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Objects with turtles: cheat sheet

Objects with turtles · In any playground, open Reference to copy these.

Many things on screen

left

Turn anticlockwise by this many degrees.

left(90)
Example
forward(60)
left(90)
forward(60)
What it draws

penup

Lift the pen so moving does not draw.

penup()
Example
forward(40)
penup()
forward(40)
pendown()
forward(40)
What it draws
Pen up for the middle part

for loop

Repeat the indented lines this many times.

for i in range(4):
    pass
Example
for i in range(5):
    forward(15)
    penup()
    forward(10)
    pendown()
What it draws
range(5): five dashes

every other

i % 2 == 0 is true on every second pass.

for i in range(8):
    if i % 2 == 0:
        pass
    else:
        pass
Example
for i in range(8):
    if i % 2 == 0:
        pencolor("red")
    else:
        pencolor("blue")
    pensize(6)
    forward(30)
    right(45)
What it draws
Red and blue sides, taking turns

counter step

Use the counter i to make each pass different.

for i in range(20):
    forward(i * 5)
    right(90)
Example
for i in range(30):
    forward(i * 5)
    right(90)
What it draws
A square spiral: each line longer than the last

list

Keep many values in order under one name. The first item is place 0.

colours = ["red", "blue", "green"]

for each item

Work through a list, one item at a time.

for c in colours:
    pass
Example
pensize(6)
for c in ["red", "blue", "green"]:
    pencolor(c)
    forward(40)
What it draws
One line per colour in the list

print

Show text or a value in the output box.

print("Hello")

random number

Pick a random whole number between two values.

import random

n = random.randint(1, 10)

onkey

Run a function every time a key is pressed. Click the picture first so it receives keys.

def go_up():
    forward(20)

onkey(go_up, "Up")
listen()
mainloop()
Example
def go_up():
    forward(20)

go_up()
go_up()
go_up()
What it draws
After pressing Up three times

str

Turn a number into text, so it can be joined: "Score: " + str(score).

str(score)

new turtle

Make another turtle with its own name. Put the name and a dot in front of a command to give it to that turtle.

star = Turtle()
star.shape("circle")
star.forward(50)

in

True if the item is in the list (or the key is in the dictionary, or the letters are in the text).

if item in bag:
    pass

goto

Jump to a position. (0, 0) is the centre; x goes right, y goes up.

goto(0, 0)
Example
goto(80, 60)
What it draws
From the centre to (80, 60)

speed

How fast the turtle moves, from 1 (slow) to 10 (fast).

speed(10)

square (loop)

Draw a square by repeating forward and right four times.

for i in range(4):
    forward(100)
    right(90)
What it draws

running total

Start the total at 0 once, then add to it on every pass.

total = 0
while total < 10:
    total += int(input("Add: "))

if / else

Do one thing or the other, depending on a condition.

if x > 0:
    pass
else:
    pass
Example
for x in [-90, -30, 30, 90]:
    penup()
    goto(x, 0)
    if x > 0:
        dot(30, "red")
    else:
        dot(30, "blue")
hideturtle()
What it draws
Red when x > 0, else blue

variable

Give a value a name, so you can reuse and change it.

size = 50
Example
length = 120
forward(length)
left(90)
forward(length / 2)
What it draws
length = 120, then half of it

grow value

Change a value every pass: read the old one, store a new one.

size = 5
for i in range(20):
    forward(size)
    right(90)
    size = size + 5
Example
size = 5
for i in range(30):
    forward(size)
    right(90)
    size = size + 5
What it draws
The same spiral, grown by an update

def (function)

Name a group of steps, then run it by calling its name.

def shape():
    pass

shape()
Example
def zigzag():
    left(45)
    forward(30)
    right(90)
    forward(30)
    left(45)

zigzag()
zigzag()
zigzag()
What it draws
Defined once, called three times

ontimer

Run a function again after a number of milliseconds. A function can start its own timer.

def tick():
    forward(5)
    ontimer(tick, 50)

tick()
mainloop()

join text

Join pieces of text into one with +. Spaces only appear where they are inside the quotes.

"Hello, " + name

add to (+=)

Add to a variable: total += 5 is short for total = total + 5.

total += 1

dictionary

Values stored under names (keys). One dictionary can hold everything about one thing.

star = {"speed": 5, "points": 1}

dictionary value

The value stored under that key. The key goes in quotes. Assign to it to change it: star["speed"] = 8.

star["speed"]

Classes

class

A new kind of object. __init__ runs when one is made; self.name = value stores a value in that object.

class Star:
    def __init__(self, x):
        self.speed = 5

s = Star(100)

method

A function inside a class. Call it on one object: star.move(). self is that object.

    def move(self):
        self.turtle.forward(self.speed)