else and finally

Errors & Exceptions, lesson 4 of 6

🛟 Errors & Exceptions, lesson 4 of 6

else and finally

Run code only on success, and clean up no matter what.

8 min

1 exercise

1 quiz

0/2 solved

Getting Python ready… examples can run in a moment.

A try statement has two optional extra parts:

else runs only if the try block raised no exception. Put your "success" code here so the try block stays small.
finally runs always: after success, after a handled error, even after return. Use it for cleanup.

try:
    risky()
except ValueError:
    handle_it()
else:
    on_success()
finally:
    clean_up()

Example

Watch the order

Add attempt("") at the end. Which blocks run?

Quiz

What does this print?

['try', 'else', 'finally']

['try', 'except', 'finally']

['try', 'except', 'else', 'finally']

['except', 'finally']

finally even beats return. If a function returns from inside try, the finally block still runs before the function exits. If an error is not caught, finally runs first and then the error keeps travelling up.

Example

finally runs before the return completes

"Closing settings." appears before "dark". Now look up a missing key like "font": the cleanup still prints, then the KeyError crashes the program.

Example

Files, with, and else

Change the second file name to "todo.txt" (a file that doesn't exist) to trigger the except branch.

Exercise

A safe average with a call log

safe_average([]) crashes. Rewrite the function with all four parts:

try: compute sum(nums) / len(nums)
except ZeroDivisionError: return None
else: return the average rounded to 1 decimal
finally: append len(nums) to calls