How-to guides

A how-to guide:

  • is goal-oriented
  • shows how to solve a specific problem
  • is a series of steps

Analogy: a recipe in a cookery book

How to install Roll

Roll requires Python 3.5+ to be installed.

It is recommended to install it within a pipenv or virtualenv.

You can install Roll through pip:

pip install roll

How to create an extension

You can use extensions to achieve a lot of enhancements of the base framework.

Basically, an extension is a function listening to events, for instance:

def cors(app, value='*'):

    async def add_cors_headers(response, request):
        response.headers['Access-Control-Allow-Origin'] = value

Here the cors extension can be applied to the Roll app object. It listens to the response event and for each of those add a custom header. The name of the inner function is not relevant but explicit is always a bonus. The response object is modified in place.

Note: more extensions are available by default. Make sure to check these out!

How to deal with content negociation

The content_negociation extension is made for this purpose, you can use it that way:


@app.route('/test', accepts=['text/html', 'application/json'])
async def get(req, resp):
    if req.headers['Accept'] == 'text/html':
        resp.headers['Content-Type'] = 'text/html'
        resp.body = '<h1>accepted</h1>'
    elif req.headers['Accept'] == 'application/json':
        resp.json = {'status': 'accepted'}

Requests with Accept header not matching text/html or application/json will be honored with a 406 Not Acceptable response.

How to return an HTTP error

There are many reasons to return an HTTP error, with Roll you have to raise an HttpError instance. Remember our base example from tutorial? What if we want to return an error to the user:

from http import HTTPStatus

from roll import Roll, HttpError
from roll.extensions import simple_server

app = Roll()

async def hello(request, response, parameter):
    if parameter == 'foo':
        raise HttpError(HTTPStatus.BAD_REQUEST, 'Run, you foo(l)!')
    response.body = f'Hello {parameter}'

if __name__ == '__main__':

Now when we try to reach the view with the foo parameter:

$ http :3579/hello/foo
HTTP/1.1 400 Bad Request
Content-Length: 16

Run, you foo(l)!

One advantage of using the exception mechanism is that you can raise an HttpError from anywhere and let Roll handle it!

How to return JSON content

There is a shortcut to return JSON content from a view. Remember our base example from tutorial?

from roll import Roll
from roll.extensions import simple_server

app = Roll()

async def hello(request, response, parameter):
    response.json = {'hello': parameter}

if __name__ == '__main__':

Setting a dict to response.json will automagically dump it to regular JSON and set the appropriated content type:

$ http :3579/hello/world
HTTP/1.1 200 OK
Content-Length: 17
Content-Type: application/json; charset=utf-8

    "hello": "world"

Especially useful for APIs.

How to subclass Roll itself

Let’s say you want your own Query parser to deal with GET parameters that should be converted as objects.

What you can do is subclass both the Roll class and the Protocol one to set your custom Query class:

from datetime import date

from roll import Roll, Query
from roll.extensions import simple_server

class MyQuery(Query):

    def date(self):
        return date(int(self.get('year')),

class MyProtocol(Roll.Protocol):
    Query = MyQuery

class MyRoll(Roll):
    Protocol = MyProtocol

app = MyRoll()

async def hello(request, response):
    response.body =

if __name__ == '__main__':

And now when you pass appropriated parameters (for the sake of brievety, no error handling is performed but hopefully you get the point!):

$ http :3579/hello/ year==2017 month==9 day==20
HTTP/1.1 200 OK
Content-Length: 10


How to deploy Roll into production

The recommended way to deploy Roll is using Gunicorn.

First install gunicorn in your virtualenv:

pip install gunicorn

To run it, you need to pass it the pythonpath to your roll project application. For example, if you have created a module in your package mypackage, where you create your application with app = Roll(), then you need to issue this command line:

gunicorn mypackage.core:app --worker roll.worker.Worker

See gunicorn documentation for more details about the available arguments.

How to run Roll’s tests

Roll exposes a pytest fixture (client), and for this needs to be properly installed so pytest sees it. Once in the roll root (and with your virtualenv active), run:

python develop

Then you can run the tests:


How to send custom events

Roll has a very small API for listening and sending events. It's possible to use it in your project for your own events.

Events are useful when you want other users to extend your own code, whether it's a Roll extension, or a full project built with Roll. They differ from configuration in that they are more adapted for dynamic modularity.

For example, say we develop a DB pooling extension for Roll. We would use a simple configuration parameter to let users change the connection credentials (host, username, password…). But if we want users to run some code each time a new connection is created, we may use a custom event.

Our extension usage would look like this:

app = Roll()
db_pool_extension(app, dbname='mydb', username='foo', password='bar')

def listener(connection):
    # dosomething with the connection,
    # for example register some PostgreSQL custom types.

Then, in our extension, when creating a new connection, we'd do something like that: app.hook('new_connection', connection=connection)

How to use a livereload development server

First, install hupper.

Then turn your Roll service into an importable module. Basically, a folder with and files and put your simple_server call within the file (see the example folder for… an example!).

Once you did that, you can run the server using hupper -m example. Each and every time you modify a python file, the server will reload and take into account your modifications accordingly.

One of the pros of using hupper is that you can even set an ipdb call within your code and it will work seamlessly (as opposed to using solutions like entr).