Functions in Python

Introduction

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

Defining a Function

def greet(name):
    """Display a greeting."""
    print(f"Hello, {name}!")

Calling a Function

greet("Alice")  # Hello, Alice!

Return Values

# Single return value
def add(a, b):
    return a + b

result = add(3, 5)  # 8

# Multiple return values (tuple)
def min_max(numbers):
    return min(numbers), max(numbers)

low, high = min_max([3, 1, 4, 1, 5])
print(low, high)  # 1 5

# Early return
def divide(a, b):
    if b == 0:
        return "Cannot divide by zero"
    return a / b

Docstrings

def calculate_bmi(weight: float, height: float) -> float:
    """Calculate Body Mass Index (BMI).

    Args:
        weight: Weight in kilograms
        height: Height in meters

    Returns:
        BMI value as a float

    Example:
        >>> calculate_bmi(70, 1.75)
        22.86
    """
    return weight / (height ** 2)

Type Hints

def process_data(name: str, age: int, scores: list[float]) -> dict[str, float]:
    """Process user data and return statistics."""
    return {
        "name": name,
        "average": sum(scores) / len(scores),
        "age_category": "adult" if age >= 18 else "minor"
    }

Best Practices

Functions should do one thing — single responsibility principle
Use descriptive names — calculate_total() not calc()
Keep functions short — ideally under 20-30 lines
Always include docstrings for non-trivial functions
Use type hints to make code self-documenting
Avoid side effects — prefer returning values over modifying globals

Practice Exercises

Exercise 1: Write a function is_palindrome(s) that checks if a string is a palindrome.

Exercise 2: Create a function that takes a list of numbers and returns the second largest number.

Exercise 3: Build a simple calculator with functions for each operation.