SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL') or 'sqlite://' + os.path.join(basedir, 'app.db')
Here is what the above code is Doing:
1. The first line imports the os module.
2. The second line gets the value of the environment variable DATABASE_URL, if it exists, or else it uses a local SQLite database at app.db.
3. The third line creates the application object as an instance of class Flask imported from the flask package.
4. The fourth line configures the application object with the configuration settings from the object called Config that I defined earlier.
5. The fifth line connects to the database specified by the DATABASE_URL environment variable or, failing that, the local SQLite database.