Is there a way in Flask to send the response to the client and then continue doing some processing? I have a few book-keeping tasks which are to be done, but I don't want to keep the client waiting.

Note that these are actually really fast things I wish to do, thus creating a new thread, or using a queue, isn't really appropriate here. (One of these fast things is actually adding something to a job queue.)

Sadly teardown callbacks do not execute after the response has been returned to the client:

import flask
import time
app = flask.Flask("after_response")

@app.teardown_request
def teardown(request):
    time.sleep(2)
    print("teardown_request")

@app.route("/")
def home():
    return "Success!\n"

if __name__ == "__main__":
    app.run()

When curling this you'll note a 2s delay before the response displays, rather than the curl ending immediately and then a log 2s later. This is further confirmed by the logs:

teardown_request
127.0.0.1 - - [25/Jun/2018 15:41:51] "GET / HTTP/1.1" 200 -

The correct way to execute after a response is returned is to use WSGI middleware that adds a hook to the close method of the response iterator. This is not quite as simple as the teardown_request decorator, but it's still pretty straight-forward:

import traceback
from werkzeug.wsgi import ClosingIterator

class AfterResponse:
    def __init__(self, app=None):
        self.callbacks = []
        if app:
            self.init_app(app)

    def __call__(self, callback):
        self.callbacks.append(callback)
        return callback

    def init_app(self, app):
        # install extension
        app.after_response = self

        # install middleware
        app.wsgi_app = AfterResponseMiddleware(app.wsgi_app, self)

    def flush(self):
        for fn in self.callbacks:
            try:
                fn()
            except Exception:
                traceback.print_exc()

class AfterResponseMiddleware:
    def __init__(self, application, after_response_ext):
        self.application = application
        self.after_response_ext = after_response_ext

    def __call__(self, environ, start_response):
        iterator = self.application(environ, start_response)
        try:
            return ClosingIterator(iterator, [self.after_response_ext.flush])
        except Exception:
            traceback.print_exc()
            return iterator

Which you can then use like this:

@app.after_response
def after():
    time.sleep(2)
    print("after_response")

From the shell you will see the response return immediately and then 2 seconds later the after_response will hit the logs:

127.0.0.1 - - [25/Jun/2018 15:41:51] "GET / HTTP/1.1" 200 -
after_response

This is a summary of a previous answer provided here.