Demonstration of the ability to switchable flask server, from the blocking dev server to the async gevent server. Instructions to test in the comment below

flask_async.py

#!/usr/bin/env python
import time

import argparse
import gevent
import gevent.monkey
from gevent.pywsgi import WSGIServer
gevent.monkey.patch_all()

from flask import Flask, request, Response, render_template

app = Flask(__name__)


def call_redis(WAIT):
    time.sleep(WAIT)


@app.route('/')    
@app.route('/<int:wait>')
def click(wait=0):
    start = time.time()
    stuff = call_redis(wait)
    wait = time.time() - start
    return Response({'took {0} seconds'.format(wait)})


def parse_arguments():
    """Gets console arguments"""
    parser = argparse.ArgumentParser(
        description='Flask dev server')
    parser.add_argument(
        "-a", "--async", default=False, choices=['yes', 'no'],
        help="Whether to run the server async"
    )
    return vars(parser.parse_args())

if __name__ == '__main__':
    args = parse_arguments()
    app.debug = True
    if args['async'] == 'yes':
        http_server = WSGIServer(('127.0.0.1', 5000), app)
        http_server.serve_forever()
    else:
        app.run()

Just call

curl localhost:5000/10

for a request that blocks for 10 seconds, then

curl localhost:5000/

for a request that does nothing but will be blocked until the first one ends, but which will finish immediately when choosing -a yes