Programming with Python - Exceptions

Chapter 10 - Exceptions

Why programs need exception-handling code

To prevent crashes when input, files, or operations fail.

What an exception is in Python

A runtime error that stops normal execution unless handled.

Purpose of handling exceptions

To catch errors, show messages, and keep the program running.

How a try/except block works

Try runs normally if error occurs, jump to except.

What happens when no exception occurs in a try block

Try runs fully and except is skipped.

Why try/except is better than manual checking

It handles unexpected issues cleanly without messy code.

Why programs use multiple except blocks

To handle different error types appropriately.

How Python chooses which except block to run

It runs the first matching except only.

Why bare except should be avoided

It hides bugs by catching every error.

What happens if an exception is not handled

The program stops and prints a traceback.

How to handle multiple exception types in one except

Use a tuple: except (ValueError, TypeError).

What the raise statement does

Stops normal flow and triggers an exception immediately.

Why raising exceptions keeps code cleaner

It separates normal logic from error-handling.

Why ValueError is used for invalid numeric input

It signals the value itself is unacceptable.

What printing an exception object shows

The custom message passed to raise.

How exceptions travel up the call stack

They exit the function and move to the caller’s except.

Why exception propagation improves structure

Keeps functions simple while callers handle errors.

Why returning special values is inferior

Sentinel values require extra checks and hide errors.

Purpose of the finally block

Code that must run no matter what happens.

Execution rules for finally

Runs after success, handled error, unhandled error, or early exit.

Why finally is essential for file handling

Ensures files close even if errors occur.

Why custom exceptions are useful

They represent specific, meaningful program errors.

How to define a custom exception type

Create a class inheriting from Exception.

Why custom exception names end with “Error”

Follows Python naming conventions.

How to raise a custom exception

Use raise CustomError("message").

How to handle a custom exception

Catch it with except CustomError as e: