates_keep_order)
{'anna': [{'age': 23, 'color': 'green', 'o': 0},
  {'age': 41, 'color': 'pink', 'o': 4}],
 'john': [{'age': 35, 'color': 'blue', 'o': 1},
  {'age': 31, 'color': 'black', 'o': 3}],
 'laura': {'age': 32, 'color': 'red', 'o': 2}}

Finally, I can recover the original order of students by using a named tuple and sorting by the order key:

class Student(NamedTuple):
    name: str
    age: int
    color: str
    o: int

studentList = []
for k, v in jobj.items(): 
    if not type(v) is list:
       studentList.append(Student(k, v['age'], v['color'], v['o']))
    else:
       for s in v:
           studentList.append(Student(k, s['age'], s['color'], s['o']))

orderedList = sorted(studentList, key=lambda s: s.o) 

Which gives me what I wanted, without changing the input and still using JSON as intermediate storage variable:

studentList
[Student(name='anna', age=23, color='green', o=0),
 Student(name='anna', age=41, color='pink', o=4),
 Student(name='john', age=35, color='blue', o=1),
 Student(name='john', age=31, color='black', o=3),
 Student(name='laura', age=32, color='red', o=2)]

orderedList
[Student(name='anna', age=23, color='green', o=0),
 Student(name='john', age=35, color='blue', o=1),
 Student(name='laura', age=32, color='red', o=2),
 Student(name='john', age=31, color='black', o=3),
 Student(name='anna', age=41, color='pink', o=4)]

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