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 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 serve HTML templates

There is an example in the examples/html folder using Jinja2 to render and return HTML views.

To run it, go to the examples folder and run python -m html. Now reach with your browser.

To run associated tests: py.test html/

How to store custom data in the request

You can use Request as a dict like object for your own use, Roll itself never touches it.

request['user'] = get_current_user()

How to deal with cookies

Request cookies

If the request has any Cookie header, you can retrieve it with the request.cookies attribute, using the cookie name as key:

value = request.cookies['name']

Response cookies

You can add cookies to response using the response.cookies attribute:

response.cookies.set(name='name', value='value', path='/foo')

See the reference for all the available set kwargs.

See also the advanced guides.

How to consume query parameters

The query parameters (a.k.a. URL parameters) are made accessible via the request.query property.

The very basic usage is:

# URL looks like http://localhost/path?myparam=blah
myparam = request.query.get('myparam', 'default-value')
assert myparam == 'blah'
other = request.query.get('other', 'default-value')
assert other == 'default-value'

You can also request the full list of values:

# URL looks like http://localhost/path?myparam=bar&myparam=foo
myparam = request.query.list('myparam', 'default-value')
assert myparam == ['bar', 'foo']

If you don't pass a default value, Roll will assume that you are getting a required parameter, and so if this parameter is not present in the query, a 400 HttpError will be raised.

The Query class has three getters to cast the value for you: bool, int and float.

# URL looks like http://localhost/path?myparam=true
myparam = request.query.bool('myparam', False)
assert myparam is True

If the parameter value cannot be casted, a 400 HttpError will be raised.

See also [/how-to/] to see how to make your own Query getters.