In python, how to make a list or dictionary persist across sessions without being overwritten immediately the code is run?

Question

This is my first question on StackOverflow so please excuse any deficiencies.

I am trying to make a leaderboard for a game. It needs to add +1 to the rankings for the player that beats the game, then save the rankings to a file.

I've tried playing around lists, dictionaries, shelve, pickle, csv file, text file so far and can't figure it out.

Here's the problem: everything I can think of has the form:

Save a structure to a file e.g. save a dictionary like {1: Name1: 5, 2: Name2: 4, 3: Name3: 4, 4: Name4: 2, 5: Name5: 1, } by using e.g. pickle
Perform the +1 operation on the data if the player beats the game
Save the structure to the file again
Exit

The problem is that when the code runs again, #1 above will overwrite #3 above.

So what I really want to do is start with an existing file and then open it up as a list or dictionary and then do #2-4 above on it.

But after lots of searching I have no idea what the right way to do this is. Is it a CSV file? A dictionary in a text file? A list in a text file? Is now the right time to learn about databases?

I'm looking for a quick and light solution. Doesn't need to scale or be particularly robust, I am a noob and this is a toy programme to learn from

You want to replace the line which contains the dictionary , or you want to overwrite the line? — Ahsanul Haque, Dec 01 '15 at 08:52

adrianus · Answer 1 · 2015-12-01T09:03:37.913

When starting the program, you could first read the leaderboard file, then run the game, and in the end, save the changes.

Here's an example with JSON:

import json

# higscores.json: {"player2": 5, "player1": 3}

# read data
try:
    with open('highscores.json', 'r') as f:
        higscores = json.load(f)
except IOError:
    # highscores not existing, create new
    higscores = {"player1": 0} 

# run the game
# ...

# edit the data, if player has beaten the game
higscores['player1'] += 1

# write it back to the file
with open('highscores.json', 'w') as f:
    json.dump(higscores, f)

score 0 · Answer 2 · answered Dec 01 '15 at 09:01

Modify your first step so it works in this way:

if file exists read it
otherwise create it with initial data

It could look something like this:

try:
    with open(path_to_file, 'r') as f:
        # read score from file
        score = ...
except FileNotFoundError:
    with open(path_to_file, 'w') as f:
        # save initial data to file
        score = initial_data

In python, how to make a list or dictionary persist across sessions without being overwritten immediately the code is run?

2 Answers2