What is purpose of init_app function in Flask?
It is used to control the integration of a package to one or more Flask applications. Depending on how you initialize the object it is usable right away or will attach as needed to a Flask application. 
In the case of db.init_app(app) the app is your flask application,the object must have been created initially.
Example: db.init_app(app)
What is purpose of init_app function in Flask? Is this like any other method?