Functions in Python: CBSE Class 12 Computer Science Notes and Questions

What Functions 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, Exception Handling in Python, 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

Types of functions

NCERT section Class XI NCERT 7.2, 7.5

Built-in functions are always available without an import: len(), type(), int(), input(), print(), max(), round(). Functions defined in modules need the module imported first: import math then math.ceil(4.2), or from random import randint then randint(1, 6). User-defined functions are the ones you write with def. A function may take arguments and may return a value; one that has no return statement returns None.

Worked example

Classify each as built-in, module or user-defined, and give its value: len('abc'), max(3, 7, 1), math.sqrt(49) (after import math), round(3.14159, 2), int('25') + 5.

Try it first, then show the solutionSolution

1. len() is built-in: 3. max() is built-in: 7.
2. math.sqrt() is a module function and returns a float: 7.0.
3. round() is built-in: 3.14. int() is built-in: int('25') + 5 = 30.

Answer: 3, 7, 7.0, 3.14, 30; only sqrt() is a module function. None of these is user-defined.

Defining and calling a function

NCERT section Class XI NCERT 7.3

A definition has a header and an indented body:

def function_name(parameter1, parameter2):
    statements
    return value

The header needs the keyword def, the name, round brackets (even if empty) and a colon. The body runs only when the function is called, e.g. function_name(10, 20). return sends a value back and ends the function at once; any lines after it in that path are skipped. If the body ends without return, the call gives None.

Worked example

Predict the output:

def f(n):
    if n % 2 == 0:
        return n // 2
    print('odd')
print(f(4), f(3))

Try it first, then show the solutionSolution

1. Both arguments of print() are evaluated first, left to right.
2. f(4): 4 is even, so it returns 2.
3. f(3): 3 is odd, so it prints 'odd' and reaches the end without return, giving None.
4. Only now does the outer print() run, showing 2 and None.

Answer: odd
2 None

Parameters, arguments, positional and default parameters

NCERT section Class XI NCERT 7.3.1-7.3.2

Parameters (formal parameters) are the names in the header; arguments (actual parameters) are the values in the call. Positional parameters are matched by order: the first argument goes to the first parameter, and so on. A default parameter has a value in the header (def power(base, exp=2):) and takes that value when the call leaves it out. Rule: once a parameter has a default, every parameter after it must also have one, so def f(a=1, b): is a SyntaxError. Too few or too many arguments in a call raise TypeError.

Worked example

Given def calc(p, q=2, r=3): return p * q + r, find calc(1), calc(1, 4) and calc(1, 4, 0). Which of the headers def g(a, b=1, c=2): and def g(a=1, b, c): is valid?

Try it first, then show the solutionSolution

1. calc(1): p = 1, q = 2, r = 3, so 1 × 2 + 3 = 5.
2. calc(1, 4): p = 1, q = 4, r = 3, so 7.
3. calc(1, 4, 0): 1 × 4 + 0 = 4.
4. In def g(a=1, b, c): non-default parameters follow a default one, so it is invalid; the first header is valid.

Answer: 5, 7, 4; only def g(a, b=1, c=2): is valid.

Returning values

NCERT section Class XI NCERT 7.3.3

A function can return any object. Writing return a, b returns one tuple (a, b), which can be unpacked: x, y = f(). A function without return (or with a bare return) gives None. Return ends the function: in a loop, return inside the loop stops the loop and the function together.

Worked example

Predict the output:

def change(a, b=5):
    a = a + b
    b = a - b
    return a, b
x, y = change(10)
print(x, y)
print(change(2, 3))

Try it first, then show the solutionSolution

1. change(10): a = 10, b = 5 (default). a becomes 15, b becomes 15 - 5 = 10. Returns (15, 10), unpacked into x and y.
2. change(2, 3): a becomes 5, b becomes 5 - 3 = 2. Returns the tuple (5, 2), which print() shows with brackets.

Answer: 15 10
(5, 2)

Flow of execution

NCERT section Class XI NCERT 7.4

Python runs a program from top to bottom, but a def statement only creates the function; its body is skipped until a call. On a call, control jumps to the function, runs its body, and comes back to the point just after the call (carrying the return value). A function can call another function defined later in the file, as long as both definitions have run before the first call happens.

Worked example

Predict the output:

def outer():
    print('A', end='')
    inner()
    print('C', end='')
def inner():
    print('B', end='')
print('S', end='')
outer()
print('E')

Try it first, then show the solutionSolution

1. The two def statements only create the functions.
2. The first statement that runs is print('S').
3. outer() prints A, calls inner() which prints B, returns, and outer prints C.
4. Back at the top level, 'E' is printed with a newline.

Answer: SABCE

Scope of a variable: local and global

NCERT section Class XI NCERT 7.4.1

A variable assigned inside a function is local: it exists only while that call runs and hides any global variable with the same name. A variable created at the top level is global and can be read inside a function. To assign to a global variable inside a function, declare it first with global name. If a function assigns to a name anywhere in its body, Python treats that name as local throughout the body, so reading it before the assignment raises UnboundLocalError.

Worked example

Predict the output:

total = 100
def add(x):
    global total
    total = total + x
    return total
def show():
    total = 5
    print('in show', total)
print(add(20), total)
show()
print(total)

Try it first, then show the solutionSolution

1. add(20) is evaluated first: it changes the global total to 120 and returns 120. Then the second argument, total, is read: 120.
2. show() creates a local total = 5, prints it, and leaves the global alone.
3. The global total is still 120.

Answer: 120 120
in show 5
120

Passing mutable and immutable arguments

NCERT section Class XI NCERT 7.3 (passing lists and strings)

Python passes a reference to the object. If the function changes a mutable object in place (L.append(), L[i] = ..., D[k] = ...), the caller sees the change. If it rebinds the parameter (L = [0], n = n * 2), only the local name changes. Immutable arguments (numbers, strings, tuples) can never be changed in place, so the caller's variable is never affected.

Worked example

Predict the output:

def update(L, n):
    L.append(n)
    n = n * 2
    L = [0]
    return n
A = [1, 2]
m = 4
print(update(A, m), A, m)

Try it first, then show the solutionSolution

1. L.append(4) changes the caller's list in place: A becomes [1, 2, 4].
2. n = n * 2 rebinds the local n to 8; m stays 4.
3. L = [0] rebinds only the local name L; A is not affected.
4. The function returns 8.

Answer: 8 [1, 2, 4] 4

Functions 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

• Returning Values and Flow of Execution
• Scope of Variables: Local and Global
• Positional and Default Parameters

Functions: syntax and key facts

Function header

def name(p1, p2=default): followed by an indented body; brackets and colon are compulsory

Parameters with defaults must come after all parameters without defaults.

Return several values

return a, b, c returns the tuple (a, b, c); unpack with x, y, z = f()

A function with no return statement returns None.

Change a global inside a function

global count as the first statement that uses count, then count = count + 1

Without global, the assignment creates a local variable.

Three kinds of function

built-in (len(), max()), module (math.sqrt(), random.randint()), user-defined (def)

Module functions need an import first.

Mutable argument rule

In-place change (append, L[i] =, D[k] =) is seen by the caller; rebinding (L = ...) is not

Strings, numbers and tuples can never be changed through a parameter.

How the board asks Functions

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

Tricks to remember Functions

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

• PA: Parameters in the header, Arguments in the call

  P comes first in the alphabet and first in the program (the def line); A comes later, at the call.

  Use it when: Definition questions on parameters and arguments.

• Defaults go to the back of the queue

  Like latecomers in a line, parameters with defaults must stand after all those without.

  Use it when: Spotting the SyntaxError in headers such as def f(a=1, b):.

• No return, gets None

  A function that falls off its end hands back None, so print(f()) prints whatever f prints and then None.

  Use it when: Output questions where a function prints instead of returning.

• Assign inside = local everywhere inside

  One assignment to a name anywhere in a function makes it local from the first line of the function.

  Use it when: UnboundLocalError questions.

• Change in place travels, rebinding stays home

  append, item assignment and += on a list travel back to the caller; L = something stays inside the function.

  Use it when: List-argument output questions.

Functions: board-level solved questions

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

def fun(s):
    k = len(s)
    m = ''
    for i in range(0, k):
        if s[i].isupper():
            m = m + s[i].lower()
        elif s[i].isalpha():
            m = m + s[i].upper()
        else:
            m = m + 'bb'
    print(m)
fun('school2@com')

def Lshift(Arr, n):
    L = len(Arr)
    for x in range(0, n):
        y = Arr[0]
        for i in range(0, L - 1):
            Arr[i] = Arr[i + 1]
        Arr[L - 1] = y
    print(Arr)
Lshift([10, 20, 30, 40, 12, 11], 2)

a = 5
def tricky(a):
    a += 1
    print(a, end=' ')
c = 0
def counter():
    global c
    c += 1
tricky(a)
print(a)
for i in range(3):
    counter()
print(c)

def area(l=10, b)
    a = l * b
    Return a
print(area(5 4))

def area(l, b=10):
    a = l * b
    return a
print(area(5, 4))

def change(a, b=5):
    a = a + b
    b = a - b
    return a, b
r = change(1)
print(type(r), r[0])

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 grace(L, g=2):
    for i in range(len(L)):
        L[i] = L[i] + g
def average(L):
    return sum(L) / len(L)
marks = [35, 48, 61]

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 Functions

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

• ✗Mistake: Printing inside the function when the question asks it to return a value (or the reverse)

  "returns the count" and "displays the count" are marked differently.

  Wrong working

  def COUNT(L):
  c = 0
  ...
  print(c)

  Right working

  def COUNT(L):
  c = 0
  ...
  return c

  ✓Do this: Underline the verb in the question: returns means return, displays or prints means print().

  Usually costs: Half to 1 mark

• ✗Mistake: Changing the function name or parameters given in the question

  The question fixes the header, e.g. EOReplace(L); a different name or an extra input() inside the function does not match it.

  ✓Do this: Copy the header exactly as given, and take data from the parameters, not from input(), unless the question asks for input.

  Usually costs: Half a mark

• ✗Mistake: Forgetting that print() evaluates all its arguments before printing

  When an argument is a call that itself prints, its output comes first, on its own line.

  Wrong working

  print(f(4), f(3)) gives 2 odd None

  Right working

  odd
  2 None

  ✓Do this: Evaluate every argument (running any prints inside) and only then write the line print() itself produces.

  Usually costs: 1 mark

• ✗Mistake: Assuming a function cannot change a list passed to it

  Students apply the rule for numbers to lists.

  Wrong working

  A = [1, 2]; after update(A, 4), A is still [1, 2]

  Right working

  L.append(n) changes the shared list, so A is [1, 2, 4]

  ✓Do this: Mark every line in the function as in-place change or rebinding before you trace.

  Usually costs: 1 mark

• ✗Mistake: Using a loop variable instead of an index to change list items

  for x in L: x = x * 2 changes only x, never the list.

  Wrong working

  for x in L:
  x = x * 2

  Right working

  for i in range(len(L)):
  L[i] = L[i] * 2

  ✓Do this: When a function must modify the list itself, loop over range(len(L)) and assign to L[i].

  Usually costs: 1-2 marks in a function-writing question

NCERT exercise guide: Functions

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.

v = 10
def g():
    print(v)
def h():
    print(w)
    w = 3
g()

Practice questions on Functions

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

1. Q1 · 2 marks · Short answerHOTS / competency

   Predict the output:

   def update(L, n):
       L += [n]
       n = n + 1
       L = L + [0]
       return n, L
   A = [1, 2]
   m = 4
   print(update(A, m), A, m)

   Hint

   Separate in-place changes from rebinding: += on a list works in place, L = L + [0] builds a new list.

   Answer

   (5, [1, 2, 4, 0]) [1, 2, 4] 4 (L += [n] extends A itself; L = L + [0] makes a new local list that only the returned tuple holds; m is never changed)

2. Q2 · 1 mark · MCQBoard level

   Which statement about the global keyword is correct? (a) It is needed to read a global variable inside a function. (b) It is needed to assign to a global variable inside a function. (c) It makes a variable visible in other modules. (d) It creates a new local variable.

   Hint

   Reading needs nothing.

   Answer

   (b) It is needed to assign to a global variable inside a function.

3. Q3 · 2 marks · Short answerBoard level

   Write a function SUMDIGIT(n) that returns the sum of the digits of a positive integer n. Show the call for 4072.

   Hint

   Use % 10 and // 10 in a loop.

   Answer

   def SUMDIGIT(n):
       s = 0
       while n > 0:
           s += n % 10
           n //= 10
       return s
   print(SUMDIGIT(4072))

   Output: 13

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output:

   def f(x, y=3):
       x = x + y
       return x * y
   print(f(2), f(2, 1), f(f(1, 1)))

   Hint

   Work out the inner call first in f(f(1, 1)).

   Answer

   15 3 15 (f(2) = (2 + 3) × 3 = 15; f(2, 1) = 3 × 1 = 3; f(1, 1) = 2 × 1 = 2, so f(f(1, 1)) = f(2) = 15)

5. Q5 · 3 marks · Short answerHarder board level

   Write a function COUNTWORDS(s) that returns a dictionary giving how many times each word occurs in the string s. Show the result for 'to be or not to be'.

   Hint

   split() then get(w, 0) + 1.

   Answer

   def COUNTWORDS(s):
       D = {}
       for w in s.split():
           D[w] = D.get(w, 0) + 1
       return D
   print(COUNTWORDS('to be or not to be'))

   Output: {'to': 2, 'be': 2, 'or': 1, 'not': 1}

6. Q6 · 2 marks · Short answerBoard level

   Identify the local and global variables in this program and give its output:

   x = 50
   def func(y):
       z = x + y
       return z
   print(func(5))

   Hint

   Parameters are local too.

   Answer

   Global: x. Local: y (parameter) and z. Output: 55

7. Q7 · 2 marks · Short answerHarder board level

   Predict the output:

   def m(s):
       r = ''
       for i in range(len(s) - 1, -1, -2):
           r = r + s[i]
       return r.upper()
   print(m('python'))

   Hint

   Indices 5, 3, 1.

   Answer

   NHY (s[5] = 'n', s[3] = 'h', s[1] = 'y')

8. Q8 · 2 marks · Short answerBoard level

   Write a function INDEX_LIST(L) that returns a list of the indices of all non-zero items of L. For L = [12, 4, 0, 11, 0, 56] it should return [0, 1, 3, 5].

   Hint

   Loop over range(len(L)).

   Answer

   def INDEX_LIST(L):
       R = []
       for i in range(len(L)):
           if L[i] != 0:
               R.append(i)
       return R
   print(INDEX_LIST([12, 4, 0, 11, 0, 56]))

   Output: [0, 1, 3, 5]

9. Q9 · 2 marks · Short answerHOTS / competency

   Predict the output and explain:

   num = 1
   def add_one():
       num = num + 1
       return num
   print(add_one())

   Hint

   The assignment makes num local.

   Answer

   UnboundLocalError: because add_one() assigns to num, num is local throughout the function, and num + 1 reads it before it has a value. Adding global num would print 2.

10. Q10 · 3 marks · Short answerHarder board level

    Write a function stats(L) that returns the minimum, maximum and average of L, and print the three values for [4, 9, 2, 7].

    Hint

    Return three values as one tuple.

    Answer

    def stats(L):
        return min(L), max(L), sum(L) / len(L)
    lo, hi, av = stats([4, 9, 2, 7])
    print(lo, hi, av)

    Output: 2 9 5.5

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 the difference between a parameter and an argument?Show answer

   A parameter is the name in the function header; an argument is the value passed in the call.

2. 2.What does a function return if it has no return statement?Show answer

   None.

3. 3.Why is def f(a=2, b): an error?Show answer

   A parameter without a default cannot follow one with a default (SyntaxError).

4. 4.What type is returned by return x, y?Show answer

   A tuple (x, y).

5. 5.When is the global keyword needed?Show answer

   When a function assigns to a global variable; reading a global needs no declaration.

6. 6.Give one example each of a built-in, a module and a user-defined function.Show answer

   len(); math.sqrt() or random.randint(); any function you write with def, such as area(r).

7. 7.Does a function body run when Python reads its def statement?Show answer

   No. def only creates the function; the body runs on each call.

8. 8.What error do you get if a function reads a name before assigning it locally?Show answer

   UnboundLocalError.

Quick revision checklist

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

• ☐Three types: built-in, module (needs import), user-defined (def).
• ☐Header: def name(params):; body indented; called by name(args).
• ☐Parameters in the header, arguments in the call; positional arguments match by order.
• ☐Default parameters must come after non-default ones.
• ☐return a, b returns a tuple; no return gives None; return ends the function.
• ☐def does not run the body; a call does, and control comes back to the caller.
• ☐Assigned inside a function = local, unless declared global.
• ☐Lists and dictionaries changed in place inside a function stay changed for the caller.

Functions: frequently asked questions