Snake Body ⭐

🎯 Learning Objectives

What students will be able to do after Step 4:

• Represent the snake as a Python list of (x, y) tuples
• Use list.insert(0, ...) and list.pop() to move without growing
• Use list.append() when the snake eats food (grow)
• Detect wall collision (head off-screen) and self-collision (head in body)
• Place food randomly using random.randint()

💡 The Big Idea — Annotated Code

import random

# Snake = list of (col, row) tuples, head first
snake = [(10, 7), (9, 7), (8, 7)]   # starting body, 3 cells

# Food position
food_x = random.randint(0, GRID_COLS - 1)
food_y = random.randint(0, GRID_ROWS - 1)

# --- Move the snake (update step) ---
new_head = (snake[0][0] + dx, snake[0][1] + dy)  # compute new head
snake.insert(0, new_head)       # add new head at front

if new_head == (food_x, food_y):  # ate food → grow
    food_x = random.randint(0, GRID_COLS - 1)
    food_y = random.randint(0, GRID_ROWS - 1)
else:
    snake.pop()                   # no food → remove tail (length stays same)

# --- Collision detection ---
hx, hy = snake[0]

# Wall collision
if hx < 0 or hx >= GRID_COLS or hy < 0 or hy >= GRID_ROWS:
    running = False

# Self collision — is head in the rest of the body?
if snake[0] in snake[1:]:
    running = False

List as a queue

insert(0, head) pushes a new head to the front; pop() removes the tail. The list always represents the full body, newest segment first. This is the "train" model: new engine added at front, last carriage uncoupled at back.

Grow = skip the pop

When food is eaten, we insert the new head but do NOT pop the tail. The list grows by 1 element. The tail stays attached — like the train picking up a new carriage.

snake[1:] — slice without the head

A slice from index 1 to end — the body without the head. Checking snake[0] in snake[1:] asks "is the head position anywhere in the body?" Using snake[0] in snake (without the slice) would always be True and the game would end immediately.

random.randint(a, b)

Returns a random integer between a and b inclusive. So randint(0, GRID_COLS-1) gives a valid column index from 0 to 19 for a 20-column grid.

🔢 Activity Answers — step-4-activity.html

Tier 1 — Check Understanding

Tier 2 — Fix the Bug

Effect: Snake shrinks from the front — it appears to "reverse eat" itself and disappears.

Fix: snake.pop() with no argument removes the last element (the tail).

Tier 3 — Extend: Show score on window title

score = len(snake) - 3   # initial length is 3
pygame.display.set_caption(f"Snake — Score: {score}")

Tier 4 — New Task: Food never spawns on the snake

def spawn_food(snake):
    while True:
        fx = random.randint(0, GRID_COLS - 1)
        fy = random.randint(0, GRID_ROWS - 1)
        if (fx, fy) not in snake:
            return fx, fy

food_x, food_y = spawn_food(snake)
# After eating:
food_x, food_y = spawn_food(snake)

Reflective Journal — Suggested Answers

⚠️ Mistakes Students Actually Make

⭐ Metacognitive Hotspot — Teaching Notes

Why this step is hard

Step 4 introduces THREE new concepts simultaneously (lists, random, collision detection) and combines them with Step 3's movement. Students are holding more state in their heads than before. This is the cognitive load peak of the tutorial.

Normalising struggle

The "train" mental model

A snake body is like a train. Each frame: a new engine is added to the front, and the last carriage is uncoupled from the back. When the train picks up a passenger (food), the last carriage stays on — the train grows by one.

Collaborative strategy

Consider asking students to trace through the list state on paper for 3–4 frames before writing code. Draw the list as boxes with (x, y) values. This externalises the mental model and surfaces the insert/pop pattern concretely.

CT connection — Data structures as design

Checkpoint before Step 5

Every student should have a working game where the snake grows on food and the game ends on wall/self collision. Do a quick visual check before allowing anyone to advance. This is a natural assessment point.