Python 3 Object-Oriented Programming - Third Edition

Raising an exception

We'll get to responding to such exceptions in a minute, but first, let's discover what we should do if we're writing a program that needs to inform the user or a calling function that the inputs are invalid. We can use the exact same mechanism that Python uses. Here's a simple class that adds items to a list only if they are even numbered integers:

class EvenOnly(list): 
    def append(self, integer): 
        if not isinstance(integer, int): 
            raise TypeError("Only integers can be added") 
        if integer % 2: 
            raise ValueError("Only even numbers can be added") 
        super().append(integer) 

This class extends the list built-in, as we discussed in Chapter 2, Objects in Python, and overrides the append method to check two conditions that ensure ...