Flask app context for sqlalchemy

Question

I am working on a small rest api in flask. Api has route that registers a request and spawn separate thread to run in background. Here is the code:

def dostuff(scriptname):
    new_thread = threading.Thread(target=executescript,args=(scriptname,))
    new_thread.start()

Thread starts but it errors out when I try to insert into db from executescript function. It complains about db object not registered with the app.

I am dynamically creating my app (with api as Blueprint).

Here is the structure of the app

-run.py ## runner script
-config
   -development.py
   -prod.py
-app
  -__init__.py
  - auth.py
  - api_v1
     - __init__.py
     - routes.py
     - models.py

here is my runner script run.py :

from app import create_app, db

if __name__ == '__main__':
    app = create_app(os.environ.get('FLASK_CONFIG', 'development'))
    with app.app_context():
        db.create_all()
    app.run()

Here is the code from app/__init__.py which creates the app:

from flask import Flask, jsonify, g
from flask.ext.sqlalchemy import SQLAlchemy

db = SQLAlchemy()

def create_app(config_name):
    """Create an application instance."""
    app = Flask(__name__)

    # apply configuration
    cfg = os.path.join(os.getcwd(), 'config', config_name + '.py')
    app.config.from_pyfile(cfg)

    # initialize extensions
    db.init_app(app)
    # register blueprints
    from .api_v1 import api as api_blueprint
    app.register_blueprint(api_blueprint, url_prefix='/api/')
    return app 

All I need to know is how do I extend app context in routes.py. I can not import app there directly and if I do the following, I get RuntimeError: working outside of application context

def executescript(scriptname):
    with current_app.app_context():
        test_run = Testrun(pid=989, exit_status=988,comments="Test TestStarted")
        db.session.add(test_run)
        db.session.commit()

Answer 1

You're running a background task in a different thread that doesn't have the application context. You should pass the app object to the background worker. Miguel Grinberg gives an example of this here:

from threading import Thread
from app import app

def send_async_email(app, msg):
    with app.app_context():
        mail.send(msg)

def send_email(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.body = text_body
    msg.html = html_body
    thr = Thread(target=send_async_email, args=[app, msg])
    thr.start()

Alternatively (and probably the best solution) would be to actually set up a thread-local scoped SQLAlchemy session instead of relying on Flask-SQLAlchemy's request context.

>>> from sqlalchemy.orm import scoped_session
>>> from sqlalchemy.orm import sessionmaker

>>> session_factory = sessionmaker(bind=some_engine)
>>> Session = scoped_session(session_factory)