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

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Growler is a web framework built atop asyncio, the asynchronous library described in PEP 3156 and added to the standard library in python 3.4. It takes a cue from the Connect & Express frameworks in the nodejs ecosystem, using a single application object and series of middleware to process HTTP requests. The custom chain of middleware provides an easy way to implement complex applications.


Growler is installable via pip:

$ pip install growler

The source can be downloaded/cloned from github at http://github.com/pyGrowler/Growler.


The pip utility allows packages to provide optional requirements, so features may be installed only upon request. This meshes well with the minimal nature of the Growler project: don't install anything the user doesn't need. That being said, there are (will be) community packages that are blessed by the growler developers (after ensuring they work as expected and are well tested with each version of growler) that will be available as extras directly from the growler package.

For example, if you want to use the popular mako html template engine, you can add support easily by adding it to the list of optionals:

$ pip install growler[mako]

This will automatically install the mako-growler packge, or growler-mako, or whatever it is named - you don't care, it's right there, and it works! Very easy!

The goal here is to provide a super simple method for adding middleware packages that the user can be sure works with that version of growler (i.e. has been tested), and has the blessing of the growler developer(s).

The coolest thing would be to describe your web stack via this command, so if you want mako, coffeescript, and some postgres ORM, your install command would look like growler[mako,coffee,pgorm]; anybody could look at that string and get the birds-eye view of your project.

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


The core of the framework is the growler.App class, which links the asyncio server to your project's middleware. Middeware can be any callable or coroutine. The App object creates a request and a response object when a client connects and passes the pair to this middleware chain. Note: The middleware are processed in the same order they are specified - this could cause unexpected behavior (errors) if a developer is not careful, so be careful! The middleware can manipulate the request and response, adding features or checking state. If any respond to the client, the middleware chain is finished. This stream/filter model makes it very easy to modularize and extend web applications with many features, backed by the power of python.

Example Usage

import asyncio

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

loop = asyncio.get_event_loop()

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

# Add some growler middleware to the application

# 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 a reference to the event loop. Requests are passed to some middleware provided by the Grower package: Logger, Static, and StringRenderer. Logger simply prints the ip address of the connecting client to stdout. Static will check a request url path against files in views/, if one of the files match, the file type is determined, proper content-type header is set, and the file content is sent. Renderer adds the 'render' method to the response object, allowing any following function to call res.render('/filename'), where filename exists in the "views" directory.

Decorators are used to add endpoints to the application, so requests with path matching '/' will call index(req, res) and requests matching '/hello' will call hello_world(req, res). Because 'app.get' is used, only HTTP GET requests will match these endpoints. Other HTTP 'verbs' (post, put, delete, etc) are available as well as 'all', which matches any method. Verb methods must match a path in full.

The 'use' method takes an optional path parameter (e.g. app.use(Static("public"), '/static')), which calls the middleware anytime the request path begins with the parameter.

The asyncio package provides a Server class which does the low-level socket handling for the developer, this is how your application should be hosted. Calling app.create_server(...) creates an asyncio Server object with the event loop given in app's constructor, and the app as the target for incomming connections; this is the recommended way to setup a server. You can't do much with the server directly, so after creation the event loop must be given control of the thread The easiest way to do this is to use loop.run_forever() after app.create_server(...). Or do it in one line with app.create_server_and_run_forever(...).


Growler introduces the virtual namespace growler_ext to which other projects may add their own growler-specific code. Of course, these packages may be imported in the standard way, but Growler provides an autoloading feature via the growler.ext module (note the '.' in place of '_') which will automatically import any packages found in the growler_ext namespace. This not only provides a standard interface for extensions, but allows for different implementations of an interface to be chosen by the environment, rather than hard-coded in. It also can reduce the number of import statements at the beginning of the file. This specialized importer may be imported as a standalone module:

from growler import App, ext

app = App()

or a module to import 'from':

from growler import App
from growler.ext import MyGrowlerExtension

app = App()

This works by replacing the 'real' ext module with an object that will import submodules in the growler_ext namespace automatically. Perhaps unfortunately, because of this there is no way I know of to allow the import growler.ext.my_extension syntax, as this skips the importer object and raises an import error. Users must use the from growler.ext import ... syntax instead.

The best practice for developers to add their middleware to growler is now to put their code in the python module growler_ext/my_extension. This will allow your code to be imported by others via from growler.ext import my_extension or the combination of from growler import ext and ext.my_extension.

An example of an extension is the indexer packge, which hosts an automatically generated index of a filesystem directory. It should implement the best practices of how to write extensions.


As it stands, Growler is single threaded, partially implemented, and not fully tested. Any submissions, comments, and issues are greatly appreciated, but I request that you please follow the Growler contributing guide.

The name Growler comes from the beer bottle keeping in line with the theme of giving python micro-web-frameworks fluid container names.


Growler is licensed under Apache 2.0.

  • Crash on request.get_body()

    Crash on request.get_body()

    Greetings. I`m trying to use some code from your examples:

    import asyncio
    from growler import App
    from growler.middleware import (Logger, Static, StringRenderer)
    loop = asyncio.get_event_loop()
    app = App('GrowlerServer', loop=loop)
    def post_test(req, res):
    Server = app.create_server(host='', port=4000)

    but when I call this handler: curl -H "Content-Type: application/json" -X POST -d '{"somekey": "somevalue"}' I`m getting this errors in console:

    ERROR:growler.router:140393215106800:Event loop is running. ERROR:growler.middleware_chain:140393229054640:Event loop is running. And then app exits with code 0.

    Python - 3.5.2 asyncio (3.4.3) growler (0.7.5)

    Do you have some advice, to aboid this problem. Cause it seems, like your framework is very interesting and perfect for my purposes. But this problem ruins all idea ;)

    Thank you.

    opened by yalosev 4
  • Add SSL Support

    Add SSL Support

    Growler would benefit from supporting SSL and HTTPS connections.

    opened by kyeatman74 4
  • Implement ETag

    Implement ETag

    ETags are a core feature of web servers that users should expect to work out of the box. This should be a non-middleware feature baked into either Application or HTTPRequest classes, with the option to disable or swap implementations if the user so chooses.

    enhancement help wanted 
    opened by akubera 3
  • Consider an integration with uvloop

    Consider an integration with uvloop

    opened by nikolay 2
  • routes using @routerclass decorator

    routes using @routerclass decorator

    Very awesome project and i appreciate your great work. I am liking the project but the docstring based routing seems a bit unusual to me. Are you guys planning to keep it or change it to more simpler straight forward approach like other framework like falcon (another python framework to build REST API) does? For the newbie programmers, would that be confusing and harder to debug?

    opened by rawbinz 2
  • Using Uvloop in Growler

    Using Uvloop in Growler

    I recently came across Growler, it looks pretty good framework to move away from Flask that I am currently using.

    But I have seen some improvements on the loop with https://github.com/MagicStack/uvloop - the benchmark seems to show it is better event loop as compared to Asyncio Event loop. so can I replace the Event loop from the current example to use Uvloop - just wanted to know if I am missing anything.

    Asyncio Loop

    loop = asyncio.get_event_loop()
    app = App('GrowlerServer', loop=loop)


     loop = uvloop.new_event_loop()
     app = App('GrowlerServer', loop=loop)

    I understand that #4 its been brought forward - but should it not be as straightforward as given above.

    opened by machbio 2
  • Are views a coroutines?

    Are views a coroutines?

    Let's take a look at the documentation example:

    from growler import App
    app = App()
    def log_file(req, res):
        print("<{ip}> {req_path}".format(ip=req.ip, req_path=req.path))
    def index_file(req, res):
    app.create_server_and_run_forever(host='', port='8080')

    Is index_file a coroutine from which I could do a yield from?

    opened by JerzySpendel 2
  • Is this project production ready?

    Is this project production ready?


    Is this project production ready? Maybe some projects already runs with Growler?

    Nice to listen use cases 👍

    opened by koddr 2
  • Handle binary data uploads

    Handle binary data uploads

    I'm running into a problem with uploading binary data (longer than 32K) to a pygrowler-based service. It looks like the responder is not handling the 'EXPECT': '100-continue' header. Can this be added in? It can probably be handled similar to the aiohttp module.

    enhancement priority-high 
    opened by kyeatman74 2
  • Example in README.md doesn't work?

    Example in README.md doesn't work?

    Thank you for creating growler, I think it looks great!

    I was trying the examples and they work fine, but I copied and pasted the example on the main README and it doesn't work with the pip version of growler (0.7.2).

    Here's the error

    File "hello.py", line 14, in <module>
        app.use(Renderer("views/", "jade"))
    TypeError: __init__() takes 2 positional arguments but 3 were given
    opened by jfmatth 1
  • Suggestion: remove growler.ext mechanism

    Suggestion: remove growler.ext mechanism

    I saw this feature in your README, and was reminded of our experience with Django and doing magical imports etc. - back in the day, your own models would end up being imported from a Django namespace, rather than your own Python modules. This was unnecessary magic that caused complications of various kinds (similar to the way that you can't do normal imports such as from growler.ext import foo). The fact that you have to mention "it doesn't work quite the way you expect" tells you that you will have trouble with it.

    The end result was that we removed this feature, along with a whole lot of unnecessary magic that required people to learn “the Django way” instead of just using "the Python way" which was perfectly adequate.

    In this case, it seems like this mechanism just adds complications - like:

    • "More than one way to do it" (you can import the middleware from the original module names or from growler.ext)
    • Requiring people to put things in certain namespaces, instead of however they want to package their code.
    • Making life hard for IDEs and other tools that want to do static analysis to provide help.

    There doesn't seem to be a reason why it is needed. If you really need a registry of plugins, that could be explicit. You could almost certainly use setuptools entrypoints for this - http://stackoverflow.com/questions/774824/explain-python-entry-points#9615473 https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points

    opened by spookylukey 2
  • Refactor pytest fixtures into a pytest-growler plugin

    Refactor pytest fixtures into a pytest-growler plugin

    I'm copying tests from growler-mako into my extension, so it's probably a good idea to eventually consolidate common fixtures into a pytest plugin at some future point.

    help wanted 
    opened by lmergner 3
  • Clarify how to use the ext namespace and RendererEngine

    Clarify how to use the ext namespace and RendererEngine

    I'm trying to build a growler-jinja extension by copying growler-jade and growler.renderer.RendererEngine. However, I'm getting an import error running from the REPL (after a pip install -e .):

    >>> from growler.ext import JinjaRenderer
    ImportError: No module named 'growler_ext.JinjaRenderer

    I don't fully understand setuptools namespaces, so this is partly my own ignorance.

    I also see that the api for subclassing RenderEngine has changed subtly. Does the growler-jade source reflect the current api?

    After using node's koa framework, I'm excited to try to port my Flask app to Growler.

    ubuntu 16 python 3.5.2 virtualenv 15.0.2 setuptools 26.0.0 growler 0.7.5

    opened by lmergner 4
  • Replace root logger.

    Replace root logger.

    This project is interesting to me.

    I think python's library should not use root logger like:

    import logging
     logging.warn('this is a warning')

    refs: http://pieces.openpolitics.com/2012/04/python-logging-best-practices/

    Would you replace root logger like:

    from logging import getLogger
    logger = getLogger(__name__)
    logger.warn('this is a warning')
    opened by c-bata 1
  • please suggest an ORM/db layer

    please suggest an ORM/db layer

    coming from HN (https://news.ycombinator.com/item?id=11634152 and https://news.ycombinator.com/item?id=11632650)

    Could you please include something like aiopg in your setup instructions - not as a requirement, but as a recommendation. Too many of us are using something like Flask with postgres and we would love to try Growler out.

    But the world of Python ORMs is problematic when it comes to asyncio compatible libraries. This includes mysql, postgresql and redis - three of the biggest ones which pretty much everyone uses.

    As someone else put it on HN - https://news.ycombinator.com/item?id=11629444

    One problem with this is that the entire ecosystem has to get on board with async. Maybe a new framework would make it compelling enough, who knows. This was/is the big issue with Tornado, IMO (and Tornado has been around for ages in framework time). Tornado is only async if the entire call stack all the way down to the http socket is async, using callbacks instead of returning values. This means that any 3rd party client library you use has to be completely written asynchronously, and none are in python. So you end up with a lot tedious work re-implementing http client libraries for Twilio or Stripe or whatever you're using. I'm curious to see where asyncio goes in python, but I'm a bit skeptical after seeing how much of a pain it was to use Tornado on a large web app. In the meantime I'll be using Gevent + Flask, which isn't perfect since it adds some magic & complexity but has the huge upside of letting you keep using all the libraries you're used to.

    opened by sandys 4
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