Built-in Exceptions in Python: What Raises Each One (Class 12 Computer Science)

Understand Built-in Exceptions

Built-in Exceptions: key facts

NCERT Table 1.1 in brief

SyntaxError	code breaks Python's rules
ValueError	right type, unsuitable value
IOError	file cannot be opened
KeyboardInterrupt	user interrupts the program
ImportError	module not found
EOFError	input() reaches end of input
ZeroDivisionError	divisor is zero
IndexError	index out of range
NameError	name not defined
IndentationError	wrong indentation
TypeError	wrong type for the operation
OverflowError	result too large for the type

NCERT Class XII Computer Science 1.4.

Statement → exception

int('4.0')	ValueError
'abc' + 5	TypeError
7 % 0	ZeroDivisionError
[1, 2][2]	IndexError
print(total) with no total	NameError
open('nofile.txt')	FileNotFoundError (an IOError)
{'a'	1}['z']: KeyError
2.0 ** 5000	OverflowError

Each one checked by running it.

Families

• •FileNotFoundError is an IOError (OSError)
• •ModuleNotFoundError is an ImportError
• •IndentationError is a SyntaxError
• •UnboundLocalError is a NameError

except on the family name also catches its members.

ValueError or TypeError?

1. 1.Operation allowed for this type, bad value
2. 2.ValueError | Operation not allowed for this type
3. 3.TypeError

int('abc') is ValueError; int(None) is TypeError.

How to answer Built-in Exceptions questions

1. 1Is the code itself badly written (missing colon, bracket, wrong indentation)? Then it is SyntaxError or IndentationError, reported before anything runs.
2. 2Is a name used that was never assigned? NameError (UnboundLocalError if it is a local read before assignment).
3. 3Is an operator or function applied to a type that cannot take it, such as str + int? TypeError.
4. 4Is the type fine but the value unsuitable, such as int('12a') or sqrt of a negative number? ValueError.
5. 5Is a divisor zero in /, // or %? ZeroDivisionError.
6. 6Is an index past the end of a sequence (IndexError) or a key missing from a dictionary (KeyError)?
7. 7Does it involve a file, input() or import? IOError/FileNotFoundError, EOFError or ImportError.

Built-in Exceptions 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 Built-in Exceptions

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

• Type is the kind, value is the content

  TypeError: 'you can't do this with that kind of thing'. ValueError: 'right kind of thing, wrong content'.

  Use it when: Choosing between ValueError and TypeError.

• Index for positions, Key for labels

  Sequences complain with IndexError; dictionaries complain with KeyError.

  Use it when: Lookup errors.

• Syntax family strikes first

  SyntaxError and IndentationError are caught at the gate, before the program runs; the rest strike during the run.

  Use it when: Questions on when an error is detected.

• Floats overflow, ints just grow

  2 5000 is fine; 2.0 5000 raises OverflowError.

  Use it when: OverflowError items.

Built-in Exceptions: solved examples

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

try:
    f = open('data.txt', 'r')
    print('opened')
except IOError:
    print('IOError caught')
except FileNotFoundError:
    print('FileNotFoundError caught')

try:
    name = input()
    print('Hello', name)
except EOFError:
    print('No input given')
except ValueError:
    print('Bad value')

Where students lose marks in Built-in Exceptions

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

• ✗Mistake: Calling int('abc') a TypeError

  int() does accept strings; the problem is the value inside the string.

  Wrong working

  int('abc') → TypeError

  Right working

  int('abc') → ValueError

  ✓Do this: Ask whether the operation is allowed for the type at all; if yes, it is a ValueError.

  Usually costs: 1 mark.

• ✗Mistake: Using IndexError for a missing dictionary key

  Dictionaries raise KeyError.

  Wrong working

  except IndexError: # for d['z']

  Right working

  except KeyError: # for d['z']

  ✓Do this: Index for sequences, Key for dictionaries.

  Usually costs: 1 mark.

• ✗Mistake: Misspelling exception names

  An unknown name in an except clause itself raises NameError when Python checks it.

  Wrong working

  except ZeroDivideError:

  Right working

  except ZeroDivisionError:

  ✓Do this: Learn the exact spellings with their capital letters.

  Usually costs: Half to 1 mark.

• ✗Mistake: Thinking SyntaxError can be caught in the same program

  A syntax error stops the program from starting, so its own try block never runs.

  ✓Do this: Say that syntax errors must be corrected in the code; exceptions are what try/except handles.

  Usually costs: 1 mark in reasoning questions.

Practice questions on Built-in Exceptions

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 exception is raised when the index of a list is out of range?
   (a) KeyError (b) ValueError (c) IndexError (d) RangeError

   Hint

   Positions in a sequence.

   Answer

   (c) IndexError

2. Q2 · 1 mark · Very short answerBoard level

   Which exception does '10' / 2 raise?

   Hint

   Can a string be divided at all?

   Answer

   TypeError

3. Q3 · 1 mark · MCQBoard level

   Which statement raises ZeroDivisionError?
   (a) 0 / 5 (b) 5 // 0 (c) 0 * 5 (d) 5 - 0

   Hint

   Zero must be the divisor.

   Answer

   (b) 5 // 0

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output:

   for x in [10, 'a', 0, None]:
       try:
           print(100 / x)
       except ZeroDivisionError:
           print('Z')
       except TypeError:
           print('T')

   Hint

   None cannot be a divisor either, but not because it is zero.

   Answer

   10.0
   T
   Z
   T

5. Q5 · 2 marks · Short answerHarder board level

   Write code that reads a number from the list vals = ['16', '-4', 'nine'], prints its square root using math.sqrt, and prints 'invalid' for any value that raises ValueError. Give the output.

   Hint

   Both int('nine') and math.sqrt(-4) raise ValueError.

   Answer

   import math
   vals = ['16', '-4', 'nine']
   for v in vals:
       try:
           print(math.sqrt(int(v)))
       except ValueError:
           print('invalid')

   Output: 4.0, invalid, invalid (one per line)

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

   Assertion (A): An IndentationError is reported before any statement of the program is executed.
   Reason (R): IndentationError is a kind of SyntaxError.
   (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

   When are syntax errors found?

   Answer

   (a) Both are true and R explains A: syntax errors, including indentation errors, are found when the code is parsed, before it runs.

7. Q7 · 2 marks · Short answerHOTS / competency

   Predict the output:

   try:
       a = 2 ** 5000
       print('int ok', len(str(a)))
       b = 2.0 ** 5000
       print('float ok')
   except OverflowError:
       print('overflow')

   Hint

   Which of the two types has a size limit?

   Answer

   int ok 1506
   overflow
   (the integer has 1506 digits and is fine; the float power exceeds the float limit).

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.Which exception does int('4.0') raise?Show answer

   ValueError.

2. 2.Which exception is raised when input() finds no data?Show answer

   EOFError.

3. 3.Which exception is raised for an undefined variable?Show answer

   NameError.

4. 4.What does 2.0 ** 5000 raise?Show answer

   OverflowError.

5. 5.Does except IOError catch FileNotFoundError?Show answer

   Yes; FileNotFoundError is a kind of IOError (OSError).

6. 6.Which exception does importing a misspelt module raise?Show answer

   ModuleNotFoundError, a kind of ImportError.

Built-in Exceptions: frequently asked questions