Accessing JSON message from IRI node API using Python 3

I am using one command tangle. The tangle is running on Ubuntu machine. From the list of APIs given for IRI node, I’m trying to broadcast a transaction. For this, the first step is to get getTransactionsToApprove which returns trunkTransaction and branchTransaction and I have to assign that value to attachToTangle API.
I am getting the correct output from getTransactionsToApprove API:

{'trunkTransaction': 'NTCYZTAJTVJIAQW9JXUJCACYPHNMESWGWTANQQZSLGHQTKWGDOYHAHBETUXKNAMMWYDTILDWLPQTOX999', 'branchTransaction': 'NTCYZTAJTVJIAQW9JXUJCACYPHNMESWGWTANQQZSLGHQTKWGDOYHAHBETUXKNAMMWYDTILDWLPQTOX999', 'duration': 2}

The problem is when I am assigning those values to attachToTangle API, I’m getting this error:

HTTP Error 500: Internal Server Error

My code is:

import urllib.request
import urllib.parse
import json
import requests


trytes = "hello world!"
url="http://mydnsname:14265"

headers = {
    'Content-Type': 'application/json',
    "Accept": "application/json",
    'X-IOTA-API-Version': '1'
}

#get transactions
command = {
   "command": "getTransactionsToApprove",
   "depth": 4
}

data= json.dumps(command).encode("utf-8")
request = urllib.request.Request(url, data, headers)
try:
    returnData = urllib.request.urlopen(request).read()
    jsonData = json.loads(returnData)
    print (jsonData)
except urllib.error.URLError as e: 
    ResponseData = e.reason
    print (e)

#attach to tangle
command = {
   "command": "attachToTangle", 
"trunkTransaction": jsonData['trunkTransaction'],
"branchTransaction": jsonData['branchTransaction'],
"minWeightMagnitude": 9,
"trytes": trytes
}

data= json.dumps(command).encode("utf-8") 
request = urllib.request.Request(url, data, headers)
try:
    returnData = urllib.request.urlopen(request).read()
    jsonData = json.loads(returnData)
    print (jsonData)
except urllib.error.URLError as e: 
    ResponseData = e.reason
    print (e)

Any help is much appreciated.