Setting up a Rock Solid Python Development Web Server

So you want to start developing a Python application do ya? Let’s assume we are planning to use a smaller framework like Flask which is a framework I’m really loving at the moment.

Update (6th of April 2014): In the past, I had found Flask’s in-built development server to be a bit unstable which is why I put together this entry but this is no longer the case. As such, I now recommend using the dev server that comes with Flask for development purposes. However, if you prefer to use another server instead, please read on.

In this article, I’ll show you how to setup various web servers to serve Flask applications using WSGI which may also be suitable for production use later on.

The requirements I have for a development server are as follows:

It must print access and error log output to stdout in realtime
It must display stdout when the print function is used in the code (which is really handy for debugging)
The server should reliably reload automatically when any code is changed
Exceptions should be printed to stderr or the browser if something goes wrong
The web server needs to be easy to setup
(Bonus) It would be ideal if the web server works with Jython 2.5.x too

So let’s get into it.

CherryPy

CherryPy’s web server is very well regarded and was one of the first I looked at. My main criticism of CherryPy was its documentation which I found extremely difficult to read through and grasp. With a bit of digging around, I managed to get everything to work. You must use the Paste library for logging.

Installation of CherryPy is super easy:

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pip install cherrypy paste

And here’s the Flask script:

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#!/usr/bin/env python

from flask import Flask
import cherrypy
from paste.translogger import TransLogger

app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"


def run_server():
    # Enable WSGI access logging via Paste
    app_logged = TransLogger(app)

    # Mount the WSGI callable object (app) on the root directory
    cherrypy.tree.graft(app_logged, '/')

    # Set the configuration of the web server
    cherrypy.config.update({
        'engine.autoreload_on': True,
        'log.screen': True,
        'server.socket_port': 5000,
        'server.socket_host': '0.0.0.0'
    })

    # Start the CherryPy WSGI web server
    cherrypy.engine.start()
    cherrypy.engine.block()

if __name__ == "__main__":
    run_server()

With a bit of extra effort, we can customise the access logging from Paste to be consistent with CherryPy if desired:

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#!/usr/bin/env python

import time

from flask import Flask
import cherrypy
from paste.translogger import TransLogger

app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"


class FotsTransLogger(TransLogger):
    def write_log(self, environ, method, req_uri, start, status, bytes):
        """ We'll override the write_log function to remove the time offset so
        that the output aligns nicely with CherryPy's web server logging

        i.e.

        [08/Jan/2013:23:50:03] ENGINE Serving on 0.0.0.0:5000
        [08/Jan/2013:23:50:03] ENGINE Bus STARTED
        [08/Jan/2013:23:50:45 +1100] REQUES GET 200 / (192.168.172.1) 830

        becomes

        [08/Jan/2013:23:50:03] ENGINE Serving on 0.0.0.0:5000
        [08/Jan/2013:23:50:03] ENGINE Bus STARTED
        [08/Jan/2013:23:50:45] REQUES GET 200 / (192.168.172.1) 830
        """

        if bytes is None:
            bytes = '-'
        remote_addr = '-'
        if environ.get('HTTP_X_FORWARDED_FOR'):
            remote_addr = environ['HTTP_X_FORWARDED_FOR']
        elif environ.get('REMOTE_ADDR'):
            remote_addr = environ['REMOTE_ADDR']
        d = {
            'REMOTE_ADDR': remote_addr,
            'REMOTE_USER': environ.get('REMOTE_USER') or '-',
            'REQUEST_METHOD': method,
            'REQUEST_URI': req_uri,
            'HTTP_VERSION': environ.get('SERVER_PROTOCOL'),
            'time': time.strftime('%d/%b/%Y:%H:%M:%S', start),
            'status': status.split(None, 1)[0],
            'bytes': bytes,
            'HTTP_REFERER': environ.get('HTTP_REFERER', '-'),
            'HTTP_USER_AGENT': environ.get('HTTP_USER_AGENT', '-'),
        }
        message = self.format % d
        self.logger.log(self.logging_level, message)


def run_server():
    # Enable custom Paste access logging
    log_format = (
        '[%(time)s] REQUES %(REQUEST_METHOD)s %(status)s %(REQUEST_URI)s '
        '(%(REMOTE_ADDR)s) %(bytes)s'
    )
    app_logged = FotsTransLogger(app, format=log_format)

    # Mount the WSGI callable object (app) on the root directory
    cherrypy.tree.graft(app_logged, '/')

    # Set the configuration of the web server
    cherrypy.config.update({
        'engine.autoreload_on': True,
        'log.screen': True,
        'server.socket_port': 5000,
        'server.socket_host': '0.0.0.0'
    })

    # Start the CherryPy WSGI web server
    cherrypy.engine.start()
    cherrypy.engine.block()

if __name__ == "__main__":
    run_server()

Gevent

http://www.gevent.org/ appears to be one of the fastest WSGI web servers out there and provides all the features we are after too!

Installing Gevent is a bit more of a pain due to the libevent dependency:

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sudo apt-get install build-essential python-dev libevent-dev
pip install gevent

And here’s the Flask script:

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#!/usr/bin/env python

from flask import Flask
import gevent.wsgi
import gevent.monkey
import werkzeug.serving

gevent.monkey.patch_all()
app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"


@werkzeug.serving.run_with_reloader
def run_server():
    ws = gevent.wsgi.WSGIServer(listener=('0.0.0.0', 5000),
                                application=app)
    ws.serve_forever()

if __name__ == "__main__":
    run_server()

Gunicorn

Gunicorn is a production-ready web server for Python. I must commend the designer of the site who shows that even Python-related sites can look beautiful! Unfortunately, Gunicorn (being a server aimed at production use) makes it a lot harder to get auto-restart capabilities as this functionality is not natively included.

Installing Gunicorn is painless:

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pip install gunicorn

To make it all happen with Gunicorn, we’re going to need supervisor and watchdog to monitor for changes and trigger a restart of Gunicorn.

These tools rely on several C libraries, so there’s a bit more to it than just using pip:

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sudo apt-get install build-essential python-dev libyaml-dev
pip install supervisor watchdog

The supervisor configuration is as follows:

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[supervisord]
logfile=test.log
loglevel=debug
nodaemon=true

[program:test]
autostart=true
command=gunicorn --pid /tmp/flask-project.pid --workers 4 --log-level debug -b 0.0.0.0:5000 test:app

[program:test-reloader]
autostart=true
autorestart=false
command=watchmedo shell-command --patterns="*.py;*.html;*.css;*.js" --recursive --command='kill -HUP $(cat /tmp/flask-project.pid)'

The Python script stays super clean and simple which is nice:

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#!/usr/bin/env python

from flask import Flask

app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"

We can now launch the web server using supervisor as follows:

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supervisord -c test.conf

Overall though, I don’t see myself using Gunicorn for development purposes due to the added complexity involved. Another point worth noting is that print statements to stdout do not appear on the console with Gunicorn, unlike the rest of the web servers tested here.

Rocket

Rocket is a newer pure Python WSGI web server which is also production ready. I thought it would be worth giving it a try too.

Installing Rocket is very simple

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pip install rocket

And here’s the Flask script:

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#!/usr/bin/env python

import logging
import sys

from flask import Flask
from rocket import Rocket

app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"


def run_server():
    # Setup logging
    log = logging.getLogger('Rocket')
    log.setLevel(logging.INFO)
    log.addHandler(logging.StreamHandler(sys.stdout))

    # Set the configuration of the web server
    server = Rocket(interfaces=('0.0.0.0', 5000), method='wsgi',
                    app_info={"wsgi_app": app})

    # Start the Rocket web server
    server.start()

if __name__ == "__main__":
    run_server()

Unfortunately, Rocket (much like Gunicorn) is primarily aimed at production deployments, so it doesn’t include an auto-restart feature.

To restart it, you may send a SIGUSR1 to the pid:

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kill -SIGUSR1 <pid>

You may shutdown the process using SIGTERM:

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kill -SIGTERM <pid>

Tornado

Tornado appears to be well respected too and has no C dependencies.

Note: The latest Tornado 3.x has a significantly changed API and therefore the code below will not work with it. I may look into rewriting the code below to work with Tornado 3.x when I have a spare moment.

Installing Tornado is as easy as CherryPy:

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pip install tornado==2.4

And here’s the Flask script:

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#!/usr/bin/env python

from flask import Flask
import tornado.wsgi
import tornado.httpserver
import tornado.ioloop
import tornado.options
import tornado.autoreload

app = Flask(__name__)
app.debug = True


@app.route("/")
def hello():
    return "Hello World!"


def run_server():
    # Create the HTTP server
    http_server = tornado.httpserver.HTTPServer(
        tornado.wsgi.WSGIContainer(app)
    )
    http_server.listen(5000)

    # Reads args given at command line (this also enables logging to stderr)
    tornado.options.parse_command_line()

    # Start the I/O loop with autoreload
    io_loop = tornado.ioloop.IOLoop.instance()
    tornado.autoreload.start(io_loop)
    try:
        io_loop.start()
    except KeyboardInterrupt:
        pass

if __name__ == "__main__":
    run_server()

Further WSGI Debugging with Werkzeug

Flask’s Werkzeug has an awesome debugging module which you will lose access to when not using the default web server. But don’t fear, we can add it back in!

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...
from werkzeug.debug import DebuggedApplication
...

def run_server():
    # Enable the Werkzeug Debugger
    app_debug = DebuggedApplication(app, evalex=True)
    ...

Now simply ensure that you pass app_debug into sebsequent functions instead of app as we did above.

Further WSGI Logging with wsgilog

An additional module you can plug in into your application is wsgilog which provides further logging options for capturing output of WSGI applications.

Installation goes something like this:

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pip install wsgilog

You may use it as follows:

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...
import wsgilog

app = Flask(__name__)
app.debug = True

app_logged_wsgi = wsgilog.WsgiLog(app, tohtml=True, tofile='wsgi.log',
                                  tostream=True, toprint=True)
...

Now when initialising the web server, pass in app_logged_wsgi instead of app.

Final Words

To summarise, the following web servers failed to meet one or more criteria above:

Gunicorn: Does not display stdout via the print statement. Gunicorn is also more work to setup for a development server compared to the rest.
Rocket: Doesn’t include auto-restart ability, but is less troublesome to work with in comparison to Gunicorn.

As far as Jython is concerned, I’m sorry to say that none of the web servers worked with it. I also tried them with 2.7b1 and still no dice.