database.py

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

def init_db(app):
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/db/user.db'
    app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False
    db.init_app(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    password = db.Column(db.String(120), nullable=False)