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4.2 USER-DEFINED FUNCTIONS IN PYTHON

What is a Function?

A function is a block of organized and reusable code designed to perform a specific task.

Functions help reduce repetition and make programs easier to manage.

 

Advantages of Functions

v  Reuse code multiple times.

v  Reduce code duplication.

v  Improve program readability.

v  Make debugging easier.

v  Support modular programming.

 

Modularity

Modularity means dividing a large program into smaller and manageable parts called functions or modules.

 

Types of Functions in Python

There are two types of functions:

v  Built-in Functions

v  User-defined Functions

 

1. Built-in Functions (Library Functions)

Functions already provided by Python are called built-in functions.

These functions are automatically available.

 

Common Built-in Functions

Function	Purpose	Examples
print( )	Displays output	print("Hello")
input( )	Takes input
len( )	Returns length	len("Python")
int( )	Converts into integer	int("5")
sum( )	Calculates total	sum([1,2,3])

 

2. User-defined Functions

A function created by the programmer using the def keyword is called a user-defined function.

Example

def greet( ):
    print("Hello")

 

Difference Between Built-in and User-defined Functions

Feature	Built-in Function	User-defined Function
Creator	Python Developers	Programmer
Availability	Available by default	Must be created
Loading	Loaded automatically	Loaded when defined
Example	print( )	add( )

Creating a User-defined Function

Syntax

def function_name(parameters):
    statements
    return value

 

Example Program

def add_numbers(x, y):
    total = x + y
    return total

print(add_numbers(5, 6))

Output

11

 

Important Rules of Functions

v  Function definition starts with def

v  Function header ends with colon :

v  Indentation is compulsory

v  Function name should be meaningful

v  Parameters are optional

v  Return statement is optional

 

Return Statement

A statement used to send a value back from a function.

Syntax

return value

 

Function Returning Value

Example

def area(length, width):
    return length * width

result = area(5, 8)

print(result)

Output

40

 

Parameters and Arguments

 

Parameter

A variable written inside the function definition is called a parameter.

Example

def area(radius):

radius is a parameter.

 

 

 

Argument

The actual value passed during the function call is called an argument.

Example

area(5)

5 is an argument.

 

Difference Between Parameter and Argument

Parameter	Argument
Placeholder	Actual value
Used in definition	Used in function call
Variable	Value

 

Example

def add(x, y):
    return x + y

add(5, 3)

Item	Type
x, y	Parameters
5, 3	Arguments

 

Scope of Variables

Scope determines where a variable can be accessed.

Types of Scope

v  Local Scope

v  Global Scope

 

Local Scope

Variables declared inside a function and accessible only within that function.

Example

def test():
    num = 5
    print(num)

test( )

 

Global Scope

Variables declared outside functions and accessible throughout the program.

Example

num = 10

def show( ):
    print(num)

show( )

 

 

 

Passing Arguments

Python supports three types of arguments.

v  Positional Arguments

v  Default Arguments

v  Keyword Arguments

 

1. Positional Arguments

Arguments are passed in the same order as parameters.

Example

def check(a, b, c):
    print(a, b, c)

check(2, 5, 7)

Output

2 5 7

 

2. Default Arguments

Parameters are assigned default values.

Example

def power(base, exp=2):
    return base ** exp

print(power(4))
print(power(2,3))

Output

16
8

 

Important Rule for Default Arguments

✅ Required parameters first
✅ Default parameters last

 

Legal Function

def interest(prin, time, rate=0.10):

Illegal Function

def interest(prin, time=2, rate):

❌ Error occurs because default arguments must come after required arguments.

 

3. Keyword Arguments

Arguments are passed using parameter names.

Order does not matter.

Example

def interest(prin, time, rate):
    pass

interest(rate=0.10, prin=2000, time=2)

 

 

Types of Functions Based on Return Value

v  Void Function

v  Non-Void Function

 

Void Function

A function that does not return any value.

Example

def greet( ):
    print("Hello")

Python automatically returns None.

Example

def greet():
    print("Hello")

a = greet( )

print(a)

Output

Hello
None

 

Non-Void Function

A function that returns a value.

Example

def add(x, y):
    return x + y

result = add(5,3)

 

Returning Multiple Values

Python can return multiple values using one return statement.

Example

def squared(x, y, z):
    return x*x, y*y, z*z

print(squared(2,5,7))

Output

(4, 25, 49)

 

Example with Variables

def squared(x, y, z):
    return x*x, y*y, z*z

a, b, c = squared(2,3,4)

print(a, b, c)

Output

4 9 16

 

Common Exam Mistakes ❌

v  Forgetting colon : after function definition

v  Wrong indentation

v  Confusing parameter and argument

v  Writing default arguments before required arguments

v  Forgetting to call the function

 

Most Important Programs for SEE Exam

Program 1: Add Two Numbers

def add(a, b):
    return a + b

print(add(5, 3))

 

Program 2: Find Area of Rectangle

def area(length, width):
    return length * width

print(area(5, 8))

 

Program 3: Default Argument

def power(base, exp=2):
    return base ** exp

print(power(4))

 

Program 4: Keyword Argument

def student(name, age):
    print(name, age)

student(age=15, name="Ram")

 

Program 5: Local and Global Variable

x = 10

def show( ):
    y = 5
    print(x)
    print(y)

show( )