database.py

from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

from flask import current_app, g

engine = create_engine('sqlite:///users.db', convert_unicode=True)
db_session = scoped_session(sessionmaker(autocommit=False,
                                        autoflush=False,
                                        bind=engine))

Base = declarative_base()
Base.query = db_session.query_property()

def init_db():
    import models
    Base.metadata.create_all(bind=engine)

def get_db():
    if 'db' not in g:
        g.db = db_session
    
    return g.db

def close_db(e=None):
    db = g.pop('db', None)

    if db is not None:
        db.close()

def init_app(app):
    init_db()
    app.teardown_appcontext(close_db)