blog.notmyidea.org/content/code/2011-12-07-introducing-cornice.md

Introducing Cornice

Wow, already my third working day at Mozilla. Since Monday, I've been working with Tarek Ziadé, on a pyramid REST-ish toolkit named Cornice.

Its goal is to take care for you of what you're usually missing so you can focus on what's important. Cornice provides you facilities for validation of any kind.

The goal is to simplify your work, but we don't want to reinvent the wheel, so it is easily pluggable with validations frameworks, such as Colander.

Handling errors and validation

Here is how it works:

service = Service(name="service", path="/service")


def is_awesome(request):
    if not 'awesome' in request.GET:
        request.errors.add('query', 'awesome',
                            'the awesome parameter is required')


@service.get(validator=is_awesome)
def get1(request):
    return {"test": "yay!"}

All the errors collected during the validation process, or after, are collected before returning the request. If any, a error 400 is fired up, with the list of problems encountered returned as a nice json list response (we plan to support multiple formats in the future)

As you might have seen, request.errors.add takes three parameters: location, name and description.

location is where the error is located in the request. It can either be "body", "query", "headers" or "path". name is the name of the variable causing problem, if any, and description contains a more detailed message.

Let's run this simple service and send some queries to it:

$ curl -v http://127.0.0.1:5000/service
> GET /service HTTP/1.1
> Host: 127.0.0.1:5000
> Accept: */*
>
* HTTP 1.0, assume close after body
< HTTP/1.0 400 Bad Request
< Content-Type: application/json; charset=UTF-8
[{"location": "query", "name": "awesome", "description": "You lack awesomeness!"}

I've removed the extra clutter from the curl's output, but you got the general idea.

The content returned is in JSON, and I know exactly what I have to do: add an "awesome" parameter in my query. Let's do it again:

$ curl http://127.0.0.1:5000/service?awesome=yeah
{"test": "yay!"}

Validators can also convert parts of the request and store the converted value in request.validated. It is a standard dict automatically attached to the requests.

For instance, in our validator, we can chose to validate the parameter passed and use it in the body of the webservice:

service = Service(name="service", path="/service")


def is_awesome(request):
    if not 'awesome' in request.GET:
        request.errors.add('query', 'awesome',
                            'the awesome parameter is required')
    else:
        request.validated['awesome'] = 'awesome ' + request.GET['awesome']


@service.get(validator=is_awesome)
def get1(request):
    return {"test": request.validated['awesome']}

The output would look like this:

curl http://127.0.0.1:5000/service?awesome=yeah
{"test": "awesome yeah"}

Dealing with "Accept" headers

The HTTP spec defines a Accept header the client can send so the response is encoded the right way. A resource, available at an URL, can be available in different formats. This is especially true for web services.

Cornice can help you dealing with this. The services you define can tell which Content-Type values they can deal with and this will be checked against the Accept headers sent by the client.

Let's refine a bit our previous example, by specifying which content-types are supported, using the accept parameter:

@service.get(validator=is_awesome, accept=("application/json", "text/json"))
def get1(request):
    return {"test": "yay!"}

Now, if you specifically ask for XML, Cornice will throw a 406 with the list of accepted Content-Type values:

$ curl -vH "Accept: application/xml" http://127.0.0.1:5000/service
> GET /service HTTP/1.1
> Host: 127.0.0.1:5000
> Accept: application/xml
>
< HTTP/1.0 406 Not Acceptable
< Content-Type: application/json; charset=UTF-8
< Content-Length: 33
<
["application/json", "text/json"]

Building your documentation automatically

writing documentation for web services can be painful, especially when your services evolve. Cornice provides a sphinx directive to automatically document your API in your docs.

rst
.. services::
   :package: coolapp
   :service: quote

Here is an example of what a generated page looks like: http://packages.python.org/cornice/exampledoc.html

Yay! How can I get it?

We just cut a 0.4 release, so it's available at http://pypi.python.org/pypi/cornice You can install it easily using pip, for instance:

$ pip install cornice

You can also have a look at the documentation at http://packages.python.org/cornice/

What's next?

We try to make our best to find how Cornice can help you build better web services. Cool features we want for the future include the automatic publication of a static definition of the services, so it can be used by clients to discover services in a nice way.

Of course, we are open to all your ideas and patches! If you feel haskish and want to see the sources, go grab them on github , commit and send us a pull request!