Python - update item in an existing json file

Question

I want to update float values inside my json file, structured like this:

{"Starbucks": {"Roads": 1.0, "Pyramid Song": 1.0, "Go It Alone": 1.0}}

So whenever I generate an already existing playlist, with the exact same items, I increment key values by +1.0.

I have a file opened with 'append' option

with open('pre_database/playlist.json', 'a') as f:
     if os.path.exists('pre_database/playlist.json'):
         #update json here
     json.dump(playlist,f)

but this 'a' method would append another dictionary to json, and it would generate parsing problems later on.

likewise, if I use 'w' method, it would overwrite the file entirely.

what is the best solution for updating the values?

Moses Koledoye · Accepted Answer · 2016-11-10 15:00:25Z

5

You could open the file in r+ mode (opens the file for both reading and writing), read in the JSON content, seek back to the start of the file, truncate it and then rewrite the modified dictionary back to the file:

if os.path.exists('pre_database/playlist.json'):
    with open('pre_database/playlist.json', 'r+') as f:
         playlist = json.load(f)
         # update json here
         f.seek(0)
         f.truncate()
         json.dump(playlist, f)

edited Nov 10, 2016 at 15:00

answered Nov 10, 2016 at 14:52

Moses Koledoye

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1 Comment

Moses Koledoye Over a year ago

@data_garden It does, but you need to read from the file first before it gets truncated, so w won't work here. You need to manually truncate.

Robin Koch · Accepted Answer · 2016-11-10 15:19:26Z

Appending means that your file is getting longer, which is neither what you what nor how JSON works.

If you want to update some value you need to load the json-file, update your value(s) and dump it back:

with open('pre_database/playlist.json', 'r') as f:
    playlist = json.load(f)
playlist[key] = value  # or whatever
with open('pre_database/playlist.json', 'w') as f:
    json.dump(playlist, f)

Also your check if your file exists should happen before you open the file and not when it's already open:

if os.path.exists('pre_database/playlist.json'):
    with open('pre_database/playlist.json', 'r') as f:
        playlist = json.load(f)
    playlist[key] = value  # or whatever
    with open('pre_database/playlist.json', 'w') as f:
        json.dump(playlist, f)

although I guess the pythonic ways would be to just try it and catch the IOError if the file didn't exist as expected.

Depending on how you continue it's probably best to do something like this:

try:
    with open('pre_database/playlist.json', 'r') as f:
        playlist = json.load(f)
except IOError, ValueError:
    playlist = default_playlist

playlist[key] = value  # or whatever

with open('pre_database/playlist.json', 'w') as f:
    json.dump(playlist, f)

Robin

Rohit · Accepted Answer · 2021-02-25 02:49:01Z

0

It is appendind new dictionary because file is opened in append mode and cursor is at end of file. You need to truncate before dumping latest dict to file

with open('pre_database/playlist.json', 'a') as f:
    if os.path.exists('pre_database/playlist.json'):
        f.seek(0)
        playlist = json.load(f)
        #Update your dict here
        playlist.update(dict({'key1':'NewValue1'}))
        f.truncate(0)
        playlist.dump(playlist,f)

edited Feb 25, 2021 at 2:49

answered Feb 24, 2021 at 16:59

Rohit

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