How to separate flask files when two files depend on each other?

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from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()
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from app.extensions import db

from flask import Flask
from app.extensions import db
from app.user import user as user_bp

def create_app():
    app = Flask(__name__)
    register_extensions(app)
    register_blueprints(app)
    return app

def register_extensions(app):
    """Register Flask extensions."""
    db.init_app(app)
    return None

def register_blueprints(app):
    """Register Flask blueprints."""
    app.register_blueprint(user_bp)
    return None

if __name__ == '__main__':
    app = create_app()
    app.run(debug=True)