Exceptions

When things go wrong

• A runtime error normally crashes the program.
• An exception is Python's signal that something went wrong.
• Exception handling lets you catch it and keep running.

try / except

• Put risky code in a try: block.
• If it raises an exception, the except: block runs instead of crashing.
• If nothing goes wrong, the except block is skipped.

try:
    number = int("not a number")
except ValueError:
    number = 0
print(number)

Catch the right error

• Name the type of error you expect: except ValueError:.
• ValueError is bad text for a number; ZeroDivisionError is divide by zero.
• Catching a specific type means you do not hide other bugs.

try:
    print(10 / 0)
except ZeroDivisionError:
    print("Cannot divide by zero")

finally and raise

• A finally: block always runs, error or not — good for cleanup.
• raise lets you signal an error on purpose.
• raise ValueError("message") stops the code unless something catches it.

try:
    raise ValueError("bad value")
except ValueError as e:
    print("Caught:", e)
finally:
    print("Always runs")

Why it matters

• A robust program should not crash on bad input.
• Handling exceptions lets it warn the user and carry on.
• It also makes sure files and other resources are closed cleanly.

Now you try

• Use try / except (and raise in the last task).
• Press Check answer to test your code.