CODEWITHSUNDEEP
pythonjson
Tony Roczz
Tony Roczz

Reputation: 2398

Python - how to remove the final comma(,) in json string

Hi I have just started experimenting with python and tornado along with mongodb(I am a newbie). I have written a simple get function to get all the values from my mongodb and return it in JSON format. The problem is when I try to write the output as a JSON string I get a trailing comma(,) after the last record from the collection.

class TypeList(APIHandler):
@gen.coroutine
def get(self):
    cursor = db.vtype.find()
    self.write("{"'"success"'": 1, "'"data"'":[")
    while (yield cursor.fetch_next):
        document = cursor.next_object()
        self.write(format(JSONEncoder().encode(document)))
        self.write(",")
    self.write("]}")

class JSONEncoder(json.JSONEncoder):
def default(self, o):
    if isinstance(o,ObjectId):
        return str(o)
    return json.JSONEncoder.default(self, o)

And my output is like

{"success": 1, "data":[{"_id": "55a5e988545779f35d3ecdf4", "name": "fgkd", "city": "fghj"},{"_id": 12345.0, "name": "adfs", "city": "asd"},]}

Can anyone tell me how can I get rid of that trailing comma(,) after my last record, because of that comma I am getting an error malformed JSON string

I have tried using json dumps

@gen.coroutine
def get(self):
    cursor = db.vtype.find({"brand": "Tata"})
    while (yield cursor.fetch_next):
        document = cursor.next_object()
        self.write(json.dumps(document,default=json_util.default))

got the output as

{"Reg": "11ts", "_id": {"$oid": "55a5e988545779f35d3ecdf4"}, "Name": "Alex"}{"Reg": "12ts", "_id": {"$oid": "55a5eac6545779f35d3ecdf5"}, "Name": "asdf"}

When using dumps[{ "data": document }]

I am getting the output as

[{"data": {"Name": "asdf", "Reg": "asdfs", "_id": {"$oid": "55a5e988545779f35d3ecdf4"}}}]

[{"data": {"Name": "qwer", "Reg": "asdff", "_id": {"$oid": "55a5eac6545779f35d3ecdf5"}}}]

but I want the output like this

{"data": [{"Name": "asdf", "Reg": "asdfs", "_id": {"$oid": "55a5e988545779f35d3ecdf4"}},{"Name": "qwer", "Reg": "asdff", "_id": {"$oid": "55a5eac6545779f35d3ecdf5"}}]}

If I am doing something wrong please tell me I dont know how to do it.

Upvotes: 5

Views: 3982

Answers (3)

Blakes Seven
Blakes Seven

Reputation: 50406

So your problem is with MongoDB ObjectId? Then maybe you should have been using bson.json_util. It's probably already installed as part of your MongoDB driver dependecies ( which all use pymongo ), but if not then install it.

import bson
import bson.json_util
from bson.json_util import dumps
from bson import ObjectId

dumps({ "a": ObjectId() })

'{"a": {"$oid": "55a782261d41c80b0432b811"}}'

Or:

dumps([{ "a": ObjectId(), "b": 1 },{ "a": ObjectId(), "b": 2 }])
'[{"a": {"$oid": "55a79543268e150463d51799"}, "b": 1}, {"a": {"$oid": "55a79543268e150463d5179a"}, "b": 2}]'

And it works just like "dumps" except all the BSON type handling is built it.

Again, no need to re-invent the wheel here and "roll your own", because people already use this.

Upvotes: 2

André Laszlo
André Laszlo

Reputation: 15537

Your implementation of the JSONEncoder works well. Just use it the way it was intended to be used:

>>> JSONEncoder().encode({'data': [ObjectId(), ObjectId()]})
'{"data": ["<objId>", "<objId>"]}'

The encoder will take care of serializing dicts, objects, lists, tuples, strings (including unicode), ints, longs, floats, booleans and None. Your implementation makes it aware of ObjectIds as well. Perfect!

Just lose the string concatenation and use encode.

Upvotes: 0

Daniel Roseman
Daniel Roseman

Reputation: 599630

There is no reason you should be building up JSON documents via text concatenation.

Python has a perfectly good json module in the standard library which you should be using. Build up your document as a Python list of dicts, then use json.dumps() to convert the whole thing to valid JSON.

Upvotes: 7

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