python – Flask throwing 'working outside of request context' when starting sub thread-ThrowExceptions

Exception or error:

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 work to be done to respond to the request.

Is it possible to set the flask request in this sub-threat to the request that came in? Reason being, our ACL on our queries to our DB (mongoengine in front of mongoDB) relies on the request’s user (it grabs it from flask’s request object) to see if they have access to the objects, and its blowing up because the request is not available in the sub-thread.

Any thoughts would be much appreciated.

Here’s pseudo code of how I am handling it now, but it is not working.

@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):
        from flask import request
        request = req
        # Do Expensive work
    thread.start_new_thread(handle_sub_view, (request))
    return "Thanks"
How to solve:

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.

Answer:

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:

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:

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'})

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