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A micro web-framework using asyncio coroutines and chained middleware.

Project description

Growler is a web framework utilizing the new asynchronous library (asyncio) described in PEP 3156 and implemented in python 3.4. It takes a cue from nodejs’s express library, using a series of middleware to process HTTP requests. The custom chain of middleware provides an easy way to implement complex applications.


When available, growler will be installable via pip:

$ pip install growler


There are optionals to the install command that will ensure that additional functionality is working. For example if you want to use the (quite pythonic) jade html template engine, you can install with growler by adding it to the list of optionals:

$ pip install growler[jade]

When multiple optionals are available, they will be listed here.


The core of the framework is the growler.App class, which acts as both server and handler. The App object creates a request and a response object when a client connects and passes the pair to a series of middleware specified when setting up the server. Note: The middleware are processed in the same order they are specified. Headers are parsed and each middleware added to the app (using app.use()), then routes are matched and functions called. The middleware manipulate the request and response objects and either respond to the client or pass to the next middleware in the chain. This stream/filter model makes it very easy to modularize and extend web applications with any features, backed by the power of python.

Example Usage

import asyncio

from growler import App
from growler.middleware import (Logger, Static, Renderer)

loop = asyncio.get_event_loop()

# Construct our application with name GrowlerServer
app = App('GrowlerServer', loop=loop)

# Add some growler middleware to the application
app.use(Renderer("views/", "jade"))

# Add some routes to the application
def index(req, res):

def hello_world(req, res):
    res.send_text("Hello World!!")

# Create the server - this automatically adds it to the asyncio event loop
Server = app.create_server(host='', port=8000)

# Tell the event loop to run forever - this will listen to the server's
# socket and wake up the growler application upon each connection

This code creates an application which is identified by ‘GrowlerServer’ (this name does nothing at this point) and has some listening options, host and port. Requests are passed to some middleware provided by the Grower package: Logger and Renderer. Logger simply prints the ip address of the connecting client, and Renderer adds the render function to the response object (used in index(req, res)).

Decorators are used to add endpoints to the application, so requests matching ‘/’ will call index(req, res) and requests matching ‘/hello’ will call hello_world(req, res).

Calling starts the asyncio event loop and calls asyncio.run_forever. This does not HAVE to be called; you can create any task which calls the coroutine app._server_listen() and pass to the event loop if you prefer.


Growler implements its own virtual namespaces to which you can add your own packages under the ‘growler’ package name. The implementation has changed such that it no-longer uses growler as a python virtual namespace (which lead to verbose import statements) but an explicit virtual package: growler_ext.

The best practice for developers to add their middleware to growler is now to put their code in the growler_ext/my_extesion. This will allow your code to be imported by web developers by import growler.ext.my_extesion. This will search through the growler_ext namespace and find your package.

There is currently one ‘official’ extension, indexer which hosts an automatically generated index of a filesystem directory. Look to it as an example of how to write extensions.


Currently Growler is single threaded, and not tested very well. Any submissions or comments would be appreciated.

The name Growler comes from the beer bottle due to the apparent convention of giving python micro-web-frameworks fluid container names.


Growler is licensed under Apache 2.0.

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