Intro to

Problem solving and

Programming in Python

(use the Space key to navigate through all slides)

Prof. Andrea Gallegati

( tuj81353@temple.edu )

CIS 1051 - Temple Rome

Interfaces Design

The Turtle Module

To check whether you have this module:

open the Python interpreter and type

                  
>>> import turtle
>>> bob = turtle.Turtle()

... and a new window should pop-up!

A small arrow represents the "turtle".

Of course, this won't happen on a Dockerized environment

...cause there's no $DISPLAY at all :)

There, import an equivalent module:

                
>>> from svg_turtle import SvgTurtle

... that will do the job!

or keep following this class clicking here below!

Open In Colab

this opens a new tab to redirect you towards an interactive version of this class

Open In Colab

then click on the play button of each code block.

                    
>>> import turtle
>>> bob = turtle.Turtle()
>>>
>>> print(bob)
                    
                
<turtle.Turtle object at 0xb7bfbf4c>
  • the turtle module (lowercase t)
  • provides a function called Turtle (uppercase T)
  • that creates a Turtle object
  • which we assign to a variable named bob
                    
>>> import turtle
>>> bob = turtle.Turtle()
>>>
>>> print(bob)
                    
                
<turtle.Turtle object at 0xb7bfbf4c>

printing bob displays that it refers to an object of Turtle type.

Once created, some methods move the Turtle around.

                    
>>> bob.fd(100) # move bob forward, by 100 pixels

... a method is a function, with slightly different syntax.

a method is associated with an object (turtle).

                    
>>> bob.fd(100) # move bob forward, by 100 pixels

it is asking: bob, move forward!

Other methods are: 
                    
>>> bob.bk(100) # move backward
                    
                
                    
>>> bob.lt(90) # turn left by an angle (in degrees)
                    
                
                    
>>> bob.rt(90) # turn right by an angle (in degrees)

Each Turtle holds a pen, which is either

                    
>>> bob.pu() # pen up
                    
                
                    
>>> bob.pd() # pen down

to leave a trail when moving the turtle.

To draw a right angle

                    
import turtle

bob = turtle.Turtle()

bob.fd(100)
bob.lt(90)
bob.fd(100)

turtle.mainloop()

put this code into a file named mypolygon.py

... right after bob, but before the mainloop:

mainloop waits for user input, e.g. close the window.

Let's modify this program to draw a square!

... the easiest way to go would be

                    
bob.fd(100) ; bob.lt(90)   # 1st side
bob.fd(100) ; bob.lt(90)   # 2nd side
bob.fd(100) ; bob.lt(90)   # 3rd side
bob.fd(100)                # last side

Simple Repetition

or more concisely with a ...

for statement

                    
for i in range(4):
    print('Hello!') # much simpler!
                    
                
Hello!
Hello!
Hello!
Hello!

with a similar syntax to functions

for statement


  • header — with an ending colon
  • body — indented statements (any number)

it's essentially a loop

Open In Colab

... the flow of execution runs through the body and then loops back to the top.

Here we draw a square with a loop:

                    
for i in range(4):
    bob.fd(100)
    bob.lt(90)

add it to mypolygon.py and try it out!

note, the extra turn it takes:

this simplifies the code since we do the same thing every time through the loop!

the original for statement can be re-written as follows: 

                    
for _ in range(4):
print('Hello!') # much simpler!


                
            Hello!
            Hello!
            Hello!
            Hello!
                

            

            

                

                    without the iterator