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Building Robust Object-Oriented Python Applications and Libraries

Raising an Exception

Explore how to raise exceptions in Python by using the raise keyword to enforce input validation. Understand when to trigger built-in errors like TypeError and ValueError and how to apply these principles to maintain robust, error-resistant programs. This lesson helps you grasp how to inform users or calling functions about invalid inputs, improving program reliability.

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Overview

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:

Python 3.10.4
from typing import List
class EvenOnly(List[int]):
    def append(self, value: int) -> None:
        if not isinstance(value, int):
            raise TypeError("Only integers can be added")
        if value % 2 != 0:
            raise ValueError("Only even numbers can be added")
        super().append(value)

When to use the raise

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