Example error handling

pycnic.errors provides several exception classes.

  • HTTPError - The parent error.
  • HTTP_400/401/403/404/405 - For 4xx client errors.
  • HTTP_500 - For 5xx server errors.

The Pycnic framework catches these built-in exceptions and returns json describing the error (defined in the exception's response method). The response method returns a dictionary which is then encoded into JSON and sent back to the client.

By default, any HTTPError subclass sends the following response to the client when the exception is caught:

    "status": self.status,
    "status_code": self.status_code,
  • status is a string like "404 Not Found".
  • status_code is the integer representation, like 404.
  • error is the error message from the exception.
  • data is a custom attribute that defaults to null.

Using Exceptions

Exceptions are wonderful and easy to use. Let's say you have a Handler that requires a message from the user on POST. If that message isn't present, you could manually set the status and return your own message, or you could do something like below:

class MessageHandler(Handler):
    def post(self):
        if not self.request.data.get("message"):
            raise HTTP_400("Message is required")
        return { 

Custom Exceptions

Pycnic makes it easy to write and use your own exceptions.

Let's say your application checks for authorization frequently. Instead of doing raise HTTP_401("You can't do that, please login", data={"loginURI":"/login"}) each time a user isn't authorized, you can create your own exception class.

class AuthError(pycnic.errors.HTTPError):

    status_code = 401 # Required
    message = "You can't do that, please login"

    def __init__(self):

    def response(self):
        return { 
            "data": { "loginURI":"/login" },
            "status":"401 Not Authorized"

You can then raise that exception when appropriate.

class SomeHandler(Handler):
    def get(self):
        if not logged_in(self.request):
            raise AuthError()

Which, if the user is not logged in, will return a 401 response with a JSON body of:

    "data": {"loginURI": "/login"}, 
    "status": "401 Not Authorized", 
    "error": "You can't do that, please login", 
    "status_code": 401