Introduction
In Flask, we can write a complete web application without the need of any third-party templating engine. For example, have a look at the following code; this is a simple Hello World application with a bit of HTML styling included:
from flask import Flask
app = Flask(__name__)
@app.route('/')
@app.route('/hello')
@app.route('/hello/<user>')
def hello_world(user=None):
    user = user or 'Shalabh'
    return '''
<html>
    <head>
      <title>Flask Framework Cookbook</title>
    </head>
      <body>
        <h1>Hello %s!</h1>
        <p>Welcome to the world of Flask!</p>
      </body>
</html>''' % user
if __name__ == '__main__':
    app.run()Is the preceding pattern of writing the application feasible in the case of large applications that involve thousands of lines of HTML, JS, and CSS code? Obviously not!
Here, templating...