The finally Block: try-except-finally in Python (Class 12 Computer Science)

Understand The finally Block

try:
    risky statements
except SomeError:
    handler
else:
    runs only if try raised nothing (optional)
finally:
    always runs

The finally Block: key facts

Order of clauses

1. 1.try
2. 2.except (one or more)
3. 3.else (optional)
4. 4.finally (optional, always last)

try...finally with no except is also valid.

What runs in each case

No exception	try, else, finally
Handled exception	try (up to the error), except, finally
Unhandled exception	try (up to the error), finally, then the exception is re-raised
	return, break or continue inside try/except: finally runs first

After an unhandled exception, statements after the try statement are skipped.

Typical uses of finally

• •closing a file: f.close()
• •closing a database connection: con.close()
• •a final message such as 'OVER AND OUT'

NCERT Program 1-6 prints a closing message from finally.

Return and finally

1. 1.return in try
2. 2.finally runs
3. 3.value reaches the caller

Prints inside finally appear before the caller prints the returned value.

How to answer The finally Block questions

1. 1Trace the try block until it ends or the first exception occurs.
2. 2If there was no exception, run else (if present) and then finally.
3. 3If there was an exception, look for a matching except. If found, run it and then finally, and continue after the statement.
4. 4If no except matches, run finally and then stop: the exception is re-raised, so only an outer handler (or a traceback) follows.
5. 5If return, break or continue happens inside try or except, run finally before carrying it out.
6. 6Inside a function called from print(), write finally's output first and the printed return value after it.
7. 7In programs, put clean-up (close, final message) in finally, not at the end of try, where it might be skipped.

The finally Block in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember The finally Block

Short memory aids. Say each one aloud once, then use it on a question the same day.

• finally = no matter what

  Success, handled error, unhandled error, return, break: finally runs in every case.

  Use it when: Any 'will this print?' question about finally.

• Last clause, last word

  finally is always written last and runs last in the try statement.

  Use it when: Order-of-clauses MCQs.

• Tidy up, don't cover up

  finally tidies up but never hides an unhandled exception; the error carries on after it.

  Use it when: Unhandled-exception outputs.

• Goodbye before the gift

  When a function returns from try, finally says goodbye before the caller opens the returned gift.

  Use it when: Order of output with return and finally.

The finally Block: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

try:
    x = 20 // 4
    print('x =', x)
except ZeroDivisionError:
    print('zero')
finally:
    print('finished')

try:
    print('start')
    y = 7 / 0
    print('y =', y)
except ZeroDivisionError:
    print('cannot divide')
else:
    print('no error')
finally:
    print('cleanup')
print('after')

try:
    n = int('4x')
    print(n)
except ZeroDivisionError:
    print('zero')
finally:
    print('done')
print('next line')

def check(x):
    try:
        return 10 / x
    except ZeroDivisionError:
        return 'inf'
    finally:
        print('checked', x)
print(check(2))
print(check(0))

count = 0
for v in [3, 0, 'a', 5]:
    try:
        r = 6 / v
    except ZeroDivisionError:
        continue
    except TypeError:
        break
    finally:
        count += 1
print(count)

Where students lose marks in The finally Block

The slips examiners see most often, and what to write instead.

• ✗Mistake: Leaving out finally's output when there is no exception

  finally runs on the success path too.

  Wrong working

  x = 5

  Right working

  x = 5
  finished

  ✓Do this: Always add finally's output at the end of the try statement.

  Usually costs: 1 mark.

• ✗Mistake: Printing the lines after an unhandled exception

  After finally runs, an unhandled exception is re-raised and the program stops.

  Wrong working

  done
  next line

  Right working

  done
  ValueError: ... (program stops)

  ✓Do this: Check whether an except clause really matches before continuing past finally.

  Usually costs: 1 mark.

• ✗Mistake: Putting finally before except

  finally must be the last clause; anything else is a SyntaxError.

  Wrong working

  try:
  ...
  finally:
  ...
  except ValueError:
  ...

  Right working

  try:
  ...
  except ValueError:
  ...
  finally:
  ...

  ✓Do this: Remember the order try, except, else, finally.

  Usually costs: Half a mark.

• ✗Mistake: Printing the returned value before finally's output

  The caller receives the value only after finally has run.

  Wrong working

  5.0
  checked 2

  Right working

  checked 2
  5.0

  ✓Do this: Run finally, then go back to the caller's print.

  Usually costs: 1 mark.

Practice questions on The finally Block

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   The finally block in Python is executed:
   (a) only when an exception occurs (b) only when no exception occurs (c) always (d) only when the exception is handled

   Hint

   No matter what.

   Answer

   (c) always

2. Q2 · 1 mark · Very short answerBoard level

   Where must the finally clause be placed in a try statement?

   Hint

   Last clause.

   Answer

   At the end, after all the except blocks and the else block (if any).

3. Q3 · 1 mark · Very short answerBoard level

   Write the output:

   try:
       print([1, 2, 3][1])
   except IndexError:
       print('I')
   finally:
       print('F')

   Hint

   Does index 1 exist?

   Answer

   2
   F

4. Q4 · 2 marks · Short answerHarder board level

   Write the output for each input value v = '8' and v = 'eight':

   try:
       n = int(v)
   except ValueError:
       print('not a number')
   else:
       print('square', n * n)
   finally:
       print('bye')

   Hint

   else runs only when try succeeds.

   Answer

   For '8': square 64 / bye. For 'eight': not a number / bye.

5. Q5 · 2 marks · Short answerHarder board level

   Write a function safe_avg(L) that returns the average of L, returns 0 if L is empty, and prints 'done' in a finally block. What does print(safe_avg([4, 6])) display?

   Hint

   finally prints before the caller prints.

   Answer

   def safe_avg(L):
       try:
           return sum(L) / len(L)
       except ZeroDivisionError:
           return 0
       finally:
           print('done')
   print(safe_avg([4, 6]))

   Output: done, then 5.0

6. Q6 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Statements written after a try...finally statement are executed even when an exception raised in the try block is not handled.
   Reason (R): The finally block is executed whether or not an exception occurs.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   What happens to an unhandled exception after finally?

   Answer

   (d) A is false (the unhandled exception is re-raised after finally, so later statements do not run) but R is true.

7. Q7 · 2 marks · Short answerHOTS / competency

   Predict the output:

   def f(n):
       try:
           if n < 0:
               return 'neg'
           print('pos')
       finally:
           print('end', n)
   print(f(-1))
   print(f(2))

   Hint

   What does f return when n is 2?

   Answer

   end -1
   neg
   pos
   end 2
   None

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.When does a finally block run?Show answer

   Always: with no exception, with a handled exception and with an unhandled one.

2. 2.What happens to an unhandled exception after finally runs?Show answer

   It is re-raised; if nothing else handles it, the program stops.

3. 3.Write the order of the clauses of a full try statement.Show answer

   try, except, else, finally.

4. 4.Does finally run if try executes return?Show answer

   Yes, before the function actually returns.

5. 5.Name a common use of finally.Show answer

   Closing a file (or a database connection) whatever happens.

6. 6.Can a try have finally without except?Show answer

   Yes; try...finally is valid.

The finally Block: frequently asked questions