Python Functions

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Python Function Introduction

A function in Python is a block of reusable code that performs a specific task. Functions help organize code, reduce repetition, and improve readability.

1. Why Use Functions?

✔ Avoid repeating code
✔ Break large programs into small logical pieces
✔ Improve readability and maintainability
✔ Make code scalable and easier to debug

2. Defining and Calling a Function

Syntax

def function_name(parameters):
    # code block
    return value

Example

def greet():
    print("Hello, welcome to Python!")
    
greet()     # Calling the function

3. Function with Parameters (Arguments)

def greet_user(name):
    print("Hello", name)

greet_user("John")

Output

Hello John

4. Function with Return Value

def add(x, y):
    return x + y

result = add(10, 5)
print(result)

Output

15

5. Default Parameters

If no value is passed, default will be used.

def info(country="India"):
    print("I am from", country)

info()
# output :   I am from India

info("Japan")
# output : I am from Japan

6. Keyword Arguments

Arguments can be passed using the parameter name.

def student(name, age):
    print(name, "is", age, "years old")

student(age=21, name="Riya")

# Output :  Riya is 21 year old.

7. Arbitrary Arguments (*args)

Used when you don’t know how many arguments will be passed.

def total_marks(*marks):
    print("Total:", sum(marks))

total_marks(80, 75, 90, 60)

# Output :  Total : 305

8. Arbitrary Keyword Arguments (**kwargs)

Accepts key-value pairs (dictionary-like).

def profile(**details):
    for key, value in details.items():
        print(key, ":", value)

profile(name="Amit", age=25, city="Delhi")

# Output :

name : Amit
age : 25
city : Delhi

9. Lambda (Anonymous) Functions

Short, one-line functions without a name.


square = lambda x: x * x
print(square(6))

# Output : 36

10. Nested Functions

Function inside another function.

def outer():
    print("Outer Function")
    
    def inner():
        print("Inner Function")
    
    inner()

outer()

11. Recursion (Function Calling Itself)

def factorial(n):
    if n == 1:
        return 1
    return n * factorial(n - 1)

print(factorial(5)) 

# Output : 120

12. Passing a Function as an Argument

def apply(func, value):
    return func(value)

def double(x):
    return x * 2

print(apply(double, 4))   # Output: 8

Best Practices

PracticeDescription
Use meaningful namescalculate_total() is better than ct()
Keep functions shortOne task per function
Document your functionWrite comments or use docstrings
Avoid global variablesPrefer passing parameters

Docstring Example

def multiply(a, b):
    """Returns the product of two numbers"""
    return a * b

print(multiply.__doc__)

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