Need to execute a function after returning the response in Flask

Question

For one request alone i need to execute a function after sending the response to the client. Because the function takes time and that ends up in connection timeout Socket error: [Errno 32] Broken pipe

Is there a way in Flask to execute function after returning the request

Answer 1

I will expose my solution.

You can use threads to compute anything after returned something in your function called by a flask route.

import time
from threading import Thread
from flask import request, Flask
app = Flask(__name__)


class Compute(Thread):
    def __init__(self, request):
        Thread.__init__(self)
        self.request = request

    def run(self):
        print("start")
        time.sleep(5)
        print(self.request)
        print("done")


@app.route('/myfunc', methods=["GET", "POST"])
def myfunc():
        thread_a = Compute(request.__copy__())
        thread_a.start()
        return "Processing in background", 200

Answer 2

You can try use streaming. See next example:

import time
from flask import Flask, Response

app = Flask(__name__)

@app.route('/')
def main():
    return '''<div>start</div>
    <script>
        var xhr = new XMLHttpRequest();
        xhr.open('GET', '/test', true);
        xhr.onreadystatechange = function(e) {
            var div = document.createElement('div');
            div.innerHTML = '' + this.readyState + ':' + this.responseText;
            document.body.appendChild(div);
        };
        xhr.send();
    </script>
    '''

@app.route('/test')
def test():
    def generate():
        app.logger.info('request started')
        for i in range(5):
            time.sleep(1)
            yield str(i)
        app.logger.info('request finished')
        yield ''
    return Response(generate(), mimetype='text/plain')

if __name__ == '__main__':
    app.run('0.0.0.0', 8080, True)

All magic in this example in genarator where you can start response data, after do some staff and yield empty data to end your stream.

For ditails look at http://flask.pocoo.org/docs/patterns/streaming/.

Answer 3

A more general solution than the flask iterator solution is to write a WSGI middleware that adds a callback to the response close method. Here we use the werkzeug ClosingIterator helper and a flask app extension to achieve this:

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

You can then use your after_response decorator like this:

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

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

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

When you curl this, you'll see that it responds immediately and curl closes, then 2s later your "after" message appears in the logs:

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

This answer is summarized from my answers here and here.

Answer 4

There is no Flask-native way to accomplish this. after_request will still run before returning the response to the client, not after.

Here is a discussion of the problem and some solutions.

Answer 5

You can use the after_request decorator or customize to create a after_this_request which only works for that particular request.

Take a look at this snippet http://flask.pocoo.org/snippets/53/

Answer 6

You can defer route specific actions with limited context by combining after_this_request and response.call_on_close. Note that request and response context won't be available but the route function context remains available. So you'll need to copy any request/reponse data you'll need into local variables for deferred access.

@app.route('/')
def index():
    # Do your pre-response work here
    msg = 'Hello World!'
    @flask.after_this_request
    def add_close_action(response):
        @response.call_on_close
        def process_after_request():
            # Do your post-response work here
            time.sleep(3.0)
            print('Delayed: ' + msg)
        return response
    return msg