3: DRY Turtle


Learning Targets

  • I can implement the basic constructs of programming languages such as variables, conditionals, loops, and flow control.

  • I can make a class wrapper that inherits from a library's class.

  • I can call a method from a library.


  • You will be submitting Trinket.io scripts demonstrating the following skills.

Class Videos

Class 1/24 Turtle drawing assignment:

What is DRY code?

Loops and functions allow skilled programmers to make more efficient code that's easier to maintain.

Parts of a for loop

A for loop has an iterator and a collection that it traverses.

# the range command will build a list you can traverse
for x in range(5):
# you could also write it using a literal list:
for x in [0, 1, 2, 3, 4]:

Parts of a function

A function has a definition that lists any parameters and has a code block underneath that gets activated when called.

def magic_spell(target):

# let's make Steve sneeze

In the example above, the magic_spell function isn't a part of a Class. It's on its own. If that ability was a part of a player in a game, we'd call it a method instead of function.

class Player(object):
    def magic_spell(self, target):
lucy = Player()
steve = Player()

# fun fact: you could also write line 8 like so:
Player.magic_spell(lucy, steve)

Code Along

We're going to jump into work with a lot of advanced concepts. This is intentional. New programmers are very often intimidated by code they don't understand. I want you to get used to that early on. Enjoy tinkering with a few lines here and there and see how things change. Get comfortable swimming in the deep end.

from turtle import Turtle

class SuperTurtle(Turtle):
  def __init__(self, x, y, color='blue', shape='turtle'):
    Turtle.__init__(self) # run the normal Turtle constructor

This class (inherits) from the Turtle class which is available since we imported the turtle library. Instead of writing from turtle import Turtle I could have also written just import turtle in which case every time I wanted to use the Turtle class, I would have had to specify the library, such as turtle.Turtle()

The only thing that our SuperTurtle does that the regular Turtle doesn't is the way it gets constructed. When a new Turtle is created or instantiated, we initialize or __init__ with some default properties.

Follow along as we go over a couple of skills. You aren't expected to be able to do this stuff yourself quite yet.

  1. Make a loop and create a bunch of turtles

  2. Add a new method to the turtle to make it go to a random location and call that method in the constructor

  3. Make a list of colors and make each turtle randomly select a color

  4. Make an empty list and add each turtle to the collection as it's being instantiated

  5. Loop through the turtle collection and modify each

We'll spend some time experimenting with this code, learning by modifying existing code.

Building Methods

Experimenting with code given to you is fun and can be very helpful, but let's slow things down now and build some basic skills. Your Trinket.io app should look something like this:

# here's where we imported the Turtle class from the turtle package
from turtle import Turtle
# now from the random package, we imported to helpful functions
from random import randint, choice

def rancolor():
  """ This method returns a string from a list of colors available to Turtle's .color() method """
  colors = ["brown", "darkorange", "maroon", "crimson", "navy", "salmon", "tomato", "khaki", "gold", "hotpink", "springgreen", "blue", "cyan", "purple", "green", "red", "pink", "yellow", "teal"]
  return choice(colors)

class SuperTurtle(Turtle):
  # our own constructor that makes Turtles more convenient
  def __init__(self, shape='turtle'):
  def goto_random(self):
    self.goto(randint(-200,200), randint(-200,200))
  def triangle(self):
    pass # replace pass with real code

  def square(self):
    pass # replace pass with real code
  def star(self):
    pass # replace pass with real code

# create a collection for our turtles
gang = []

# loop through a set range of 20 and 
# add a Turtle to our collection during each iteration   
for x in range(20):
# loop through my gang and change each one
for x in gang:

A method or function is a group of commands that can be called. If I tell you to please clap, that's me activating or calling the command that's already defined in your head. I may give you additional parameters or arguments like, "clap 5 times loudly". In Python that may input("What's your name?") but that doesn't save the variable, so we write it as:

name = input("What's your name?") written as joe.clap(5, "loudly")

What's the difference between a function and a method?

They both start with def and will execute their pre-programmed commands when called. However, we'll refer to a function as something floating outside of a class and a method as an ability built within an object. For example, a method would be if an instance of the Human class, let's call him Joe, had to sneeze: joe.sneeze(). But a function would be an anonymous helper like therancolor() function programmed above that just returns a random string in a list of colors.

Novel Design

Look at the documentation, other student examples, and Google searches to modify open-source code and customize your own design. You can use one or many turtles but the end result has to have multiple colors over a fun design.

Taking Controls

Let's make our app more interactive. Just like the print("message") function we've been using, there's another one that takes a string input called input()

input("What's your name?") will work but that doesn't save the user's response anywhere. So let's create a variable that will store the answer in this example:

name = input("What's your name?")

Testing the input

if "circle" in user_input:
elif "triangle" in user_input:

To make sure we are asked the question over again, we'll put the input within a while True: loop.

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