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?
Maybe I missed the question, but why not:
class MyException(Exception):
pass
Edit: to override something (or pass extra args), do this:
class ValidationError(Exception):
def __init__(self, message, errors):
# Call the base class constructor with the parameters it needs
super(ValidationError, self).__init__(message)
# Now for your custom code...
self.errors = errors
That way you could pass dict of error messages to the second param, and get to it later with e.errors
Python 3 Update: In Python 3+, you can use this slightly more compact use of super()
:
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