Default message in custom exception - Python

The solution is given by the bellow code:

class CustomException(Exception):
    def __init__(self, *args, **kwargs):
        default_message = 'This is a default message!'

        # if any arguments are passed...
        # If you inherit from the exception that takes message as a keyword
        # maybe you will need to check kwargs here
        if args:
            # ... pass them to the super constructor
            super().__init__(*args, **kwargs)
        else: # else, the exception was raised without arguments ...
                 # ... pass the default message to the super constructor
                 super().__init__(default_message, **kwargs)

An equivalent but more succinct solution is:

class CustomException(Exception):
    def __init__(self, *args, **kwargs):
        default_message = 'This is a default message!'

        # if no arguments are passed set the first positional argument
        # to be the default message. To do that, we have to replace the
        # 'args' tuple with another one, that will only contain the message.
        # (we cannot do an assignment since tuples are immutable)
        # If you inherit from the exception that takes message as a keyword
        # maybe you will need to check kwargs here
        if not args: args = (default_message,)

        # Call super constructor
        super().__init__(*args, **kwargs)

An even more succinct but restricted solution, in a way that you can only raise the CustomException with no arguments is:

class CustomException(Exception):
     def __init__(self):
         default_message = 'This is a default message!'
         super().__init__(default_message)

You can of course save one line, in each of the above solutions, if you just pass the string literal to the constructor rather than using the default_message variable.

If you want the code to be Python 2.7 compatible, then you just replace the: super() with super(CustomException, self).

Now running:

>>> raise CustomException

will output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: This is a default message!

and running:

raise CustomException('This is a custom message!')

will output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.CustomException: This is a custom message!

This is the output that the first 2 solutions' code will produce. The last solution, differs in that calling it with at least one argument, like:

raise CustomException('This is a custom message!')

it will output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() takes 1 positional argument but 2 were given

because it does not permit any arguments to be passed to the CustomException when it is raised.

This is the simplest solution IMHO how to define custom exception with a default message that can be overridden if needed:

class CustomException(Exception):
    def __init__(self, msg='My default message', *args, **kwargs):
        super().__init__(msg, *args, **kwargs)

Usage example:

In [10]: raise CustomException
---------------------------------------------------------------------------
CustomException                           Traceback (most recent call last)
<ipython-input-10-259ae5202c8e> in <module>
----> 1 raise CustomException

CustomException: My default message

In [11]: raise CustomException()
---------------------------------------------------------------------------
CustomException                           Traceback (most recent call last)
<ipython-input-11-c1921a8781a6> in <module>
----> 1 raise CustomException()

CustomException: My default message

In [12]: raise CustomException('Foo bar')
---------------------------------------------------------------------------
CustomException                           Traceback (most recent call last)
<ipython-input-12-7efbf94f7432> in <module>
----> 1 raise CustomException('Foo bar')

CustomException: Foo bar