Exceptions and Error Handling
try/except catches errors so programs don't crash.
About this topic
An exception is an error that occurs while a program runs, such as dividing by zero, opening a file that does not exist, or converting the word hello to a number. Without handling, an exception stops the program with an error message. Error handling lets the program detect the problem and respond gracefully instead of crashing.
Most languages use try and except (or catch) blocks. The code that might fail goes in the try block, and if an exception happens, control jumps to the matching except block, which can show a friendly message, use a default value or ask the user again. A finally block runs no matter what, which is useful for closing files. You can also raise your own exceptions to signal problems.
For example, a program reading a number might catch a value error and ask for input again. A common mistake is catching every error silently, which hides real bugs. Another is using exceptions for ordinary decisions where an if statement is clearer. Handle specific errors, give helpful messages, and never reveal sensitive details to users. Reliable software expects things to go wrong.
Key ideas
- try, catch, finally
- Raise exceptions
- Handle bad input gracefully
Quick check
Use cases: where Exceptions and Error Handling helps
- Homework: review the key idea behind Exceptions and Error Handling before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Exceptions and Error Handling without notes.
- Visual learning: play with the interactive lab to see how Exceptions and Error Handling changes when you adjust its inputs.
- Teaching a friend: explain Exceptions and Error Handling in your own words, then compare your version with the key ideas.
Computing ideas run phones, search engines, games and the wider internet.
Frequently asked questions
What is Exceptions and Error Handling?
try/except catches errors so programs don't crash.
What will I learn in Exceptions and Error Handling?
try, catch, finally. Raise exceptions. Handle bad input gracefully.
Is Exceptions and Error Handling part of Computer Science?
Yes. It is one of 80 Computer Science topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Exceptions and Error Handling lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Exceptions and Error Handling?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.