Python Turtle

Turtle basics

The numbers on the left are line numbers and don't need to be typed in. The codes on the right are Python commands and they have to be copied exactly as written. Make sure you copy all the symbols, and that everything is in the correct case.

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from turtle import * forward(100) done()

Useful turtle commands

The parts of the line that come after a # are called comments, and are ignored by Python. You don't need to type them in.

left(90)        #turn left by 90 degrees
right(120)      #turn right by 120 degrees
pencolor("red") #change pen colour to red - note 'colour' is spelt 'color'
pensize(5)      #change the pen thickness to 5
penup()         #lift the pen up so we can move without drawing
pendown()       #put the pen down again
speed(0)        #draw as fast as possible

Important data types

• An int (short for integer) stores an integer number, eg. 5
• A float (short for floating point) stores a non-integer eg. 5.111
• A string stores some text eg. "hello"

Operators

Basic mathematical operators:

 a + b
 a - b
 a * b
 a / c

Conversions

Converting from one data type to another:

#converts a string to a float
float("1.54")

#converts a string to an int
int("3")

#converts a float or an int to a string
str(2.5)

Variables

We can store some data in a variable using the = operator:

a = 10
b = "my stuff"

Then we can recall them by using their names:

print(a+5)
print(b)

And also update them:

a = a + 5
b = "hands off " + b

User Input

We can ask the user for a string (and store it in a variable called answer):

answer = raw_input("what is your name?")

Conditionals

This is how we write a program that can respond differently depending on the conditions in which it runs:

time = 12
if time < 12:
    print("morning!")
elif time >= 12 and time < 18:
    print("afternoon!")
else:
    print("evening!")

We have these to choose from:

• == exactly the same as. 2 equals signs for comparison, 1 for variable assignment.
• >= more than or equal to
• > more than
• <= less than or equal to
• < less than
• != not equal to

Loops

To loop forever:

while True:
    print "hello!!"

To loop a certain number of times we can use while. while will only loop the code after the : while its condition is True:

#make a variable to keep count
loops = 0

#keep running the code while the loops variable is less than 10
while loops < 10:
    print(loops)

    #increase the loops variable by 1
    loops = loops + 1

Libraries

We can use libraries to get extra functionality in our programs. For example, to sleep for some time:

import time
time.sleep(5)

Or to get a random number between 1 and 10:

import random
random.randint(1,10)

Functions

If we are copying and pasting the same code over and over, we can use a function to save time and improve readability. In this example the function is called my_funcand it needs 2 arguments (arg1, and arg2).

• When we call the function we need to provide the right number of arguments
• We can only call a function after it's been defined with the def keyword

#define the function
def my_func(arg1,arg2):
    print(arg1 * arg2)

#call it
my_func(10,100)