You'll can use string formatting for that.

In your JSON string, replace random_uuid with %s, than do:

payload = payload % random_uuid

Another option is to use json.dumps to create the json:

payload_dict = {
    'id': random_uuid,
    ...
}

payload = json.dumps(payload_dict)

Use the json standard library. Turn your json into a dictionary, modify it, and serialize it back to json. The functions you're interested in are dumps for Dump-String, which converts native python data structures to json strings and loads, for Load-String, which is the inverse.

For instance

import json
data = json.loads(x)
y = ['cd', 'pid', 'pod']
for obj in data:
    if obj['parameter'] == 'PARA':
        obj['parameter'] = y.pop()

json_data = json.dumps(data)

You're escaping the inner double quote " in your string. It should be:

b"{\"Machine Name\":\""+hostname+"\"}", None, True)

In python you can also use single quotes ' for strings - and you don't need to escape double quotes inside single quoted strings

b'{"Machine Name":"'+hostname+'"}', None, True)

There are two better ways of doing this though. The first is string formatting which inserts a variable into a string:

b'{"Machine Name":"%s"}' % hostname # python 2.x (old way)
b'{{"Machine Name":"{0}"}}'.format(hostname) # python >= 2.6 (new way - note the double braces at the ends)

The next is with the Python JSON module by converting a python dict to a JSON string

>>> hostname = "machineA.host.com"
>>> data = {'Machine Name': hostname}
>>> json.dumps(data)
'{"Machine Name": "machineA.host.com"}'

This is probably the preferred method as it will handle escaping weird characters in your hostname and other fields, ensuring that you have valid JSON at the end.

Is there a reason you're using a bytestring

You can build the data structure from the JSON string using a dictionary comprehension, like this:

import json
from datetime import datetime

json_string = '''{"Type1":["2015-03-12","2015-04-08"],"Type2":["2015-01-04","2015-03-09","2015-05-25"]}'''

daytypes = {k: [datetime.strptime(x, '%Y-%m-%d').date() for x in v] for k,v in json.loads(json_string).items()}

>>> daytypes
{'Type1': [datetime.date(2015, 3, 12), datetime.date(2015, 4, 8)], 'Type2': [datetime.date(2015, 1, 4), datetime.date(2015, 3, 9), datetime.date(2015, 5, 25)]}

If you must have tuples in your dictionary then you can convert the list into a tuple in the dict comprehension:

daytypes = {k: tuple(datetime.strptime(x, '%Y-%m-%d').date() for x in v) for k,v in json.loads(json_string).items()}
>>> daytypes
{u'Type1': (datetime.date(2015, 3, 12), datetime.date(2015, 4, 8)), u'Type2': (datetime.date(2015, 1, 4), datetime.date(2015, 3, 9), datetime.date(2015, 5, 25))}

You could use the older modulo formatting. Replace the gte field with %s:

json_data_string = "{\"size\":0,\"query\":{\"bool\":{\"must\":[{\"range\":{\"@timestamp\":{\"time_zone\":\"+09:00\",\"gte\":\"%s\",\"lt\":\"2023-01-25T00:00:00.000Z\"}}},{\"term\":{\"serviceid.keyword\":{\"value\":\"430011397\"}}}]}},\"aggs\":{\"by_day\":{\"auto_date_histogram\":{\"field\":\"@timestamp\",\"minimum_interval\":\"minute\"},\"aggs\":{\"agg-type\":{\"terms\":{\"field\":\"nxlogtype.keyword\",\"size\":1000},\"aggs\":{\"my-sub-agg-name\":{\"avg\":{\"field\":\"size\"}}}}}}}}"

Then use % to format the string

for n in range(minutesDiff):
    print(json_data_string % (datetime.strptime(str(d1 + timedelta(minutes=n)),fmt)))

The reason your string isn't working is because you used double quotes " for the string instead of single quotes '. Since json format requires double quotes, you only should be using double quotes inside the string itself and then use the single quotes to denote the start/end of a string. (That way you don't need to keep using those \" unnecessarily.
Also, .format() can help with putting variables inside strings to make them easier.

This should fix your json string:

targetTemp = 17

payload = '{\n    "nodes": [{\n        "attributes": {\n            "targetHeatTemperature": {\n                "targetValue": {},\n            }\n        }\n    }]\n}'.format(targetTemp)

However, using the json module makes things a lot easier because it allows you to pass in a python dictionary which can be converted from/to a json string.

Example using the json package:

import json

targetTemp = 17

payload = {
    "nodes": [{
        "attributes": {
            "targetHeatTemperature": {
                "targetValue": targetTemp
            }
        }
    }]
}

payload_json = json.dumps(payload)  # Convert dict to json string

You can do something like this.

import json 
with open('/path/to/josn_file.json', 'r') as file:
     json_data = json.load(file)
     for item in json_data:
           if item['ParameterKey'] in ["Shell","Type"]:
              item['ParameterKey'] = "new value"
with open('/path/to/josn_file.json', 'w') as file:
    json.dump(json_data, file, indent=2)