Sep 21, 2009

Add a web api to your app with django-piston

More and more often I found myself in a situation where I would like to add a web api to my django applications. I have recently tried to used django-piston for this and I found the learning curve a bit steep. It is not particularly hard but it requires you to understand few things before being able to enjoy it.

This post should help you to understand how to create the handlers to read, create, update, delete an object and see how you can call this web api from the command line using curl. I have forked django-piston to extend the example "blogserver". I would recommend you to also read the README inside the example

You can grab the code like this :

hg clone

curl is a command line tool to transfer data from or to a server, using one of the supported protocols (HTTP, HTTPS, FTP, FTPS, SCP, SFTP, TFTP, DICT, TELNET, LDAP or FILE). The command is designed to work without user interaction.

Here it is the 4 things that I found a bit hard to understand.

1. @require_extended

This decorator is handy if you want to restrict the access to your handler to only the request that have have one of the header listed below :
• application/json
• application/x-yaml
• text/xml
• application/python-pickle

The direct effect on the curl command line is that you will need to add a -H 'Content-Type:', this will give you something like :

$ curl -u testuser:foobar -H 'Content-Type:application/json'

If the header is omitted django-piston will return a 'Bad Request'.

2. Request Method

The handler for your resource can be composed of the following method : read, create, update, delete that are respectively mapped to the following request method : GET, POST, PUT, DELETE. "-X " is used to specify the method you want to use. Here it is an example that execute the update method of the blogserver :

$ curl -u testuser:foobar -H 'Content-Type:application/json' -X PUT -d '{"content": "Update test", "title": "Update test"}'

3. Passing data to your handler

In this example I am going to demonstrate how to pass JSON string with curl. In order to do this you should use the -d followed by the JSON string :

$ curl -u testuser:foobar -H 'Content-Type:application/json' -X PUT -d '{"content": "Update test", "title": "Update test"}'

django-piston will automatically turn this string into a python dictionary that is ready to be used by your handler. You will find this data in '' the raw string is available in "request.raw_post_data"

4. How to pass parameter to your handler

It took me a while to understand that the same handler can be mounted to several urls this will allow you to pass additional parameters like the primary key ID or the slug :

blogposts = Resource(handler=BlogpostHandler, authentication=auth)
urlpatterns = patterns('',
url(r'^posts/$', blogposts),
url(r'^post/(?P.+)/$', blogposts),

In the example above "blockposts" is mapped to 2 different urls, the second one will be particularly handy to read, update, delete a particular post. Additional parameters from the URL will be passed to the method : read, create, update, delete. I encourage you to check out the code from bitbucket to see how you could take advantage of this technique.

This post barely scratch the surface of how to use django-piston, to put it in a nutshell this reusable application makes creating a web api for your own project simple. It avoids you to write a lot of boilerplate code because it abstracts all the machinery and let you focus on important things.

I would be glad to hear from you how you use it and what is your favorite trick.

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