Functions

🎯 Learning Objectives

• Define functions with def and a parameter list
• Explain the DRY (Don’t Repeat Yourself) principle with a concrete example
• Distinguish between parameters (definition) and arguments (call)
• Understand local vs. global scope
• Refactor drawing and collision code into three named functions

💡 The Big Idea — Annotated Code

def draw_snake(surface, snake):
    """Draw every body segment; head is darker green."""
    for i, (col, row) in enumerate(snake):
        colour = DKGREEN if i == 0 else GREEN
        rect = (col * CELL_SIZE, row * CELL_SIZE, CELL_SIZE - 1, CELL_SIZE - 1)
        pygame.draw.rect(surface, colour, rect)

def draw_food(surface, food_x, food_y):
    """Draw the food square."""
    rect = (food_x * CELL_SIZE, food_y * CELL_SIZE, CELL_SIZE - 1, CELL_SIZE - 1)
    pygame.draw.rect(surface, RED, rect)

def check_wall_collision(hx, hy):
    """Return True if the head is outside grid boundaries."""
    return hx < 0 or hx >= GRID_COLS or hy < 0 or hy >= GRID_ROWS

# ── Main loop — clean and readable ──
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    screen.fill(BG)
    draw_snake(screen, snake)
    draw_food(screen, food_x, food_y)
    pygame.display.flip()
    clock.tick(8)

    hx, hy = snake[0]
    if check_wall_collision(hx, hy) or snake[0] in snake[1:]:
        running = False

def function_name(params):

Defines the function. The body is indented below.

Parameters vs arguments

surface and snake are parameters (placeholders in the definition); screen and snake (the actual list) are arguments passed at the call.

DRY — Don’t Repeat Yourself

Drawing code was scattered through the loop. Functions give it a name — draw_snake() tells you WHAT it does without reading HOW.

enumerate(snake)

Yields (index, value) pairs — lets us colour the head (index 0) differently.

Docstrings """..."""

The string immediately after def is Python’s built-in documentation mechanism.

return value

check_wall_collision returns True or False so the caller can decide what to do.

🔢 Activity Answers (step-6-activity.html)

Tier 1 — Check Understanding

Tier 2 — Fix the Bug

Error: TypeError: draw_snake() takes 1 positional argument but 2 were given

Fix: def draw_snake(surface, snake):

Tier 3 — Extend

Add draw_score(surface, score) that renders the score as text.

def draw_score(surface, score):
    font = pygame.font.SysFont("monospace", 20)
    text = font.render(f"Score: {score}", True, WHITE)
    surface.blit(text, (8, 8))

# In main loop after screen.fill():
draw_score(screen, score)

Tier 4 — New Task

Create game_over(surface) that displays “GAME OVER” centred and waits 2 seconds.

def game_over(surface):
    font = pygame.font.SysFont("monospace", 36, bold=True)
    msg = font.render("GAME OVER", True, RED)
    rect = msg.get_rect(center=(WINDOW_W // 2, WINDOW_H // 2))
    surface.blit(msg, rect)
    pygame.display.flip()
    pygame.time.wait(2000)

# After running = False:
game_over(screen)

Reflective Journal — Suggested Strong Answers

⚠️ Mistakes Students Actually Make

🗣️ Teaching & Facilitation Notes

Opening hook

Display the Step 5 loop (messy, long) next to the Step 6 loop (short, readable). Ask: “Which is easier to read and debug?” The visual difference makes DRY concrete without explanation.

The recipe analogy

A function is a recipe. Write it once; follow it as many times as needed. The ingredient list = parameters; making the dish = calling the function.

Key pedagogical moment — scope

Demonstrate: define x = 5 inside a function, then try to print x outside it → NameError. Local variables exist only while the function runs.

CT connection — Abstraction

Functions hide complexity behind a name. Ask: “Now that we have draw_snake(), do you need to know HOW it draws to use it in the loop?” This is abstraction.

Pacing

This step is mostly refactoring — no new visual features. Students who expect to see something change may feel disappointed. Help them see the value in code organisation: “Clean code is a gift to your future self.”