json_append.py

import json

def bad_json_append_please_donʼt(filename, data):
    with open(filename, 'rb+') as f:
        f.seek(-1, 2)
        if f.tell() > 1:
            f.write(b',')
        f.write(json.dumps(data).encode('utf-8'))
        f.write(b']')

with open('database.json', 'w') as f:
    f.write('[]')

bad_json_append_please_donʼt('database.json', {'a': 'b'})
bad_json_append_please_donʼt('database.json', {'c': 'd'})

with open('database.json') as f:
    print(f.read())