Merge two json object in python

Question

I am merging two json in python

I'm doing

import json

json_obj = json.dumps({"a": [1,2]})
json_obj1 = json.dumps({"a": [3,4]})

json_obj += json_obj1

print(json_obj)

I am expecting the output as

{"a": [1, 2,3,4]}

but i got

{"a": [1, 2]}{"a": [3, 4]}

How to get the earlier one?

Answer 1

In json module, dumps convert python object to a string, and loads convert a string into python object. So in your original codes, you just try to concat two json-string. Try to code like this:

import json

from collections import defaultdict


def merge_dict(d1, d2):
    dd = defaultdict(list)

    for d in (d1, d2):
        for key, value in d.items():
            if isinstance(value, list):
                dd[key].extend(value)
            else:
                dd[key].append(value)
    return dict(dd)


if __name__ == '__main__':
    json_str1 = json.dumps({"a": [1, 2]})
    json_str2 = json.dumps({"a": [3, 4]})

    dct1 = json.loads(json_str1)
    dct2 = json.loads(json_str2)
    combined_dct = merge_dict(dct1, dct2)

    json_str3 = json.dumps(combined_dct)

    # {"a": [1, 2, 3, 4]}
    print(json_str3)

Answer 2

json.dumps() converts a dictionary to str object, not a json(dict) object.

So, adding some dumps statement in your code shows that the type is changed to str after using json.dumps() and with + you are effectively concatenating the two string and hence you get the concatenated output.

Further, to merge the two dictionaries for your simple case, you can just use the append:

import json

json_obj = json.dumps({"a": [1,2]})
json_obj1 = json.dumps({"a": [3,4]})

print(type(json_obj1))  # the type is `str`

json_obj += json_obj1   # this concatenates the two str objects

json_obj = {"a": [1,2]}
json_obj1 = {"a": [3,4]}

json_obj["a"].extend(json_obj1["a"])
 
print(json_obj)