Unit 11.3A · Term 3

Defining Functions

A function is a reusable block of code that performs a specific task. Functions make programs shorter, more organized, and easier to debug by breaking complex problems into smaller, named pieces.

Learning Objectives

11.3.2.1 Create functions using the def keyword

Lesson Presentation

11.3A-lesson-17-functions-define.pdf · Slides for classroom use

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Conceptual Anchor

The Recipe Analogy

A function is like a recipe: you define it once (write the recipe), then use it whenever needed (cook the dish). The ingredients are parameters, and the finished dish is the return value. You don't rewrite the recipe every time you cook — you just call it by name!

Rules & Theory

Function Syntax

# Defining a function
def greet(name):         # def keyword, function name, parameters
    """Greets a person by name."""   # Docstring (optional)
    print(f"Hello, {name}!")

# Calling a function
greet("Ali")       # Hello, Ali!
greet("Dana")      # Hello, Dana!

# Function with no parameters
def say_hello():
    print("Hello, World!")

say_hello()        # Hello, World!

Parameters & Arguments

# Multiple parameters
def add(a, b):
    return a + b

result = add(10, 20)    # 30

# Default parameter values
def power(base, exp=2):
    return base ** exp

print(power(5))         # 25 — uses default exp=2
print(power(5, 3))      # 125 — overrides exp=3

# Keyword arguments
def greet(name, greeting="Hello"):
    print(f"{greeting}, {name}!")

greet("Ali")                        # Hello, Ali!
greet("Ali", greeting="Hi")         # Hi, Ali!
greet(greeting="Hey", name="Dana")  # Hey, Dana!

Return Values

# Return a single value
def square(n):
    return n ** 2

result = square(5)    # 25

# Return multiple values (as tuple)
def divide(a, b):
    quotient = a // b
    remainder = a % b
    return quotient, remainder

q, r = divide(17, 5)
print(f"17 ÷ 5 = {q} remainder {r}")  # 17 ÷ 5 = 3 remainder 2

# Return vs Print
def bad_add(a, b):
    print(a + b)       # Prints but returns None!

def good_add(a, b):
    return a + b        # Returns the value

x = bad_add(3, 4)      # Prints 7, but x = None!
y = good_add(3, 4)     # y = 7

Concept	Description	Example
Parameter	Variable in function definition	`def f(x):` — x is parameter
Argument	Value passed in function call	`f(5)` — 5 is argument
Return	Send a value back to caller	`return x + 1`
None	Returned if no explicit return	Functions with only print()

return vs print

return sends a value back to the code that called the function. print() just displays text on screen. A function that print()s without returning gives None when you try to use its result.

Worked Examples

1 Temperature Converter

def celsius_to_fahrenheit(c):
    return c * 9/5 + 32

def fahrenheit_to_celsius(f):
    return (f - 32) * 5/9

temp = float(input("Temperature: "))
unit = input("C or F? ").upper()

if unit == "C":
    result = celsius_to_fahrenheit(temp)
    print(f"{temp}°C = {result:.1f}°F")
else:
    result = fahrenheit_to_celsius(temp)
    print(f"{temp}°F = {result:.1f}°C")

2 Grade Calculator

def get_grade(score):
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"

def get_feedback(grade):
    feedback = {
        "A": "Excellent!", "B": "Good job!",
        "C": "Satisfactory", "D": "Needs improvement",
        "F": "Please see the teacher"
    }
    return feedback.get(grade, "Unknown")

score = int(input("Enter score: "))
grade = get_grade(score)
print(f"Grade: {grade} — {get_feedback(grade)}")

3 Input Validation Function

def get_int(prompt, low=None, high=None):
    """Get a valid integer from user within optional range."""
    while True:
        try:
            value = int(input(prompt))
            if low is not None and value < low:
                print(f"Must be at least {low}")
            elif high is not None and value > high:
                print(f"Must be at most {high}")
            else:
                return value
        except ValueError:
            print("Please enter a valid integer")

age = get_int("Enter age (1-120): ", 1, 120)
print(f"Your age is {age}")

Pitfalls & Common Errors

Calling Before Defining

You must define a function before calling it. Python reads top to bottom — calling greet() above its def causes NameError.

Forgetting return

If your function doesn't have a return statement, it returns None. Writing x = my_func() where my_func only prints gives x = None.

Forgetting Parentheses

greet is the function object. greet() actually calls it. Without (), the function is not executed.

Pro-Tips for Exams

Function Tips

A function should do one thing well — keep functions focused
Use return for values you need to use later; print for output to screen
Default parameters must come after required parameters
A function stops executing at the first return it reaches
Use descriptive function names: calculate_average() not calc()

Graded Tasks

Remember

What are the 3 parts of a function definition? What is the difference between a parameter and an argument?

Understand

Explain why x = print("hello") sets x to None.

Apply

Write a function is_even(n) that returns True if n is even, False otherwise.

Apply

Write a function factorial(n) that returns n! using a loop.

Analyze

Trace: def f(x, y=2): return x * y. What is f(3)? f(3, 4)? f(y=5, x=2)?

Create

Create a mini-calculator with separate functions for addition, subtraction, multiplication, and division. Use a menu loop.

Self-Check Quiz

1. What keyword defines a function in Python?

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2. What does a function return if there is no return statement?

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3. What is the difference between def f(x) and f(5)?

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4. Can a function return more than one value?

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5. What happens if you call a function without parentheses?

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