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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.


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

from klein import run, route

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

run("localhost", 8080)

Static files

Helpfully you can also return a twisted.web.resource.IResource such as static.File. If the branch=True is passed to route the returned IResource will also be allowed to handle all children path segments. So http://localhost:8080/static/img.gif should return an image and http://localhost:8080/static/ should return a directory listing.

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

@route('/static/', branch=True)
def static(request):
    return File("./static")

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 '
        '>Hello, <span t:render="name"></span>!</h1>'))

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

    def name(self, request, tag):
        return self._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. A Deferred 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

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

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

Handling POST

The route decorator supports a methods keyword which is the list of HTTP methods as strings. For example methods=['POST'] will cause the handler to be invoked when an POST request is received. If a handler can support multiple methods the current method can be distinguished with request.method.

Here is our "Hello, world!" example extended to support setting the name we are saying Hello to via a POST request with a name argument.

This also demonstrates the use of the redirect method of the request to redirect back to '/' after handling the POST.

The most specific handler should be defined first. So the POST handler must be defined before the handler with no methods.

from twisted.internet.defer import succeed
from klein import run, route


@route('/', methods=['POST'])
def setname(request):
    global name
    name = request.args.get('name', ['world'])[0]
    return succeed(None)

def hello(request):
    return "Hello, {0}!".format(name)

run("localhost", 8080)

The following curl command can be used to test this behaviour:

curl -v -L -d name='bob' http://localhost:8080/

Non-global state

For obvious reasons it may be desirable for your application to have some non-global state that is used by the your route handlers.

Below we have created a simple ItemStore class that has an instance of Klein as a class variable app. We can now use @app.route to decorate the methods of the class.

import json

from klein import Klein

class ItemStore(object):
    app = Klein()

    def __init__(self):
        self._items = {}

    def items(self, request):
        request.setHeader('Content-Type', 'application/json')
        return json.dumps(self._items)

    @app.route('/<string:name>', methods=['PUT'])
    def save_item(self, request, name):
        request.setHeader('Content-Type', 'application/json')
        body = json.loads(request.content.read())
        self._items[name] = body
        return json.dumps({'success': True})

    @app.route('/<string:name>', methods=['GET'])
    def get_item(self, request, name):
        request.setHeader('Content-Type', 'application/json')
        return json.dumps(self._items.get(name))

if __name__ == '__main__':
    store = ItemStore()
    store.app.run('localhost', 8080)