Python Function Definition and Calling

Python Functions


What is a Function in Python?

A function in Python is a reusable block of code that performs a specific task. Functions help reduce redundancy, increase modularity, and make code easier to maintain.


Python def Keyword - Function Definition

To define a function, use the def keyword, followed by the function name and parentheses ().

Syntax:

def function_name(parameters):
    # function body
    return value  # optional

Example: Define a Simple Function

def greet():
    print("Hello, welcome to Python!")


Calling a Function in Python

To execute the function code, simply call the function by its name followed by parentheses.

greet()

Output:

Hello, welcome to Python!

Function with Parameters

Functions can accept inputs called parameters to perform operations dynamically.

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

Calling:

greet_user("Alice")

Output:

Hello Alice

Function with Return Statement

The return statement sends a result back to the caller.

def add(a, b):
    return a + b

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

Output:

15

Function with Default Parameters

You can provide default values for parameters.

def greet(name="Guest"):
    print("Welcome", name)

Calls:

greet("John")   # Output: Welcome John
greet()         # Output: Welcome Guest


Function with Multiple Return Values

Python allows functions to return multiple values using tuples.

def get_user():
    name = "Alice"
    age = 25
    return name, age

n, a = get_user()
print(n, a)

Output:

Alice 25

Types of Function Arguments

Type Description Example
Positional Passed in correct position add(2, 3)
Keyword Passed using key=value format add(b=3, a=2)
Default Uses default if no value is passed greet(name="Sam")
Variable Length Accepts many arguments using *args def add(*nums):

Example: Variable-Length Arguments

def add_all(*numbers):
    total = sum(numbers)
    return total

print(add_all(1, 2, 3, 4))  # Output: 10

Best Practices for Writing Python Functions

  • Use meaningful function names - e.g., calculate_area()
  • Keep functions small - one task per function
  • Use docstrings - describe what the function does
  • Avoid side effects - return values instead of printing
  • Use default and keyword arguments for flexibility