Circular import of db reference using Flask-SQLAlchemy and Blueprints

Question

I am using Flask-SQLAlchemy and Blueprints and I cannot help myself from using circular imports. I know I can write imports inside functions and make it work but it sounds nasty, I'd like to confirm with the community if there is a better way to do this.

The problem is I have a module (blueprints.py) where I declare the database and import the blueprints but those blueprints need to import the database declaration at the same time.

This is the code (excerpt of the important parts):

application.apps.people.views.py

from application.blueprints import db  people = Blueprint('people', __name__,                  template_folder='templates',                  static_folder='static')  class User(db.Model):     id = db.Column(db.Integer, primary_key=True)     username = db.Column(db.String(80), unique=True)  @people.route('/all') def all():     users = User.query.all() 

application.blueprints.py

from application.apps.people.views import people  app = Flask(__name__) app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:////tmp/test.db' db = SQLAlchemy(app) app.register_blueprint(people, url_prefix='/people') 

I have read the documentation and the questions I found on this topic, but I still cannot find the answer I am looking for. I have found this chapter (https://pythonhosted.org/Flask-SQLAlchemy/contexts.html) where it suggest to put the initialization code inside a method but the circular import still persist.

Edit I fixed the problem using the pattern Application Factory

Answer 1

I fixed the problem with the help of the Application Factory pattern. I declare the database in a third module and configure it later in the same module in which I start the application.

This results in the following imports:

• database.py → app.py
• views.py → app.py
• database.py → views.py

There is no circular import. It is important to make sure that the application was started and configured before calling database operations.

Here is an example application:

app.py

from database import db from flask import Flask import os.path from views import User from views import people   def create_app():     app = Flask(__name__)     app.config['DEBUG'] = True     app.config['SQLALCHEMY_DATABASE_URI'] = "sqlite:////tmp/test.db"     db.init_app(app)         app.register_blueprint(people, url_prefix='')     return app    def setup_database(app):     with app.app_context():         db.create_all()     user = User()     user.username = "Tom"     db.session.add(user)     db.session.commit()       if __name__ == '__main__':     app = create_app()     # Because this is just a demonstration we set up the database like this.     if not os.path.isfile('/tmp/test.db'):       setup_database(app)     app.run() 

database.py

from flask_sqlalchemy import SQLAlchemy  db = SQLAlchemy() 

views.py

from database import db from flask.blueprints import Blueprint   people = Blueprint('people', __name__,                  template_folder='templates',                  static_folder='static')   class User(db.Model):     id = db.Column(db.Integer, primary_key=True)     username = db.Column(db.String(80), unique=True)   @people.route('/') def test():   user = User.query.filter_by(username="Tom").first()   return "Test: Username %s " % user.username 

Answer 2

Circular imports in Flask are driving me nuts. From the docs: http://flask.pocoo.org/docs/0.10/patterns/packages/

... Be advised that this is a bad idea in general but here it is actually fine.

It is not fine. It is deeply wrong. I also consider putting any code in __init__.py as a bad practice. It makes the application harder to scale. Blueprints is a way to alleviate the problem with circular imports. I think Flask needs more of this.