[FIXED] Circular import of db reference using Flask-SQLAlchemy and Blueprints

Issue

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

Solution

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

Answered By - S182