1.2 Modules and turtle graphics

Big ideas

• A module is a file of ready-made code. import math then math.sqrt(2); or from math import sqrt to use sqrt(2) directly; or import random as r to shorten the name.
• math has sqrt, floor, ceil, pi, sin, cos; random has randint(a, b) (both ends included), random() (0 up to 1) and choice(list).
• The turtle module draws by moving a pen: forward(d), backward(d), left(deg), right(deg), penup()/pendown(), goto(x, y), circle(r), color(...), begin_fill()/end_fill().
• The turtle starts at (0, 0) facing east (right); angles turn counter-clockwise for left. The screen’s y axis points up, unlike most graphics systems.
• Events make drawings interactive: screen.onkey(func, "Up"), screen.onclick(func), screen.ontimer(func, ms), then screen.listen() and screen.mainloop(). You pass the function name, without parentheses.

Code you should know

import turtle

t = turtle.Turtle()
t.color("black", "gold")
t.begin_fill()
for _ in range(4):
    t.forward(100)
    t.left(90)
t.end_fill()
turtle.done()

A gold square appears in the picture pane.

import math
import random

print(math.sqrt(144), math.pi, math.floor(3.7), math.ceil(3.2))
roll = random.randint(1, 6)
print("You rolled", roll)
print(random.choice(["rock", "paper", "scissors"]))

12.0 3.141592653589793 3 4
You rolled 4
paper

import turtle

t = turtle.Turtle()
t.pensize(3)
t.color("purple")
for _ in range(5):
    t.forward(150)
    t.right(144)
t.hideturtle()

A five-pointed star.

Worked example

Common mistakes

• Writing screen.onkey(move(), "Up") with parentheses: that calls move once and registers its return value (None). Pass move.
• Forgetting screen.listen(), so key events never arrive.
• Calling math.sqrt after from math import sqrt, or sqrt after plain import math.
• randint(1, 6) includes 6; range(1, 6) does not.