We’re going to build a program that uses a turtle in python to simulate the traffic lights.
There will be four states in our traffic light: Green, then Green and Orange together, then Orange only, and then Red. The light should spend 3 seconds in the Green state, followed by one second in the Green+Orange state, then one second in the Orange state, and then 2 seconds in the Red state.
import turtle # Allows us to use turtles
turtle.setup(400, 600) # Determine the window size
wn = turtle.Screen() # Creates a playground for turtles
wn.title('traffic light using different turtles') # Set the window title
wn.bgcolor('skyblue') # Set the window background color
tess = turtle.Turtle() # Create a turtle, assign to tess
alex = turtle.Turtle() # Create alex
henry = turtle.Turtle() # Create henry
def draw_housing():
""" Draw a nice housing to hold the traffic lights"""
tess.pensize(3) # Change tess' pen width
tess.color('black', 'white') # Set tess' color
tess.begin_fill() # Tell tess to start filling the color
tess.forward(80) # Tell tess to move forward by 80 units
tess.left(90) # Tell tess to turn left by 90 degrees
tess.forward(200)
tess.circle(40, 180) # Tell tess to draw a semi-circle
tess.forward(200)
tess.left(90)
tess.end_fill() # Tell tess to stop filling the color
draw_housing()
def circle(t, ht, colr):
"""Position turtle onto the place where the lights should be, and
turn turtle into a big circle"""
t.penup() # This allows us to move a turtle without drawing a line
t.forward(40)
t.left(90)
t.forward(ht)
t.shape('circle') # Set tutle's shape to circle
t.shapesize(3) # Set size of circle
t.fillcolor(colr) # Fill color in circle
circle(tess, 50, 'green')
circle(alex, 120, 'orange')
circle(henry, 190, 'red')We’re going to use the concept of state machine.
A state machine is a system that can be in one of a few different states.
This idea is not new: for example, when first turning on a cellphone, it goes into a state which we could call “Awaiting PIN”. When the correct PIN is entered, it transitions into a different state — say “Ready”. Then we could lock the phone, and it would enter a “Locked” state, and so on.
A traffic light is a kind of state machine with four states: Green, then Green+Orange, then Orange only, and then Red. We number these states 0, 1, 2 and 3. When the machine changes state, we change turtle’s position and its color.
# This variable holds the current state of the machine
state_num = 0
def advance_state_machine():
"""A state machine for traffic light"""
global state_num # Tells Python not to create a new local variable for state_num
if state_num == 0: # Transition from state 0 to state 1
henry.color('darkgrey')
alex.color('darkgrey')
tess.color('green')
wn.ontimer(advance_state_machine, 3000) # set the timer to explode in 3 sec
state_num = 1
elif state_num == 1: # Transition from state 1 to state 2
henry.color('darkgrey')
alex.color('orange')
wn.ontimer(advance_state_machine, 1000)
state_num = 2
elif state_num == 2: # Transition from state 2 to state 3
tess.color('darkgrey')
wn.ontimer(advance_state_machine, 1000)
state_num = 3
else: # Transition from state 3 to state 0
henry.color('red')
alex.color('darkgrey')
wn.ontimer(advance_state_machine, 2000)
state_num = 0
advance_state_machine()Now we need to tell the window to start listening for events.
wn.listen() # Listen for events
wn.mainloop() # Wait for user to close windowOur traffic light will look like this:
Below is the same program in interactive mode with minor modifications:
Resources:
The above program is an exercise from the book Think Python.