From the app.config dictionary, the database connection string is passed to the create_engine function. Once engine is bound to the sessionmaker, a session can be initiated:

from application import app
# Connect to the database called chapter11 using SQLAlchemy functions
engine = create_engine(app.config['SQLALCHEMY_DATABASE_URI'])
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base()

A Python class called Base is created from the declarative_base() function. The Base class is then used to subclass all application classes.