Exception Handling in Python: CBSE Class 12 Computer Science

What Exception Handling in Python is about

Weightage: This chapter is part of Unit 1, Computational Thinking and Programming - 2, which carries 40 of the 70 theory marks together with Revision of Python Topics from Class XI, Functions, File Handling in Python and Stack.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 1 Computational Thinking and Programming - 2

Key concepts

Syntax errors and exceptions

NCERT section NCERT 1.2, 1.3

A syntax error breaks the rules of the language: a missing colon, an unclosed bracket, wrong indentation. Python finds it while reading the program, so not even the first line runs. An exception happens while a correct program is running, when an operation cannot be carried out: 10 / 0, int('abc'), L[10] on a short list. When an exception is not handled, Python stops the program and prints a traceback with the exception's name and message. Strictly, SyntaxError and IndentationError are also exception classes, but they are reported before execution and cannot be caught by the program that contains them.

Worked example

Classify each as a syntax error or an exception (and name it): (i) if x > 5 print(x) (ii) print(10 / 0) (iii) int('12a') (iv) a line indented without reason.

Try it first, then show the solutionSolution

1. (i) The colon is missing: syntax error (SyntaxError), reported before running.
2. (ii) The statement is valid but division by zero fails at run time: ZeroDivisionError.
3. (iii) Valid statement, but '12a' is not a valid integer: ValueError at run time.
4. (iv) Wrong indentation is caught while reading: IndentationError.

Answer: (i) and (iv) are syntax errors; (ii) ZeroDivisionError and (iii) ValueError are exceptions.

Built-in exceptions

NCERT section NCERT 1.4

Python names each kind of run-time error. The ones to know:

Exception	Raised when
ZeroDivisionError	the divisor in /, // or % is zero
ValueError	the type is right but the value is not, e.g. int('abc')
TypeError	an operation gets the wrong type, e.g. 5 + '5'
NameError	a variable or function name is not defined
IndexError	a sequence index is out of range
KeyError	a dictionary key does not exist
IOError / FileNotFoundError	a file cannot be opened, e.g. it does not exist
ImportError	a module or name in an import cannot be found
EOFError	input() or a read hits end of file with no data, e.g. pickle.load() at the end of a binary file
KeyboardInterrupt	the user interrupts the program (Ctrl+C)
OverflowError	a calculation is too large to represent
IndentationError, SyntaxError	the code breaks the language rules

Worked example

Name the exception each statement raises: (a) print(x) where x was never assigned (b) [1, 2][2] (c) {'a': 1}['b'] (d) '5' + 5 (e) float('3.5.1')

Try it first, then show the solutionSolution

1. (a) The name x is not defined: NameError.
2. (b) Valid indices are 0 and 1: IndexError.
3. (c) 'b' is not a key: KeyError.
4. (d) str and int cannot be added: TypeError.
5. (e) A string is the right type for float() but this value is not a number: ValueError.

Answer: (a) NameError (b) IndexError (c) KeyError (d) TypeError (e) ValueError

try and except

NCERT section NCERT 1.6.2, 1.6.3

Put the statements that may fail in a try block and the response in an except block:

try:
    statements that may raise an exception
except ExceptionName:
    what to do if that exception occurs

If no exception occurs, the except block is skipped. If one occurs, the rest of the try block is skipped and Python looks for a matching except. One try can have several except blocks (checked top to bottom, and only the first match runs), an except can name a tuple of exceptions except (ValueError, TypeError):, and a bare except: (with no name) catches any exception, so it must come last. If no except matches, the exception stays unhandled and the program stops.

Worked example

Predict the output:

L = [10, 20, 30]
for i in [1, 3, 'x']:
    try:
        print(L[i] // 10)
    except IndexError:
        print('Index')
    except TypeError:
        print('Type')
    finally:
        print('Done')

Try it first, then show the solutionSolution

1. i = 1: L[1] // 10 = 2 is printed; no exception; finally prints Done.
2. i = 3: L[3] raises IndexError; 'Index' is printed; finally prints Done.
3. i = 'x': a list index must be an integer, so TypeError; 'Type' is printed; then Done.

Answer: 2
Done
Index
Done
Type
Done

The finally block

NCERT section NCERT 1.7

finally comes after all the except blocks and always runs: when the try block succeeds, when an exception is handled, when an exception is not handled (finally runs, then the program stops), and even when try or except executes return. It is used for clean-up such as closing a file or a database connection. Order of blocks: try, then any except blocks, then finally. A try must be followed by at least one except or a finally.

Worked example

Predict the output:

def div(a, b):
    try:
        r = a // b
        print('A', end=' ')
    except ZeroDivisionError:
        print('B', end=' ')
        r = -1
    finally:
        print('C', end=' ')
    return r
print(div(7, 2))
print(div(7, 0))

Try it first, then show the solutionSolution

1. div(7, 2): r = 3, prints A, no exception, finally prints C, returns 3. The outer print shows 3 on the same line.
2. div(7, 0): 7 // 0 raises ZeroDivisionError before 'A' can print; except prints B and sets r = -1; finally prints C; returns -1.

Answer: A C 3
B C -1

finally with return

NCERT section NCERT 1.7

When the try or except block reaches return, Python first runs the finally block and only then leaves the function with the return value. So any print inside finally appears before the caller prints the returned value.

Worked example

Predict the output:

def f(x):
    try:
        return int(x) * 2
    except ValueError:
        return 0
    finally:
        print('cleanup', end=' ')
print(f('5'))
print(f('five'))

Try it first, then show the solutionSolution

1. f('5'): the try block is about to return 10; finally runs first and prints 'cleanup '; then print shows 10.
2. f('five'): int('five') raises ValueError; except is about to return 0; finally prints 'cleanup '; then 0 is shown.

Answer: cleanup 10
cleanup 0

Unhandled exceptions and exceptions inside except

NCERT section NCERT 1.6.3

Except blocks only guard their own try block. If an exception is raised inside an except block, the sibling except blocks of the same try do not catch it; it goes up as an unhandled exception (after any finally runs). Likewise, an exception whose type is not listed in any except is not caught. Use a general except (or except Exception:) last if every error must be caught.

Worked example

What happens when this code runs?

try:
    print(1 / 0)
except ZeroDivisionError:
    print(int('abc'))
except ValueError:
    print('inner')

Try it first, then show the solutionSolution

1. 1 / 0 raises ZeroDivisionError, and the first except block starts.
2. Inside it, int('abc') raises ValueError. The except ValueError below belongs to the same try, so it does not guard the except block.
3. The ValueError is unhandled, so the program stops with a traceback; 'inner' is never printed.

Answer: Nothing is printed by the program; it stops with ValueError: invalid literal for int() with base 10: 'abc'.

Exception Handling in Python in real life

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

Topics in this chapter

• try and except Blocks
• Built-in Exceptions
• The finally Block

Exception Handling in Python: syntax and key facts

Full structure

1. 1.try: risky code
2. 2.except Name1: handler
3. 3.except (Name2, Name3): handler
4. 4.except: any other
5. 5.finally: always runs

A bare except must be the last except.

When does each block run?

No exception	try (whole) + finally
Handled exception	try (up to the error) + the first matching except + finally
Unmatched exception	try (up to the error) + finally, then the program stops

Only one except block ever runs for one exception.

finally and return

On return inside try or except, finally runs first, then the function returns

Output from finally appears before the caller uses the return value.

Common exception names

ZeroDivisionError, ValueError, TypeError, NameError, IndexError, KeyError, IOError / FileNotFoundError, ImportError, EOFError, KeyboardInterrupt, OverflowError

Spelling counts: 'Error' is part of every name except KeyboardInterrupt.

How the board asks Exception Handling in Python

The question patterns to expect, and the method that works for each.

Tricks to remember Exception Handling in Python

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

• T-E-F: Try, Except, Finally

  "Try Eating Fruit": the blocks always appear in this order.

  Use it when: Writing the structure and spotting wrongly ordered blocks.

• finally never misses

  No error, handled error, unhandled error, return: finally runs in all four.

  Use it when: Which-block-runs MCQs.

• Type versus Value

  Wrong kind of thing = TypeError ('5' + 5). Right kind, wrong content = ValueError (int('5a')).

  Use it when: Naming the exception.

• First match wins

  Except blocks are checked top to bottom, and only the first matching one runs; a bare except at the top would swallow everything, so Python requires it last.

  Use it when: Order-of-except questions.

Exception Handling in Python: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

def test(a, b):
    try:
        print(a / b, end=' ')
    except ZeroDivisionError:
        print('Zero', end=' ')
    except TypeError:
        print('Type', end=' ')
    finally:
        print('#')
test(6, 3)
test(6, 0)
test('6', 3)

try:
    print(1 / 0)
except ZeroDivisionError:
    print(int('abc'))
except ValueError:
    print('inner')

D = {'a': 10}
for k in ['a', 'b']:
    try:
        print(D[k] / (len(k) - 1))
    except KeyError:
        print('missing', k)
    except Exception:
        print('other error')

L = [5, 0, 'a']
total = 0
for x in L:
    try:
        total += 10 // x
        print('ok', total)
    except ZeroDivisionError:
        total += 1
    except:
        total += 100
print(total)

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

def monthly(fee_text, months):
    try:
        fee = int(fee_text)
        result = fee // months
    except ValueError:
        result = 'Invalid fee'
    except ZeroDivisionError:
        result = 'Months cannot be 0'
    finally:
        print('Calculation attempted')
    return result

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Exception Handling in Python

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

• ✗Mistake: Forgetting the finally output when a function returns

  Students print the return value first because return looks like the last word.

  Wrong working

  print(f('5')) gives
  10
  cleanup

  Right working

  print(f('5')) gives
  cleanup 10

  ✓Do this: When try or except returns, run finally first, then hand the value back to the caller.

  Usually costs: 1 mark

• ✗Mistake: Assuming the rest of the try block runs after an exception

  Control jumps straight to the matching except as soon as the error happens.

  Wrong working

  r = 7 // 0 raises the error, then print('A') still runs

  Right working

  print('A') is skipped; control goes to except ZeroDivisionError

  ✓Do this: Draw a line under the statement that raises the exception: nothing below it in the try block runs.

  Usually costs: 1 mark

• ✗Mistake: Confusing syntax errors with exceptions

  Both show red error messages, but a syntax error stops the program before any line runs.

  ✓Do this: In a definition answer write 'detected before execution' for syntax errors and 'detected during execution' for exceptions, with one example each.

  Usually costs: 1 mark in a 2-mark difference question

• ✗Mistake: Misspelling exception names

  Names such as ZeroDivisonError or IndexErrors are not valid, so the except would not match (and the answer is marked wrong).

  ✓Do this: Learn the exact names: ZeroDivisionError, ValueError, TypeError, NameError, IndexError, KeyError, EOFError, IOError, ImportError.

  Usually costs: Half to 1 mark

• ✗Mistake: Thinking an except block also guards the other except blocks

  An exception raised inside a handler is not caught by its sibling handlers.

  ✓Do this: If code inside an except can fail, put it in its own nested try.

  Usually costs: 1 mark in an output or assertion question

NCERT exercise guide: Exception Handling in Python

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Exception Handling in Python

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

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

   Hint

   Only grammar errors are found while reading the code.

   Answer

   (c) SyntaxError

2. Q2 · 1 mark · Very short answerBoard level

   Name the exception raised by: (a) L = [1, 2, 3]; print(L[3]) (b) import maths (c) print(Total) when Total is not defined.

   Hint

   One each: index, import, name.

   Answer

   (a) IndexError (b) ImportError (ModuleNotFoundError) (c) NameError

3. Q3 · 2 marks · Short answerHOTS / competency

   Predict the output:

   def g(n):
       try:
           return 10 // n
       except ZeroDivisionError:
           return 'inf'
       finally:
           print('F', end='')
   print(g(2), g(0))

   Hint

   Both calls finish (printing F) before print() shows anything.

   Answer

   FF5 inf

4. Q4 · 2 marks · Short answerBoard level

   Rewrite the code so that it prints 'Invalid' instead of crashing when the text is not a number, and always prints 'Bye':

   n = int(text)
   print(n * 2)

   Hint

   One except for ValueError and a finally.

   Answer

   text = '4a'
   try:
       n = int(text)
       print(n * 2)
   except ValueError:
       print('Invalid')
   finally:
       print('Bye')

   Output for '4a': Invalid, then Bye.

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): A try block can be followed directly by a finally block without any except block.
   Reason (R): Every try block must have at least one except block.

   Hint

   try needs at least one except or a finally.

   Answer

   (c) A is true but R is false.

6. Q6 · 2 marks · Short answerHarder board level

   Predict the output:

   values = ['3', '', '4']
   for v in values:
       try:
           print(10 / int(v))
       except (ValueError, ZeroDivisionError):
           print('skip')

   Hint

   int('') is not a valid number.

   Answer

   3.3333333333333335
   skip
   2.5

7. Q7 · 3 marks · Short answerHarder board level

   Write a function GET_ITEM(L, i) that returns L[i], or the string 'No item' if i is out of range, or 'Bad index' if i is not an integer. Show calls for L = [4, 8], i = 1, 5 and 'a'.

   Hint

   IndexError and TypeError.

   Answer

   def GET_ITEM(L, i):
       try:
           return L[i]
       except IndexError:
           return 'No item'
       except TypeError:
           return 'Bad index'
   L = [4, 8]
   print(GET_ITEM(L, 1), GET_ITEM(L, 5), GET_ITEM(L, 'a'))

   Output: 8 No item Bad index

8. Q8 · 2 marks · Short answerHarder board level

   Explain why the order of these except blocks is wrong, and correct it:

   try:
       x = int(input())
   except:
       print('Error')
   except ValueError:
       print('Not a number')

   Hint

   A bare except catches everything.

   Answer

   A bare except catches every exception, so a later named except could never run; Python reports a SyntaxError ('default except: must be last'). Put except ValueError: first and the bare except: last.

9. Q9 · 2 marks · Short answerHarder board level

   Predict the output:

   try:
       d = {'x': 1}
       print(d['x'])
       print(d['y'])
       print('end of try')
   except KeyError:
       print('key missing')
   finally:
       print('finally')

   Hint

   The second lookup fails.

   Answer

   1
   key missing
   finally

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 is a syntax error detected?Show answer

   Before the program runs, when Python reads (parses) the code.

2. 2.Which exception does int('3.5') raise?Show answer

   ValueError.

3. 3.Which exception is raised when you open a file that does not exist in read mode?Show answer

   FileNotFoundError (a kind of IOError / OSError).

4. 4.Can a try block exist without except?Show answer

   Yes, if it has a finally block. A try needs at least one except or a finally.

5. 5.How many except blocks run for one exception?Show answer

   Only one: the first one that matches.

6. 6.Does finally run if the exception is not handled?Show answer

   Yes. finally runs, and then the program stops with the traceback.

7. 7.Which exception does pickle.load() raise at the end of a binary file?Show answer

   EOFError.

8. 8.Where must a bare except: be placed?Show answer

   After all the named except blocks (it must be last).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Syntax error: found before running. Exception: raised while running.
• ☐An unhandled exception stops the program with a traceback.
• ☐try holds risky code; at the first exception the rest of try is skipped.
• ☐Except blocks are checked top to bottom; only the first match runs; a bare except goes last.
• ☐If no except matches, finally runs and then the program stops.
• ☐finally always runs, even when try or except returns.
• ☐ValueError = right type, wrong value; TypeError = wrong type.
• ☐ZeroDivisionError, IndexError, KeyError, NameError, FileNotFoundError / IOError, EOFError, ImportError, KeyboardInterrupt, OverflowError.

Exception Handling in Python: frequently asked questions