Error Handling

Why Handle Errors?

Every program encounters errors. Without error handling, your program crashes and users see scary tracebacks. With error handling, your program responds gracefully — showing a helpful message, trying again, or logging the issue.

Common Python Exceptions

Python has many built-in exception types. Here are the most common:

Exception	When It Happens
ValueError	Wrong value (e.g., int("hello"))
TypeError	Wrong type (e.g., "hello" + 5)
ZeroDivisionError	Division by zero
IndexError	List index out of range
KeyError	Dictionary key not found
FileNotFoundError	File doesn't exist
AttributeError	Object has no such attribute

Basic try/except

The try block contains code that might fail. The except block runs if an exception occurs:

Catching Multiple Exceptions

You can handle different exceptions differently:

The else and finally Clauses

• else runs if no exception occurred
• finally ALWAYS runs — whether an exception happened or not

Raising Your Own Exceptions

Use raise to signal that something went wrong in your own code:

Best Practices

1. Be specific — catch specific exceptions, not bare except:
2. Don't swallow errors silently — at minimum, log them
3. Use finally for cleanup — files, connections, locks
4. Raise meaningful errors — clear messages help debugging
5. Handle errors at the right level — close to where they can be resolved

Key Takeaways

• try/except prevents crashes and allows graceful error handling
• Catch specific exceptions to handle different problems differently
• else runs on success, finally always runs (perfect for cleanup)
• raise lets you create your own exceptions
• Error handling is what separates scripts from professional software