Proper way to declare custom exceptions in modern Python? //For all the versions?

Question

What's the proper way to declare custom exception classes in modern Python? My primary goal is to follow whatever standard other exception classes have, so that (for instance) any extra string I include in the exception is printed out by whatever tool caught the exception.

By "modern Python" I mean something that will run in Python 2.5 but be 'correct' for the Python 2.6 and Python 3.* way of doing things. And by "custom" I mean an Exception object that can include extra data about the cause of the error: a string, maybe also some other arbitrary object relevant to the exception.

I was tripped up by the following deprecation warning in Python 2.6.2:

>>> class MyError(Exception): 

... 

def __init__(self, message): 

... 

self.message = message

... 

>>> MyError("foo") 

_sandbox.py:3: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6

It seems crazy that BaseException has a special meaning for attributes named message. I gather from PEP-352 that attribute did have a special meaning in 2.5 they're trying to deprecate away, so I guess that name (and that one alone) is now forbidden? Ugh.

I'm also fuzzily aware that Exception has some magic parameter args, but I've never known how to use it. Nor am I sure it's the right way to do things going forward; a lot of the discussion I found online suggested they were trying to do away with args in Python 3.

Update: two answers have suggested overriding __init__, and __str__/__unicode__/__repr__. That seems like a lot of typing, is it necessary?

Answer 1

• There are many ways to declare the custom exception in Python 2 version:-

  • One way you can use super() method to declare the custom exception :

class ValidationError(Exception):

def __init__(self, message, errors):

# Call the base class constructor with the parameters it needs 

        super().__init__(message) 

# Now for your custom code...

       self.errors = errors

• Exceptions in modern Python such as Python 3+, don't need to abuse .message, or override .__str__() or .__repr__() or any of it. If all you want is an informative message when your exception is raised, do this:

class MyException(Exception): 

     Pass

raise MyException("ABC")