To add onto what Anton said, you can read the json file into a dictionary. Afterwards use a.update(b) like he has, and overwrite the file.

If you open the file to append, and do json dump like you have it will just create another line with the new json data.

Hope this helps.

Assuming a and b are the dictionaries you want to merge:

c = {key: value for (key, value) in (a.items() + b.items())}

To convert your string to python dictionary you use the following:

import json
my_dict = json.loads(json_str)

Update: full code using strings:

# test cases for jsonStringA and jsonStringB according to your data input
jsonStringA = '{"error_1395946244342":"valueA","error_1395952003":"valueB"}'
jsonStringB = '{"error_%d":"Error Occured on machine %s in datacenter %s on the %s of process %s"}' % (timestamp_number, host_info, local_dc, step, c)

# now we have two json STRINGS
import json
dictA = json.loads(jsonStringA)
dictB = json.loads(jsonStringB)

merged_dict = {key: value for (key, value) in (dictA.items() + dictB.items())}

# string dump of the merged dict
jsonString_merged = json.dumps(merged_dict)

But I have to say that in general what you are trying to do is not the best practice. Please read a bit on python dictionaries.

Alternative solution:

jsonStringA = get_my_value_as_string_from_somewhere()
errors_dict = json.loads(jsonStringA)

new_error_str = "Error Ocurred in datacenter %s blah for step %s blah" % (datacenter, step)
new_error_key = "error_%d" % (timestamp_number)

errors_dict[new_error_key] = new_error_str

# and if I want to export it somewhere I use the following
write_my_dict_to_a_file_as_string(json.dumps(errors_dict))

And actually you can avoid all these if you just use an array to hold all your errors.

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)

This is a more generic approach, where d1 is updated with changes contained in d2. Of course, you should provide more examples of updates from d2 to see if my solution works in all cases.

#!/usr/bin/env python3

d1 = {"CA": [{"Marin": [{"zip": 1}], "population": 10000}]}
d2 = {"CA": {"Marin": {"zip": 2}}}


# read update data from d2 and append new values to d1
def update(d1, d2):
    for key, value in d2.items():
        if key in d1:
            for key1, value1 in value.items():
                if key1 in d1[key][0]:
                    d1[key][0][key1].append(value1)
                else:
                    d1[key][0][key1] = [value1]
        else:
            d1[key] = [value]

    return d1


print(update(d1, d2))

Not sure if I'm missing something, but I think this works (tested in python 2.5) with the output you specify:

import simplejson

finalObj = { 'obj1': obj1, 'obj2': obj2 }
simplejson.dumps(finalObj)

You can use the builtin json library

import json

list_of_dicts = [{},{}]

json_rep = json.dumps(list_of_dicts)