Introduction – Getting Started

Klein is a micro-framework for developing production-ready web services with Python, built off Werkzeug and Twisted. The purpose of this introduction is to show you how to install, use, and deploy Klein-based web applications.

This Introduction

This introduction is meant as a general introduction to Klein concepts.

Everything should be as self-contained, but not everything may be runnable (for example, code that shows only a specific function).


Klein is available on PyPI. Run this to install it:

pip install klein


Since Twisted is a Klein dependency, you need to have the requirements to install that as well. You will need the Python development headers and a working compiler - installing python-dev and build-essential on Debian, Mint, or Ubuntu should be all you need.

Hello World

The following example implements a web server that will respond with “Hello, world!” when accessing the root directory.

from klein import run, route

def home(request):
    return 'Hello, world!'

run("localhost", 8080)

This imports run and route from the Klein package, and uses them directly. It then starts a Twisted Web server on port 8080, listening on the loopback address.

This works fine for basic applications. However, by creating a Klein instance, then calling the run and route methods on it, you are able to make your routing not global.

from klein import Klein
app = Klein()

def home(request):
    return 'Hello, world!'"localhost", 8080)

By not using the global Klein instance, you can have different Klein routers, each having different routes, if your application requires that in the future.

Adding Routes

Add more decorated functions to add more routes to your Klein applications.

from klein import Klein
app = Klein()

def pg_root(request):
    return 'I am the root page!'

def pg_about(request):
    return 'I am a Klein application!'"localhost", 8080)

Variable Routes

You can also make variable routes. This gives your functions extra arguments which match up with the parts of the routes that you have specified. By using this, you can implement pages that change depending on this – for example, by displaying users on a site, or documents in a repository.

from klein import Klein
app = Klein()

def pg_user(request, username):
    return 'Hi %s!' % (username,)"localhost", 8080)

If you start the server and then visit http://localhost:8080/user/bob, you should get Hi bob! in return.

You can also define what types it should match. The three available types are string (default), int and float.

from klein import Klein
app = Klein()

def pg_string(request, arg):
    return 'String: %s!' % (arg,)

def pg_float(request, arg):
    return 'Float: %s!' % (arg,)

def pg_int(request, arg):
    return 'Int: %s!' % (arg,)"localhost", 8080)

If you run this example and visit http://localhost:8080/somestring, it will be routed by pg_string, http://localhost:8080/1.0 will be routed by pg_float and http://localhost:8080/1 will be routed by pg_int.

Route Order Matters

But remember: order matters! This becomes very important when you are using variable paths. You can have a general, variable path, and then have hard coded paths over the top of it, such as in the following example.

from klein import Klein
app = Klein()

def pg_user(request, username):
    return 'Hi %s!' % (username,)

def pg_user_bob(request):
    return 'Hello there bob!'"localhost", 8080)

The later applying route for bob will overwrite the variable routing in pg_user. Any other username will be routed to pg_user as normal.

Static Files

To serve static files from a directory, set the branch keyword argument on the route you’re serving them from to True, and return a t.w.static.File with the path you want to serve.

from twisted.web.static import File
from klein import Klein
app = Klein()

@app.route('/', branch=True)
def pg_index(request):
    return File('./')"localhost", 8080)

If you run this example and then visit http://localhost:8080/, you will get a directory listing.

Streamlined Apps With HTML and JSON

For a typical web application, the first order of business is generating some simple HTML pages that users can interact with and that search engines can easily index.

In such an app, you’ll want a consistent frame for all pages, something that puts appropriate things into the <head> tag, like a title, references to stylesheets and JavaScript functions, and so on. Then, each page has its own distinct content.

While just a little HTML might have been fine for the 90s, modern web apps quickly - sometimes immediately - outgrow HTML though; soon you’ll want some way to get just the data from your backend out via a JSON API, often from a dynamic JavaScript or Python front-end in the browser.

Klein provides for this general pattern with klein.Plating.

Let’s build a little app that gives us some fake (random) information about places you can go and foods you can get there. You can download the full example here in order to run it.

First, we’ll create a top-level Plating for the site. This takes a twisted.web.template template, defined with the objects from twisted.web.template.tags, with one special slot, named Plating.CONTENT, in the spot where you want the content of each page to appear. That’ll look something like this:

from twisted.web.template import tags, slot
from klein import Klein, Plating
app = Klein()

myStyle = Plating(
        tags.body(tags.h1(slot("pageTitle"), Class="titleHeading"),
    defaults={"pageTitle": "Places & Foods"}

Notice that we have defined a "pageTitle" slot in the template - individual pages must each provide a value for the title themselves in order to use the myStyle frame. Nothing’s special about "pageTitle", by the way; you may define whatever slots you want in your page template.

You can also specify a dictionary of default values to fill slots with.

Next, you want to create a route that is plated with that Plating, by using the Plating.routed decorator. @myStyle.routed takes a route from the Klein instance, in this case app, and then a template for the content portion (the Plating.CONTENT slot) of the page. The decorated function must then return a dictionary of the values to populate the slots in the template with.

Let’s start with a really simple page that just has a static template to fill the content slot.

        tags.h2("Sample Places:"),
        tags.ul([["/places/", place])(place))
                 for place in ["new york", "san francisco", "shanghai"]]),
        tags.h2("Sample Foods:"),
        tags.ul([["/foods/", food])(food))
                 for food in ["hamburgers", "cheeseburgers", "hot dogs"]]),
def root(request):
    return {}

This page generates some links to various sub-pages which we’ll get to in a moment. But first, if you load http://localhost:8080/, you’ll see that the template specified for root is inserted at the point in the template for myStyle specified the content should go.

Next, we should actually try injecting some data.

              tags.table(border="2", style="color: blue")(
                "rating"), " stars")),
def one_food(request, food):
    random = random_from_string(food)
    return {"name": food,
            "pageTitle": "Food: {}".format(food),
            "rating": random.randint(1, 5),

Here you can see the /foods/... route for showing information about a food. In the content template, we’ve got slots for "name", "rating", and "carbohydrates", the three primary properties which define a food. The decorated function then returns a dictionary that returns values for each of those slots, as well as a value for "pageTitle".

Each of these slots is only filled with a single item, though. What if you need to put multiple items into the template? The route for /places/... can show us:

    tags.div(style="color: green")(
        tags.h1("Place: ", slot("name")),
        tags.div(slot("latitude"), "° ", slot("longitude"), "°"),
        tags.div(tags.h2("Foods Found Here:"),
                     tags.a(href=["/foods/", slot("item")])(slot("item")))))))
def one_place(request, place):
    random = random_from_string(place)
    possible_foods = ["hamburgers", "cheeseburgers", "hot dogs", "pizza",
                      "叉烧", "皮蛋", "foie gras"]
    return {"name": place,
            "pageTitle": "Place: {}".format(place),
            "latitude": random.uniform(-90, 90),

Here you can see the special <slotname>:list renderer in use. By specifying the render= attribute of a tag (in this case, a li tag) to be foods:list, we invoke a twisted.web.template renderer that repeats the tag it is the renderer for, inserting each element of that list into the special "item" slot.

You can view each of these pages in a web browser now, and you can see their contents; we’ve built a little website that generates random values for these types of data. But we’ve also built a JSON API. If you access, for example, http://localhost:8080/places/chicago, you’ll see an HTML view, but if you add the query parameter json=1 (e.g. http://localhost:8080/places/chicago?json=1) you will see a JSON result like this:

    "foods": [
        "hot dogs"
    "latitude": -32.610538480748815,
    "longitude": -9.38433633489143,
    "name": "chicago",
    "pageTitle": "Place: chicago"

Any route decorated by @routed will similarly give you structured data if you ask for it via ?json=1, so you can build your JSON API and your HTML frontend at the same time.


Since it’s all just Twisted underneath, you can return Deferreds, which then fire with a result.

import treq
from klein import Klein
app = Klein()

@app.route('/', branch=True)
def google(request):
    d = treq.get('' + request.uri)
    return d"localhost", 8080)

This example here uses treq (think Requests, but using Twisted) to implement a Google proxy.

Return Anything

Klein tries to do the right thing with what you return. You can return a result (which can be regular text, a Resource, or a Renderable) synchronously (via return) or asynchronously (via Deferred). Just remember not to give Klein any unicode, you have to encode it into bytes first.


That covers most of the general Klein concepts. The next chapter is about deploying your Klein application using Twisted’s tap functionality.