Circular import of db reference using Flask-SQLAlchemy and Blueprints

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

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.

Circular import of db reference using Flask-SQLAlchemy and Blueprints

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