Flask circular dependency

Question

I am developing a Flask application. It is still relatively small. I had only one app.py file, but because I needed to do database migrations, I divided it into 3 using this guide:

https://realpython.com/blog/python/flask-by-example-part-2-postgres-sqlalchemy-and-alembic/

However, I now can't run my application as there is a circular dependency between app and models.

app.py:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask import render_template, request, redirect, url_for
import os

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = os.environ['DB_URL']
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

app.debug = True

db = SQLAlchemy(app)

from models import User

... routes ...    

if __name__ == "__main__":
  app.run()

models.py:

from app import db
class User(db.Model):
  id = db.Column(db.Integer, primary_key=True)
  username = db.Column(db.String(80), unique=True)
  email = db.Column(db.String(120), unique=True)

  def __init__(self, username, email):
    self.username = username
    self.email = email

  def __repr__(self):
    return self.username

manage.py:

from flask_script import Manager
from flask_migrate import Migrate, MigrateCommand
from app import app, db

migrate = Migrate(app, db)
manager = Manager(app)

manager.add_command('db', MigrateCommand)

if __name__ == "__main__":
  manager.run()

They are all in the same directory. When I try to run python app.py to start the server, I receive an error which definitely shows a circular dependency (which is pretty obvious). Did I make any mistakes when following the guide or is the guide wrong? How can I refactor this to be correct?

Thanks a lot.

EDIT: Traceback

Traceback (most recent call last):
  File "app.py", line 14, in <module>
    from models import User
  File "/../models.py", line 1, in <module>
    from app import db
  File "/../app.py", line 14, in <module>
    from models import User
ImportError: cannot import name User

Answer 1

I propose the following structure:

# app/extensions.py
from flask_sqlalchemy import SQLAlchemy
db = SQLAlchemy()
...


# app/app.py
from app.extensions import db

def create_app(config_object=ProdConfig):
    app = Flask(__name__.split('.')[0])
    app.config.from_object(config_object)
    register_extensions(app)
    ...

def register_extensions(app):
    db.init_app(app)
    ...

# manage.py
from yourapp.app import create_app
app = create_app()
app.debug = True
...

In this case, database, app, and your models are all in separate modules and there are no conflicting or circular imports.