Python Flask App (simple SPA, no modules)

app.py

from flask import Flask, request, redirect, url_for, render_template
import sys, os

# this creates our Flask app
app = Flask(__name__)


# define all your routes here

@app.route('/', methods = ['POST', 'GET'])
def home():
	if request.method == 'GET':
		title = "Main Page"
		data = "Hello"
		return render_template('main.html.j2', title = title, data = data)





# this is the port assignment and app running; do not change anything here
def main():
	port = 8000
	try:
		port = int(sys.argv[1])
	except:
		pass
	app.run(debug = True, port = port, host = '127.0.0.1')

if __name__ == '__main__':
	main()

You have to additionally create a templates folder that will house all your templates, viz. main.html.j2 and so on. Remember to name them with the .j2 extension so as to have proper indentation and syntax highlighting