Appending a list of dictionaries within a file

Question

I’m trying to take a dictionary popped from a list and append it to another list and save it as a new file.

If the file is blank I can create a new list of dictionaries and save the file, but if there is an existing list of dictionaries dont know how to check the file for a list (it will either be blank or a list of dicts), and if there is, append the file. I can’t just append the file because its a list of dictionaries and the trailing ] would break the list.

My question is how do you check the file if it already contains a list and if it does, how do you append it? (ie do you have to loop through the exiting list in the file, therefore creating a new list, and append that list with the pop and saving it?… or is there a better way?)

import json
import os

def main():

  with open('file1.json', 'r') as fin:
    user_list = json.load(fin)
    temp = user_list.pop(0)

  open_if_not_exists("processed.json")
  
  
  with open('processed.json', 'w') as processed_file:  
    my_list = []
    try:
      data = json.load(processed_file)
      for i in data:
        my_list.append(i)
      my_list.append(temp)
    except (IOError):
      my_list.append(temp)
    
    json.dump(my_list, processed_file)
      
      
  with open('file1.json', 'w') as fout:
    json.dump(user_list, fout)
    
def open_if_not_exists(filename):
  try:
      fd = os.open(filename, os.O_CREAT | os.O_EXCL | os.O_WRONLY)
  except OSError as e:
      if e.errno == 17:
          return None
      else:
          raise
  else:
      return os.fdopen(fd, 'w')
      
    
if __name__ == "__main__":
    main()

Sample data

[{"id": 1234, "screen_name": "UserName"}, {"id": 5678, "screen_name":
"Bobo2022"}, {"id": 9101, "screen_name": "ReallyDude"}, {"id": 1213,
"screen_name": "SMH"}, {"id": 1415, "screen_name": "IAskQuestions"}]

Asked By: IAskQuestions

||

Source

Answer 1

As a quick modification to your code…

It still naively assumes that if a file opens, it contains a valid json array

import json
import os

def main():

  processed_list = []

  new_list = []
  new_dict = {}

  with open('file1.json', 'r') as fin:
    new_list = json.load(fin)
    new_dict = new_list.pop(0)

  try:
    with open('processed.json', 'r') as processed_file:  
      processed_list = json.load(processed_file)
  except FileNotFoundError:
    processed_list = [ new_dict ]
  else:
    processed_list.append(new_dict)

  with open('processed.json', 'w') as fout:
    json.dump(processed_list, fout)

  with open('file1.json', 'w') as fout:
    json.dump(new_list, fout)


if __name__ == "__main__":
    main()

Answered By: MatBailie

Appending a list of dictionaries within a file

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