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

With modern Python Exceptions, you 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("My hovercraft is full of eels")

That will give a traceback ending with MyException: My hovercraft is full of eels.

If you want more flexibility from the exception, you could pass a dictionary as the argument:

raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})

However, to get at those details in an except block is a bit more complicated. The details are stored in the args attribute, which is a list. You would need to do something like this:

try:
    raise MyException({"message":"My hovercraft is full of animals", "animal":"eels"})
except MyException as e:
    details = e.args[0]
    print(details["animal"])

It is still possible to pass in multiple items to the exception and access them via tuple indexes, but this is highly discouraged (and was even intended for deprecation a while back). If you do need more than a single piece of information and the above method is not sufficient for you, then you should subclass Exception as described in the tutorial.

class MyError(Exception):
    def __init__(self, message, animal):
        self.message = message
        self.animal = animal
    def __str__(self):
        return self.message

“Proper way to declare custom exceptions in modern Python?”

This is fine, unless your exception is really a type of a more specific exception:

class MyException(Exception):
    pass

Or better (maybe perfect), instead of pass give a docstring:

class MyException(Exception):
    """Raise for my specific kind of exception"""

Subclassing Exception Subclasses

From the docs

Exception

All built-in, non-system-exiting exceptions are derived from this class. All user-defined exceptions should also be derived from this class.

That means that if your exception is a type of a more specific exception, subclass that exception instead of the generic Exception (and the result will be that you still derive from Exception as the docs recommend). Also, you can at least provide a docstring (and not be forced to use the pass keyword):

class MyAppValueError(ValueError):
    '''Raise when my specific value is wrong'''

Set attributes you create yourself with a custom __init__. Avoid passing a dict as a positional argument, future users of your code will thank you. If you use the deprecated message attribute, assigning it yourself will avoid a DeprecationWarning:

class MyAppValueError(ValueError):
    '''Raise when a specific subset of values in context of app is wrong'''
    def __init__(self, message, foo, *args):
        self.message = message # without this you may get DeprecationWarning
        # Special attribute you desire with your Error, 
        # perhaps the value that caused the error?:
        self.foo = foo         
        # allow users initialize misc. arguments as any other builtin Error
        super(MyAppValueError, self).__init__(message, foo, *args) 

There’s really no need to write your own __str__ or __repr__. The builtin ones are very nice, and your cooperative inheritance ensures that you use it.

Critique of the top answer

Maybe I missed the question, but why not:

class MyException(Exception):
    pass

Again, the problem with the above is that in order to catch it, you’ll either have to name it specifically (importing it if created elsewhere) or catch Exception, (but you’re probably not prepared to handle all types of Exceptions, and you should only catch exceptions you are prepared to handle). Similar criticism to the below, but additionally that’s not the way to initialize via super, and you’ll get a DeprecationWarning if you access the message attribute:

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

It also requires exactly two arguments to be passed in (aside from the self.) No more, no less. That’s an interesting constraint that future users may not appreciate.

To be direct – it violates Liskov substitutability.

I’ll demonstrate both errors:

>>> ValidationError('foo', 'bar', 'baz').message

Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    ValidationError('foo', 'bar', 'baz').message
TypeError: __init__() takes exactly 3 arguments (4 given)

>>> ValidationError('foo', 'bar').message
__main__:1: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
'foo'

Compared to:

>>> MyAppValueError('foo', 'FOO', 'bar').message
'foo'

see how exceptions work by default if one vs more attributes are used (tracebacks omitted):

>>> raise Exception('bad thing happened')
Exception: bad thing happened

>>> raise Exception('bad thing happened', 'code is broken')
Exception: ('bad thing happened', 'code is broken')

so you might want to have a sort of “exception template“, working as an exception itself, in a compatible way:

>>> nastyerr = NastyError('bad thing happened')
>>> raise nastyerr
NastyError: bad thing happened

>>> raise nastyerr()
NastyError: bad thing happened

>>> raise nastyerr('code is broken')
NastyError: ('bad thing happened', 'code is broken')

this can be done easily with this subclass

class ExceptionTemplate(Exception):
    def __call__(self, *args):
        return self.__class__(*(self.args + args))
# ...
class NastyError(ExceptionTemplate): pass

and if you don’t like that default tuple-like representation, just add __str__ method to the ExceptionTemplate class, like:

    # ...
    def __str__(self):
        return ': '.join(self.args)

and you’ll have

>>> raise nastyerr('code is broken')
NastyError: bad thing happened: code is broken

As of Python 3.8 (2018, https://docs.python.org/dev/whatsnew/3.8.html), the recommended method is still:

class CustomExceptionName(Exception):
    """Exception raised when very uncommon things happen"""
    pass

Please don’t forget to document, why a custom exception is neccessary!

If you need to, this is the way to go for exceptions with more data:

class CustomExceptionName(Exception):
    """Still an exception raised when uncommon things happen"""
    def __init__(self, message, payload=None):
        self.message = message
        self.payload = payload # you could add more args
    def __str__(self):
        return str(self.message) # __str__() obviously expects a string to be returned, so make sure not to send any other data types

and fetch them like:

try:
    raise CustomExceptionName("Very bad mistake.", "Forgot upgrading from Python 1")
except CustomExceptionName as error:
    print(str(error)) # Very bad mistake
    print("Detail: {}".format(error.payload)) # Detail: Forgot upgrading from Python 1

payload=None is important to make it pickle-able. Before dumping it, you have to call error.__reduce__(). Loading will work as expected.

You maybe should investigate in finding a solution using pythons return statement if you need much data to be transferred to some outer structure. This seems to be clearer/more pythonic to me. Advanced exceptions are heavily used in Java, which can sometimes be annoying, when using a framework and having to catch all possible errors.

You should override __repr__ or __unicode__ methods instead of using message, the args you provide when you construct the exception will be in the args attribute of the exception object.

No, “message” is not forbidden. It’s just deprecated. You application will work fine with using message. But you may want to get rid of the deprecation error, of course.

When you create custom Exception classes for your application, many of them do not subclass just from Exception, but from others, like ValueError or similar. Then you have to adapt to their usage of variables.

And if you have many exceptions in your application it’s usually a good idea to have a common custom base class for all of them, so that users of your modules can do

try:
    ...
except NelsonsExceptions:
    ...

And in that case you can do the __init__ and __str__ needed there, so you don’t have to repeat it for every exception. But simply calling the message variable something else than message does the trick.

In any case, you only need the __init__ or __str__ if you do something different from what Exception itself does. And because if the deprecation, you then need both, or you get an error. That’s not a whole lot of extra code you need per class. ;)

See a very good article “The definitive guide to Python exceptions“. The basic principles are:

• Always inherit from (at least) Exception.
• Always call BaseException.__init__ with only one argument.
• When building a library, define a base class inheriting from Exception.
• Provide details about the error.
• Inherit from builtin exceptions types when it makes sense.

There is also information on organizing (in modules) and wrapping exceptions, I recommend to read the guide.

Try this Example

class InvalidInputError(Exception):
    def __init__(self, msg):
        self.msg = msg
    def __str__(self):
        return repr(self.msg)

inp = int(input("Enter a number between 1 to 10:"))
try:
    if type(inp) != int or inp not in list(range(1,11)):
        raise InvalidInputError
except InvalidInputError:
    print("Invalid input entered")

To define your own exceptions correctly, there are a few best practices that you need to follow:

• Define a base class inheriting from Exception. This will allow to catch any exception related to the project (more specific exceptions should inherit from it):

  class MyProjectError(Exception):
      """A base class for MyProject exceptions."""
  

  Organizing these exception classes in a separate module (e.g. exceptions.py) is generally a good idea.

• To pass extra argument(s) to your exception, define a custom __init__() method with a variable number of arguments. Call the base class’s __init__() passing any positional arguments to it (remember that BaseException/Exception expect any number of positional arguments):

  class CustomError(MyProjectError):
      def __init__(self, *args, **kwargs):
          super(CustomError, self).__init__(*args)
          self.foo = kwargs.get('foo')
  

  To raise such exception with an extra argument you can use:

  raise CustomError('Something bad happened', foo='foo')
  

  This design actually adheres to the Liskov substitution principle, since you can replace an instance of a base exception class with an instance of a derived exception class.