CODEWITHSUNDEEP
pythonmultithreadingflask
Radoslav Bodó
Radoslav Bodó

Reputation: 661

Why flask app context is lost in child thread when application factory pattern is used?

Somewhat a mystery question.

Spawning a thread from flask app works well works well when all code is placed in the single app.py file, but application context gets lost when factory pattern is used.

Exception in thread Thread-2:
Traceback (most recent call last):
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/sqlalchemy/util/_collections.py", line 1008, in __call__
127.0.0.1 - - [07/Jun/2022 14:06:53] "GET / HTTP/1.1" 200 -
    return self.registry[key]
KeyError: <greenlet.greenlet object at 0x7f4f541ad040 (otid=0x7f4f54199d00) current active started main>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/XXX/flaskthreading/app2/views.py", line 12, in printer
    print(User.query.all())
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/flask_sqlalchemy/__init__.py", line 552, in __get__
    return type.query_class(mapper, session=self.sa.session())
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/sqlalchemy/orm/scoping.py", line 47, in __call__
    sess = self.registry()
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/sqlalchemy/util/_collections.py", line 1010, in __call__
    return self.registry.setdefault(key, self.createfunc())
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 4225, in __call__
    return self.class_(**local_kw)
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/flask_sqlalchemy/__init__.py", line 174, in __init__
    self.app = app = db.get_app()
  File "/XXX/flaskthreading/venv/lib/python3.9/site-packages/flask_sqlalchemy/__init__.py", line 1042, in get_app
    raise RuntimeError(
RuntimeError: No application found. Either work inside a view function or push an application context. See http://flask-sqlalchemy.pocoo.org/contexts/.

Any idea why is this happening or what docs we have missed ? MVP provided https://github.com/bodik/flaskthreading-experiments/

git clone
pip install -r requirements.txt

# working app
FLASK_APP=app1 flask run
curl http://localhost:5000/

# produces error
FLASK_APP=app2.app flask run
curl http://localhost:5000/

Upvotes: 3

Views: 3286

Answers (1)

aaron
aaron

Reputation: 43103

From https://flask.palletsprojects.com/en/2.1.x/reqcontext/#lifetime-of-the-context:

When a Flask application begins handling a request, it pushes a request context, which also pushes an app context. When the request ends it pops the request context then the application context.

The context is unique to each thread (or other worker type). ...

In the case of a global app and db = SQLAlchemy(app), db always has self.app.
In the case of application factory pattern, db gets the app context from the thread.

Pass an AppContext to be pushed onto the thread:

# def printer():           # Change this
def printer(app_context):  # to this
    app_context.push()     #
    print(User.query.all())

@blueprint.route('/')
def index():

    # athread = Thread(target=printer, args=(), daemon=True)                          # Change this
    athread = Thread(target=printer, args=(current_app.app_context(),), daemon=True)  # to this
    athread.start()

    return 'ok'

Upvotes: 6

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