dump_to_txt.py

import json


whitelist = []

def add_user_whitelist(user_id):
  whitelist.append(user_id)
  
add_user_whitelist(12345678000)

with open('local.txt', 'w') as file:
  json_encoded = json.dumps(whitelist)
  file.write(json_encoded)

read_json_txt.py

import json

with open('local.txt', 'r') as file:
  whitelist = json.loads(file.read())
  print(whitelist) # [ 12345678000 ]