Source: 📖 Python Cookbook ch14.8 p578

Creating custom exceptions

Custom exceptions can be easily created by defining a class that inherits from Exception, or any other standard exception as follows:

class MyCustomError(Exception):
	pass

This custom exception can now be raised in the usual way. The reason to define custom exceptions is that it can provide a more detailed account of what is going wrong in your program to help users debug. For example, you could define PassedAsStringError to inherit from the standard ValueError which tells the detail more clearly what has gone wrong.

You can also create inheritance hierarchies to help users handle custom errors with more malleability:

class MyCustomBaseError(Exception):
	pass

class MyError1(MyCustomBaseError):
	pass

class MyError2(MyCustomBaseError):
	pass

In the above example, MyError1 and MyError2 can both be caught with an except clause that catches MyCustomBaseError. Use this to intuitively group together related custom exceptions.