Python: Replace string in json file, from python shell command

Question

I'm new to python and what I'm trying to do is replace text/string in a json file, from a python os shell command. I'm somewhat getting the results I'm looking for, but it's appending extra whitespace/creating a new line in the json file. This is basically what I'm trying to accomplish:

1. I have a static json file (add.json)
2. I'm running two OS shell commands within python, and storing that output into separate text files.
3. I then want to take the values in those two txt files, and replace the two strings in a json file.

Below is what I currently have (to make it simple I've replaced the true aws cli commands with simple commands)

import os
import fileinput

cmd = 'hostname > host.txt'
cmd2 = 'echo mama > echo.txt'

os.system(cmd)
os.system(cmd2)

file = open('host.txt')
contents = file.read()
with open("out.json", "wt") as fout:
with open("add.json", "rt") as fin:
    for line in fin:
        fout.write(line.replace('dns',contents))

file2 = open('echo.txt')
contents2 = file2.read()
with open("out2.json", "wt") as fout2:
    with open("out.json", "rt") as fin2:
    for line in fin2:
        fout2.write(line.replace('ip', contents2))

And this is the result that it's yielding:

{
"Comment": "A new record set for the zone.",
"Changes": [
 {
  "Action": "CREATE",
  "ResourceRecordSet": {
    "Name": "WildburritoPC
 ",
    "Type": "A",
    "TTL": 60,
    "ResourceRecords": [
      {
        "Value": "mama 
"
      }
     ]
   }
  }
 ]
}

As you can see, after Name and Value, it indeed replaces the values, but adds a new line and produces invalid json.

This is the file I'm replacing the values in:

{
"Comment": "A new record set for the zone.",
"Changes": [
 {
  "Action": "CREATE",
  "ResourceRecordSet": {
    "Name": "dns",
    "Type": "A",
    "TTL": 60,
    "ResourceRecords": [
      {
        "Value": "ip"
      }
    ]
   }
  }
 ]
}

Thank you in advance for any answers. I know what I have above is very dirty, and I'm sure there must be a better/cleaner way of accomplishing what I'm trying to do, but ultimately I know we all have to start somewhere and I can't even begin to explain how grateful I am with this community for all the help it's provided so far.

Answer 1

There's json module in python's standard library, it'll be much more error proof to use it rather than replacing strings.

To load json file:

import json
with open("add.json", "r") as fout2:
    json_data = json.load(fout2)
    for change in json_data["Changes"]:
        # strip the contents of trailing white spaces (new line)
        change["Name"] = change["Name"].strip()

# dump json to another file
with open("out.json", "w") as fout:
    fout.write(json.dumps(json_data))

I guess you got the idea. json module will take care that your json data are not corrupted (or at least it'll fail with exception when that occurs).