Traffic Light Stack

By the end of this lab you'll be able to:

• Break a real-world object into drawing primitives (rectangle + circles)
• Write a light(y, color) function and call it three times to assemble a scene
• Position shapes vertically using a y-coordinate argument

A classic three-light traffic signal stands on screen — red on top, amber in the middle, green at the bottom — built from one rectangle and three function calls.

Welcome to the Traffic Light!

Every complex drawing is really just simple shapes stacked together. In this lab you'll decompose a traffic light into primitives and assemble it with functions! Let's code it together!

How the Light Works

The housing is a tall dark rectangle drawn first. Then light(y, color) draws one filled circle at vertical position y. Three calls to light() with different y values and colors stack the three signals inside the housing.

Sample Code

import turtle
monty = turtle.Turtle()
monty.speed(0)
monty.hideturtle()

def rect(x, y, w, h, color):
    monty.penup()
    monty.goto(x, y)
    monty.pendown()
    monty.color(color)
    monty.begin_fill()
    for side in [w, h, w, h]:
        monty.forward(side)
        monty.left(90)
    monty.end_fill()

def light(y, color):
    monty.penup()
    monty.goto(0, y - 25)
    monty.dot(50, color)

rect(-35, -140, 70, 290, 'dimgray')
light(100,  'red')
light(  0,  'orange')
light(-100, 'green')

What Do You Think Will Happen?

light(y, color) draws a dot at (0, y - 25). Why subtract 25? Think about where dot() is centered before clicking Run — then check your answer!

Try It Now

▶ Run ↻ Reset

dot() is centered at the turtle's position. Subtracting 25 (half the dot diameter of 50) nudges the center to align visually with the y value passed in. Were you right?

How It Works

Drawing the housing rectangle first ensures it appears behind the lights. Because fills are opaque, the colored dots drawn afterward appear on top of the gray rectangle — correct layering without any extra tricks.

The rect() helper reuses the same for side in [w, h, w, h] pattern seen in the brick wall and skyline labs — once you learn a pattern, it works everywhere.

Explanation Table

Line	What it does
rect(x, y, w, h, color)	Draw a filled rectangle — reusable for any box shape
rect(-35, -140, 70, 290, 'dimgray')	The housing: 70px wide, 290px tall
def light(y, color)	Draw one signal dot at the given y position
monty.dot(50, color)	Filled circle of diameter 50 at current position
light(100, 'red')	Top light (highest y = highest on screen)

Learning Check

Your Turn — Add a Fourth Signal

Some railway signals have a fourth white light at the very bottom. Add one more light() call to put a white dot at y = -180, and extend the housing to fit.

▶ Run ↻ Reset

Add light(-150, 'white') as the last line.

Experiments

1. Make a pedestrian signal. Replace the three colored lights with two — light(60, 'red') and light(-60, 'green'). Make the housing taller and narrower. You'll know it worked when a walk/don't-walk signal appears.

2. Add a pole. Before drawing the housing, draw a thin tall gray rectangle from the bottom of the housing to the bottom of the canvas. You'll know it worked when the traffic light appears mounted on a pole.

3. Add glow rings. Before each dot() call, draw a slightly larger dot in a lighter shade of the same color. You'll know it worked when each light appears to have a halo.

4. Show only one active light. Change two of the light colors to 'darkgray' so only one is "on". You'll know it worked when two lights are dim and one is bright.

Great Work!

You decomposed a real-world object into layers of primitives and assembled it with functions — the exact workflow used by every UI designer and game artist! Up next: Fish School — drawing a creature shape with a function.