In Python, how to replace a json subobject based on multiple attribute value settings?

Ok, assuming this the python script is in the same directory as a json file called test.json and this is what the json looks like (not the same as yours since yours has both null and None values for some reason so I wasn't sure what to do with it):

[
    {
        "builtin_name": "custom_template",
        "fields": [
            {
                "id": 10012,
                "field_type": "OBJECT_SET",
                "tooltip_text": "",
                "name_plural": "",
                "name_singular": "reference",
                "backref_name": "reference",
                "backref_tooltip_text": "",
                "allow_multiple": false,
                "allowed_otypes": [
                    "schema",
                    "table",
                    "attribute",
                    "user",
                    "groupprofile",
                    "groupprofile"
                ],
                "options": null,
                "builtin_name": null
            },
            {
                "id": 8,
                "field_type": "OBJECT_SET",
                "tooltip_text": null,
                "name_plural": "Stewards",
                "name_singular": "Steward",
                "backref_name": "Steward",
                "backref_tooltip_text": null,
                "allow_multiple": true,
                "allowed_otypes": ["user", "groupprofile", "groupprofile"],
                "options": null,
                "builtin_name": "steward"
            }
        ],
        "id": 16,
        "title": "Custom template"
    },
    {
        "builtin_name": "new_template",
        "fields": [
            {
                "id": 10011,
                "field_type": "PICKER",
                "tooltip_text": "",
                "name_plural": "",
                "name_singular": "status",
                "backref_name": "",
                "backref_tooltip_text": "",
                "allow_multiple": false,
                "allowed_otypes": null,
                "options": [
                    {
                        "old_index": 0,
                        "article_id": null,
                        "tooltip_text": null,
                        "in_use": true,
                        "title": "Done"
                    }
                ],
                "builtin_name": null
            }
        ],
        "id": 1899,
        "title": "New Template"
    }
]

Then, this code works to load and change fields:

import json


with open("test.json") as json_file:
    jdata = json.load(json_file)

jdata_copy = jdata

for t in jdata: if t["id"] == 16 and t["title"] == "Custom template": pprint(jdata) print() t["fields"] = ["howdy", "hello"] print() pprint(jdata)

However let me know if this is the case for you, I wasn't sure from your question exactly what you were working with.

Edit

So, changing the code to:

import json
from pprint import pprint


with open("test.json") as json_file:
    jdata = json.load(json_file)

jdata_copy = jdata

for t in jdata:
    if t["id"] == 16 and t["title"] == "Custom template":
        pprint(jdata)

        t["fields"] = ["howdy", "hello"]

        pprint(jdata)

With this I can see that the first print statement returns the json as above just as a python dictionary, and the second print statement does indeed show jdata has had the fields section of the correct dictionary modified.

Then, I can export the dictionary back out to a .json:

with open("test1.json", "w") as json_file:
    json.dump(jdata, json_file)

This all works fine for me, which section here is not working for you? Sorry if I have misunderstood.