Default message in custom exception - Python

Question

I want to create a custom exception in Python, that when raised without any arguments, it will print a default message.

Code Example:

class CustomException(Exception):     pass # some code  raise CustomException() 

and get the below output:

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

Answer 1

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 

Answer 2

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.