Flask throwing 'working outside of request context' when starting sub thread

Question

I am trying to start a new thread in Python inside of a Flask application. I am doing background work that gets triggered by the request, but I don\'t need to wait for the

Answer 1

Since version 0.10 there is a supported way of doing this: http://flask.pocoo.org/docs/api/#flask.copy_current_request_context

If you want the before_request hooks to run you must call current_app.preprocess_request() inside of the decorated function.

Answer 2

As @runfalk pointed out, you'll need to use @copy_current_request_context. Here's a working code snippet:

import threading

from flask import request, jsonify, copy_current_request_context


@app.route('/foo')
def get_foo():
    @copy_current_request_context
    def foo_main():
        # insert your code here
        print(request.url)

    threading.Thread(target=foo_main).start()

    return jsonify({'status': 'started'})

Answer 3

You can copy the desired info and pass it on:

@app.route('/my_endpoint', methods=['POST'])
def my_endpoint_handler():
    #do tracking in sub-thread so we don't hold up the page
    def handle_sub_view(data):
        # Use the data in subprocess
    data = request.get_json()  # copy the data
    thread.start_new_thread(handle_sub_view, data)
    return "Thanks"

Answer 4

A more cleaner way is using Flask built-in executor which wraps app context, see https://flask-executor.readthedocs.io/en/latest/

Answer 5

Wrap your thread code in a test_request_context so you have access to context locals:

@app.route('/my_endpoint', methods=['POST'])
def my_endpoint_handler():
    #do tracking in sub-thread so we don't hold up the page
    def handle_sub_view(req):
        with app.test_request_context():
            from flask import request
            request = req
            # Do Expensive work
    thread.start_new_thread(handle_sub_view, (request))
    return "Thanks"

Edit: it's worth pointing out that the thread will have a different context than the original request. You need to extract any interesting request data, such as the user ID, before spawning the thread. You can then grab a (different) user object in the sub-thread using the ID.