try / except / finally

Errors are raised, caught, handled. Don't swallow exceptions silently — at least log them.

script.py v3.1

def divide(a, b):
    try:
        result = a / b
    except ZeroDivisionError as e:
        print(f'  cannot divide by zero: {e}')
        result = None
    except TypeError as e:
        print(f'  wrong type: {e}')
        result = None
    else:
        # runs ONLY if no exception was raised
        print(f'  {a} / {b} = {result}')
    finally:
        # ALWAYS runs — even if exception was raised
        print('  (divide() called)')
    return result

print('Test 1: divide(10, 2)')
divide(10, 2)

print('\nTest 2: divide(10, 0)')
divide(10, 0)

print('\nTest 3: divide(10, "two")')
divide(10, 'two')

# Raising your own exceptions
def check_age(age):
    if age < 0:
        raise ValueError(f'age cannot be negative, got {age}')
    return age

try:
    check_age(-5)
except ValueError as e:
    print(f'\nCaught: {type(e).__name__}: {e}')

Python output click Run to execute

Click "Run" to execute the code on the left.
The output will appear here.

🧠 How it works

try: runs code that might fail. except: catches specific exception types. else: runs only if NO exception was raised (rare but useful). finally: ALWAYS runs, even on exception — use for cleanup (closing files, releasing locks). Catch SPECIFIC exceptions (except ValueError), not bare except: (catches everything including KeyboardInterrupt). raise ValueError('message') throws an exception. raise by itself re-raises the current one. Common exceptions: ValueError, TypeError, KeyError, IndexError, FileNotFoundError, AttributeError. You can create custom exceptions: class MyError(Exception): pass.

✏️ Try these changes

Change divide(10, 0) to divide(10, 5)
→ no ZeroDivisionError
Remove the 'as e' from except ZeroDivisionError:
→ still works but you can't print the message
Add except Exception: print('unknown error') at the end
→ catches any other exception
Change raise ValueError to raise RuntimeError
→ different exception type caught (or not)

← Previous lab

Classes — __init__, self, methods

Next lab →

Strings — f-strings, methods, slicing