Append a value to a json file in place on disk.

append_json.py

import datetime
import json
import os

outfile_name = "data-{}.json".format(datetime.date.today().isoformat())

if os.path.exists(outfile_name):
    append_to_json_file(outfile_name, data)  # get `data` from somewhere
else:
    with open(outfile_name, 'w') as f:
        json.dump([data], f, sort_keys=True)

def append_to_json_file(filename, data):
    """Append a value to a json file in place on disk.
    Assumes it's appending an object to an array."""
    outfile = open(filename, 'rb+')  # binary mode is required for the seek operation
    last_char_idx = 0
    endchar = None
    while endchar != b']':
        # this loop finds the last json character, skipping over whitespace
        last_char_idx -= 1
        outfile.seek(last_char_idx, 2)
        endchar = outfile.read(1)
    outfile.seek(last_char_idx, 2)
    outfile.write(',\n {}'.format(json.dumps(data, sort_keys=True)).encode('UTF-8'))
    outfile.write(endchar)
    outfile.close()