REST exception handling with decorators

Raising REST exceptions

I can’t decide if this is the “right” way of doing this, but it seems relatively succinct, so I’ll attempt to describe how I handled exceptions for a REST api in django.

Say you need to return user info and the info given for user lookup fails.

import users
import exceptions
import rest_exceptions as rest_excs
import decorators.rest as rest

@rest.exceptions
def get_user_info(input_name):
try:
    user = users.get_user(input_name)
except exceptions.NoSuchUser
    raise rest_exc.NoSuchUser

• The exceptions import is for general error catching and it allows you to collect all your exception definitions for specific modules.
• decorators.rest holds… well, decorator functions specific to REST stuff. You don’t need a separate directory decorators but I happened to have more decorator types than were sensible to put in the main modules dir..
• rest_exceptions is similar to exceptions but these will need to define a status code to return and inherit from a exception class we define called RestException

Exception Decorator

So, catch the general exception, raise the REST exception… but then what? This is where the decorator comes in. It handles when a rest exception is raised and returns an appropriate response.

You’d define the base exception class, RestException as:

class RestException(Exception):
    code = 500
    message = {"reason" : "failed"}
    response = None

Define all the specific exceptions with this as the base class (ex. PermissionDenied(RestException)). This way you can specify the status code and response for each type of exception.

The JSONResponse object:

Basically just returning an HttpResponse encoded properly as json. You’d want to check kwargs for a status as well.

If multiple modules have a REST api this would help to generalize the exception handling. I’m sure there are other/better ways of doing this and I’d love to know what they are. Even if it is a crap way of doing it, maybe you can use the idea to implement something else.

Bottom line: Decorators are fun