Proper way to declare custom exceptions in modern Python?

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 instan

Answer 1

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

• Define a base class inheriting from Exception. This will allow to easily catch any exceptions related to the project:

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

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

• To create a specific exception, subclass the base exception class.

• To add support for extra argument(s) to a custom 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().__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 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. Also, it allows you to create an instance of a derived class with the same parameters as the parent.