Functions — Defining and Using

Why read this? As programs grow, you can't keep everything in one long script. Functions are your way of organizing, reusing, and testing code. Think of them as your own personal commands — you define them once and use them everywhere.

🎯 Learning Objectives

1. Define functions with def and return values with return
2. Understand parameters vs arguments
3. Understand local vs global variable scope
4. Write functions that solve specific problems
5. Document functions with docstrings

📋 Prerequisites

• All previous topics — functions use everything you've learned.

📖 Core Content

21.1 What Problem Do Functions Solve?

21.2 Defining and Calling Functions

python
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# runnable
def greet():
    """Print a greeting."""  # docstring
    print("Hello! Welcome to Python.")
# Call the function
greet()
greet()  # can call multiple times

pseudo
Hello! Welcome to Python.
Hello! Welcome to Python.

21.3 Functions with Parameters

python
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# runnable
def greet_person(name):
    """Greet a specific person."""
    print(f"Hello, {name}!")
greet_person("Alice")
greet_person("Bob")
def add(a, b):
    """Return the sum of a and b."""
    result = a + b
    return result
total = add(5, 3)
print(f"5 + 3 = {total}")

21.4 Return Values

python
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# runnable
def square(n):
    return n ** 2
def is_even(n):
    return n % 2 == 0
# Functions can return any type
def get_stats(numbers):
    return min(numbers), max(numbers), sum(numbers) / len(numbers)
print(square(5))                # 25
print(is_even(7))               # False
print(get_stats([1, 2, 3, 4, 5]))  # (1, 5, 3.0)

21.5 Variable Scope

python
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# runnable
x = 10  # GLOBAL variable
def my_func():
    y = 5   # LOCAL variable
    print(f"Inside: x={x}, y={y}")
my_func()
print(f"Outside: x={x}")
# print(y)  # NameError: y is not defined
def modify_global():
    global x  # needed to modify global
    x = 20
modify_global()
print(f"After modify: x={x}")  # 20

1. Local — inside current function
2. Enclosing — outer functions (for nested functions)
3. Global — module level
4. Built-in — Python's built-in names

21.6 Docstrings

python
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# runnable
def calculate_bmi(weight, height):
    """
    Calculate Body Mass Index.
    Args:
        weight: Weight in kilograms
        height: Height in meters
    Returns:
        BMI as a float
    """
    return weight / (height ** 2)
print(calculate_bmi(70, 1.75))
print(calculate_bmi.__doc__)  # prints the docstring

21.7 Worked Example 1: Temperature Converter Functions

python
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# runnable
def celsius_to_fahrenheit(c):
    return c * 9/5 + 32
def fahrenheit_to_celsius(f):
    return (f - 32) * 5/9
print(f"100°C = {celsius_to_fahrenheit(100):.1f}°F")
print(f"212°F = {fahrenheit_to_celsius(212):.1f}°C")

21.8 Worked Example 2: Prime Checker Function

python
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# runnable
def is_prime(n):
    """Check if n is prime."""
    if n < 2:
        return False
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:
            return False
    return True
# Test
for n in range(1, 20):
    if is_prime(n):
        print(n, end=" ")

21.9 Worked Example 3: Functions Calling Functions

python
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# runnable
def factorial(n):
    result = 1
    for i in range(1, n + 1):
        result *= i
    return result
def combination(n, r):
    """nCr = n! / (r! * (n-r)!)"""
    return factorial(n) // (factorial(r) * factorial(n - r))
print(f"5C2 = {combination(5, 2)}")  # 10
print(f"10C3 = {combination(10, 3)}")  # 120

21.10 Worked Example 4: Validation Wrapper

python
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# runnable
def get_positive_number(prompt):
    """Keep asking until user enters a positive number."""
    while True:
        try:
            num = float(input(prompt))
            if num > 0:
                return num
            print("Number must be positive.")
        except ValueError:
            print("Invalid input. Enter a number.")
radius = get_positive_number("Enter radius: ")
area = 3.14159 * radius ** 2
print(f"Area: {area:.2f}")

⚠️ Common Pitfalls

Pitfall 1: Forgetting return

python
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def add(a, b):
    result = a + b  # no return!
print(add(5, 3))  # None

Pitfall 2: Modifying Global Variables Inside Functions

Pitfall 3: Mutable Default Arguments

📝 Practice Questions

Q1: What does this function return?

python
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def mystery(x, y):
    return x * 2 + y
print(mystery(3, 4))

Answer: 10 (3*2 + 4 = 10) Q2: Write a function is_palindrome(s) that checks if a string is a palindrome.

Answer:

python
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def is_palindrome(s):
    s = s.lower().replace(" ", "")
    return s == s[::-1]

Q3-10: Additional function questions.

(Following pattern.)

🔗 Cross-References

• Next Topic: Function Arguments
• Previous Topic: Dict Operations
• Reference: Python for Everybody, Chapter 4 — "Functions"
• Video: L51: Introduction to functions, L49: More examples of functions Join Discord Previous20. Dict OperationsNext22. Function Arguments