[Python] Appending to .json file does not work... error code: json.decoder.JSONDecodeError

Currently working on optimizing my passwort manager. Heres the code:

from tkinter import *
from tkinter import messagebox
import random
import string
import json

# ---------------------------- PASSWORD GENERATOR ------------------------------- #
def generate_password():
password_list = []

s1 = list(string.ascii_lowercase)
s2 = list(string.ascii_uppercase)
s3 = list(string.digits)
s4 = list(string.punctuation)

for x in range(4):
password_list.append(s1[x])
password_list.append(s2[x])
password_list.append(s3[x])
password_list.append(s4[x])

random.shuffle(password_list)
password = "".join(password_list)
password_entry.delete(0, END)
password_entry.insert(0, password)


# ---------------------------- SAVE PASSWORD ------------------------------- #


def add_info():
website = website_entry.get()
email_username = email_username_entry.get()
password = password_entry.get()
data_dict = {
website: {
"email": email_username,
"password": password,
}
}
try:
with open("data.json", mode="r") as data_file:
for line in data_file:
if website in line:
messagebox.showerror("ERROR", "Data for this website already exists.")
data_file.close()
else:
with open("data.json", mode="r") as data_file:
data = json.load(data_file)
data.update(data_dict)
with open("data.json", mode="w") as data_file:
data.dump(data_file, data_dict, indent=4)

except FileNotFoundError:
with open("data.json", mode="w+") as data_file:
json.dump(data_dict, data_file, indent=4)
finally:
with open("data.json", mode="r") as data_file:
data = json.load(data_file)
data.update(data_dict)
with open("data.json", mode="w") as data_file:
json.dump(data_dict, data_file, indent=4)



Problem is that I cant manage to write to the json file/append new passwords and usernames to it. Any help would be HIGHLY appreciated as im about to crash out man :/

Tried many variants of if/else, exceptions, finally etc, nothing helped me. When I deleted the file it put the first password/username combo in and then screwed everything over again.

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