try and except Blocks: Handling Exceptions in Python (Class 12 Computer Science)

Understand try and except Blocks

try:
    statements that may raise an exception
except ExceptionName:
    statements run only if that exception occurs

try and except Blocks: key facts

Key terms

Syntax error	detected before the program runs
Exception	error detected while the program runs
Throwing (raising)	creating the exception object and passing it to the runtime system
Catching	running the handler that matches
Call stack	list of functions searched for a handler, from the current one back to the main program

NCERT Class XII Computer Science 1.2, 1.3 and 1.6.2.

What runs when

No exception in try	whole try runs; all except blocks skipped
Exception in try	rest of try skipped → first matching except runs → continue after the statement
No matching except	exception passed to the caller; program stops if unhandled

Control never goes back into the try block.

Clause order

1. 1.try
2. 2.except Specific1
3. 3.except Specific2
4. 4.except (no name, last)
5. 5.else (optional)
6. 6.finally (optional, last)

A bare except placed before others is a SyntaxError.

Input-validation pattern

1. 1.while True:
2. 2.try: n = int(input())
3. 3.break
4. 4.except ValueError: print a message and loop again

The loop ends only when int() succeeds.

How to answer try and except Blocks questions

1. 1Run the try block line by line and write each output as it happens.
2. 2At the first line that raises an exception, stop the try block. Name the exception (ZeroDivisionError, ValueError, IndexError, KeyError, TypeError, NameError ...).
3. 3Check the except clauses from top to bottom; run only the first one that matches (a bare except matches anything).
4. 4If none matches, the exception leaves this try: look for a handler in the caller; if there is none, the program stops and no later line prints.
5. 5After the except block, continue with the first statement after the whole try...except, or with the next loop pass if it is inside a loop.
6. 6When the code is inside a function called in a print(), finish all the calls (and their prints) before the outer print shows the returned values.
7. 7In programs you write, name specific exceptions where you can and keep only the risky statements inside try.

try and except Blocks 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 try and except Blocks

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

• Try, trip, jump

  Python tries each line; on the first trip it jumps out of try to the matching except and never comes back.

  Use it when: Tracing output when an exception occurs mid-block.

• First match wins

  except clauses are checked like a queue at a counter: the first one that fits serves the exception.

  Use it when: Multiple except clauses.

• Catch-all comes last

  The bare except is the goalkeeper: it stands at the back, after all the specific defenders.

  Use it when: Order of except clauses.

• Syntax before, exception during

  Syntax errors stop the program before it starts; exceptions stop it while it is running.

  Use it when: Distinguishing the two in 1-2 mark questions.

try and except Blocks: solved examples

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

try:
    x = int('12a')
    print('converted')
except ValueError:
    print('bad number')
print('end')

def safe_div(a, b):
    try:
        q = a / b
        print('ok', end=' ')
        return q
    except ZeroDivisionError:
        print('zero', end=' ')
        return 0
    except TypeError:
        print('type', end=' ')
        return -1
print(safe_div(9, 3), safe_div(5, 0), safe_div('5', 2))

def get(L, i):
    print('fetching', i)
    return L[i]
try:
    print(get([1, 2], 1))
    print(get([1, 2], 5))
    print('after')
except IndexError:
    print('caught in caller')

for v in ['4', 'zero', '0']:
    try:
        print(10 // int(v))
    except ZeroDivisionError:
        print('div by 0')
    except:
        print('other')

Where students lose marks in try and except Blocks

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

• ✗Mistake: Printing the lines after the error inside try

  Once an exception occurs, the rest of the try block is skipped.

  Wrong working

  bad number
  converted
  end

  Right working

  bad number
  end

  ✓Do this: Stop the try block at the failing line.

  Usually costs: 1 mark.

• ✗Mistake: Assuming any except catches any exception

  A named clause catches only that exception; others pass on and may stop the program.

  Wrong working

  except IndexError catches d['b'] → prints 'index'

  Right working

  d['b'] raises KeyError, which except IndexError does not catch

  ✓Do this: Name the exception the faulty line really raises before matching clauses.

  Usually costs: 1 mark.

• ✗Mistake: Putting the bare except first

  Python requires the default except to be the last clause.

  Wrong working

  except:
  ...
  except ValueError:
  ...

  Right working

  except ValueError:
  ...
  except:
  ...

  ✓Do this: Specific clauses first, catch-all last.

  Usually costs: Half to 1 mark.

• ✗Mistake: Wrapping the whole program in one try

  One error then skips everything after it, and examiners expect the handler around the risky statement.

  ✓Do this: Keep only the conversion, division or lookup inside try, especially inside loops.

  Usually costs: Half a mark in 3-mark programs.

Practice questions on try and except Blocks

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

   In Python, the statements that may raise an exception are written in the:
   (a) except block (b) try block (c) finally block (d) else block

   Hint

   You try the risky part.

   Answer

   (b) try block

2. Q2 · 1 mark · MCQBoard level

   Which of these is detected before the program starts running?
   (a) ZeroDivisionError (b) ValueError (c) SyntaxError (d) IndexError

   Hint

   Parsing happens first.

   Answer

   (c) SyntaxError

3. Q3 · 1 mark · Very short answerBoard level

   Write the output:

   try:
       print(int('7') + 1)
   except ValueError:
       print('V')
   print('next')

   Hint

   Does anything fail?

   Answer

   8
   next

4. Q4 · 2 marks · Short answerHarder board level

   Write the output:

   L = [5, 0, 2]
   for x in L:
       try:
           print(10 / x, end=' ')
       except ZeroDivisionError:
           print('inf', end=' ')

   Hint

   The loop continues after the except block.

   Answer

   2.0 inf 5.0

5. Q5 · 2 marks · Short answerHarder board level

   Write a function to_int(s) that returns int(s), or None if s cannot be converted. Show print(to_int('25'), to_int('2 5'), to_int('-8')).

   Hint

   Return inside try, return None inside except.

   Answer

   def to_int(s):
       try:
           return int(s)
       except ValueError:
           return None
   print(to_int('25'), to_int('2 5'), to_int('-8'))

   Output: 25 None -8

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

   Assertion (A): When an exception occurs in a try block, the remaining statements of the try block are not executed.
   Reason (R): Control jumps to the matching except block.
   (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

   Try, trip, jump.

   Answer

   (a) Both are true and R explains A.

7. Q7 · 2 marks · Short answerHOTS / competency

   What does this code display? Explain.

   try:
       d = {'a': 1}
       print(d['b'])
   except IndexError:
       print('index')
   print('done')

   Hint

   Which exception does a missing key raise?

   Answer

   Nothing is printed by the program itself: d['b'] raises KeyError, which does not match except IndexError and has no other handler, so the program stops with KeyError: 'b' and 'done' is never printed.

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.What is throwing an exception?Show answer

   Creating the exception object and handing it to the runtime system to find a handler.

2. 2.What is catching an exception?Show answer

   Executing the handler (except block) that matches it.

3. 3.Where must a bare except clause be placed?Show answer

   After all the other except clauses.

4. 4.After an except block runs, where does execution continue?Show answer

   At the statement after the whole try...except, not in the try block.

5. 5.What happens if no except clause matches?Show answer

   The exception goes to the caller; if no handler is found anywhere, the program stops.

6. 6.What does int('3.5') raise?Show answer

   ValueError.

try and except Blocks: frequently asked questions