What is the difference between a parameter and an argument?

A parameter is the variable name in the function definition. An argument is the actual value you pass when calling the function.

What does a function return if it has no return statement?

It returns None, Python's value that represents 'nothing'.

Python Functions: Arguments, Return Values, and Scope

Introduction

Functions let you package a piece of logic under a name so you can reuse it instead of copying code. This tutorial covers defining functions, parameters and default values, returning results, and how Python decides which variables a function can see.

Prerequisites

Python 3.8 or newer installed
Basic Python syntax (variables, print, indentation)

Step-by-Step Guide

Step 1: Define and call a function

Use the def keyword, a name, parentheses for parameters, and an indented body. Call it by name with arguments.

def greet(name):
    return "Hello, " + name

print(greet("Ava"))   # Hello, Ava

Step 2: Use default parameter values

Give a parameter a default so callers can leave it out. Parameters with defaults must come after those without.

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

print(power(5))       # 25
print(power(2, 3))    # 8

Step 3: Return multiple values

A function can return several values separated by commas. Python packages them in a tuple, which you can unpack into separate variables.

def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([4, 9, 1, 7])
print(low, high)      # 1 9

Step 4: Accept any number of arguments

Prefix a parameter with * to collect extra positional arguments into a tuple.

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

print(total(1, 2, 3, 4))   # 10

Step 5: Understand variable scope

Variables created inside a function are local: they exist only while the function runs. Prefer returning values over modifying global variables.

def double(x):
    result = x * 2    # result exists only inside this function
    return result

print(double(4))      # 8
# print(result)       # NameError: result is not defined here

FAQs

What is the difference between a parameter and an argument?: A parameter is the variable name in the function definition. An argument is the actual value you pass when calling the function.
What does a function return if it has no return statement?: It returns None, Python's value that represents 'nothing'.

Summary

Functions keep code reusable and readable. Use clear names, give optional parameters sensible defaults, return results instead of printing them inside the function, and keep variables local wherever possible.