__init__.py

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
import os

db = SQLAlchemy()
APP_ROOT = os.path.dirname(os.path.abspath(__file__))
UPLOAD_FOLDER = os.path.join(APP_ROOT,'uploads')

def create_app():
    app = Flask(__name__, static_url_path='')

    app.config['SECRET_KEY'] = 'ALOHOMORA!'
    app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = True
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///db.sqlite'
    app.config['UPLOAD_FOLDER'] = UPLOAD_FOLDER

    db.init_app(app)

    login_manager = LoginManager()
    login_manager.login_view = 'auth.login'
    login_manager.init_app(app)

    from .models import User

    @login_manager.user_loader
    def load_user(user_id):
        # since the user_id is just the primary key of our user table, use it in the query for the user
        return User.query.get(int(user_id))

    # blueprint for auth routes in our app
    from .auth import auth as auth_blueprint
    app.register_blueprint(auth_blueprint)

    from .main import main as main_blueprint
    app.register_blueprint(main_blueprint)

    from .operation import operation as operation_blueprint
    app.register_blueprint(operation_blueprint)

    return app