Klein is a micro-framework for developing production ready web services with python. It is 'micro' in that it has an incredibly small API similar to bottle and flask. It is not 'micro' in that it depends on things outside the standard library. This is primarily because it is built on widely used and well tested components like werkzeug and Twisted.

A Klein bottle is an example of a non-orientable surface, and a glass Klein bottle looks like a twisted bottle or twisted flask. This, of course, made it too good of a pun to pass up.

Examples -------

Here are some basic klein handler functions that return some strings.

from klein import run, route

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

run("localhost", 8080)

Helpfully you can also return a twisted.web.resource.IResource such as static.File. If the URL passed to route ends in a / then the returned IResource will also be allowed to handle all children path segments.

from twisted.web.static import File
from klein import run, route

@route('/static/')
def static(request):
    return File("./static")

@route('/')
def home(request):
    return '<img src="/static/img.gif">'

run("localhost", 8080)

You can also make easy use of twisted.web.templates by returning anything that implements twisted.web.template.IRenderable such as twisted.web.template.Element in which case the template will be rendered and the result will be sent as the response body.

from twisted.web.template import Element, XMLString, renderer
from klein import run, route

class HelloElement(Element):
    loader = XMLString((
        '<h1 '
        'xmlns:t="http://twistedmatrix.com/ns/twisted.web.template/0.1"'
        '>Hello, <span t:render="name"></span>!</h1>'))

    def __init__(self, name):
        self._name = name

    @renderer
    def name(self, request, tag):
        return self._name


@route('/hello/<string:name>')
def home(request, name='world'):
    return HelloElement(name)

run("localhost", 8080)

And of course, this is twisted. So there is a wealth of APIs that return a twisted.internet.defer.Deferred. Deferreds may also be returned from handler functions and their result will be used as the response body.

Here is a simple Google proxy.

from twisted.web.client import getPage
from klein import run, route

@route('/')
def google(request):
    return getPage('https://www.google.com' + request.uri)


run("localhost", 8080)

Another very important integration point with Twisted is the twistd application runner. It provides rich logging support, daemonization, reactor selection, profiler integration, and many more incredibly useful features.

To provide access to these features (and others like HTTPS) klein provides the resource function which returns a valid twisted.web.resource.IResource for your application.

Here is our "Hello, World!" application again in a form that can be launched by twistd.

from klein import resource, route

@route('/')
def hello(request):
    return "Hello, world!"

To run the above application we can save it as helloworld.py and use the twistd web plugin.

twistd -n web --class=helloworld.resource