Class 5

Objectives for today

  1. Introduce the turtle module
  2. Explain when loops are used
  3. Describe the execution of a for loop
  4. Use a for loop to iterate through a range


In the lab this week we are going to use the turtle module. turtle mimics an old programming language Logo (Logo was actually my first programming language!).

from turtle import *

Recall that this imports all the functions, etc. from the turtle module into our symbol table.

turtle works by moving a “pen” around the screen, e.g.

def draw_square():

for loops

That repetition is very tedious (and not very DRY). We know we want to move and turn 4 times. Can we loop over those statements 4 times? Yes. With a for loop. A for loop does exactly what it sounds like. For a specific set of iterations, execute the statements in the body. Note that we also convert that fixed side length into function parameter that we can change (making our function more flexible and useful).

def draw_square_with_loop(side_length):
    for i in range(4):

Note that “for” and “in” are reserved words (just like def) and therefore they cannot be used as variable or function names.

A for-loop in Python generally:

  1. Defines a loop variable that is set each iteration. For indexes, the convention is to use i, j, and k although you can and should use more descriptive variable names when relevant.
  2. The loop variable is set from a sequence. In this case the range function generates a sequence of integers starting at 0 up to but not including the supplied stop parameter (and exclusive end), e.g. 0, 1, 2, 3. So there will be 4 iterations. As we will see in future classes there are other kinds of sequences.
  3. Each iteration the loop variable is set to the next value from the sequence and the for-loop body is executed in its entirety. The body of the for loop is indicated by indentation.

Let’s look at another example using loops, this time to print numbers:

def print_loop(n):
    """ Print numbers from 0 until (but not including) n """
    print("Begin list of numbers")
    for i in range(n):
    print("End list of numbers")
>>> print_loop(5)
Begin list of numbers
End list of numbers

We can use the above output to remind us that there are three “regions” in and around our loop:

  1. Before the loop
  2. Inside the loop body
  3. After the loop

The statements inside the loop body will execute on every loop iteration, while the “after” statements will only execute once and only after all of the loop iterations are complete.

PI Questions (For loops)1

Recall in the first class that we said the computers were the right tool for the job when we needed to do something more than once. As you might imagine for-loops are one of our key tools for doing anything more than once. For example, when performing a computation “for each” data point in a file, or simulating a process “for” a set of different inputs.

for loops and scope

We saw that functions create a new scope, that is variables defined in a function are not visible outside that function. Do for loops similarly create a new scope? No. For simplicity, the designers of Python decided that only modules, classes (which we will learn about later) and functions will create a new scope. Thus the loop variable and any variables we define inside the loop body will be visible from that point to the end of the function, module, etc..

Warning about turtle errors

You may encounter errors with turtle, including rendering again after a bye() call or closing the drawing window. The graphics system that underlies turtle can only be “fired” once per Python console session. You can restart the Python console by clicking the Stop sign icon.

Turtle Enhancements

We saw that we can control the direction and angle of the drawing pen. What else can we control to enhance our square? Lets check out the docs. Here we change the color of the line to be red and set the color of the interior of our square to be yellow. The key for the “fill” is invoking begin_fill before you draw the shape and then end_fill after you draw the shape.




What about more interesting shapes? Specifically a Fibonacci spiral. A Fibonacci spiral is created by inscribing quarter circles inside squares whose edge length increases as the Fibonacci sequence. Let’s implement a function golden_spiral that has two parameters, the starting radius of the spiral and the number segments, and draws a Fibonacci spiral.

We can do so with a three step process:

  1. Define the Fibonacci sequence, and calculate Fibonacci numbers with a loop
  2. Identify a function in turtle for drawing portions of a circle
  3. Integrate circle drawing into the Fibonacci loop

Try it out before peeking at the code.

def golden_spiral(radius, segments):
    Draw a Fibonacci spiral using Turtle. turtle package must be imported into namespace.
        radius: Starting radius of the spiral
        segments: Number of quarter circle segments to draw after initial quarter circle.
            Must be >= 2.
    a = radius
    circle(a, 90)    
    b = radius
    circle(b, 90)
    # range can take two arguments, start and stop. We set the start at two since the first two
    # segments are already drawn    
    for i in range(2, segments): 
        c = a + b        
        circle(c, 90)
        # Prepare for the next iteration of the loop
        a = b
        b = c

Note that if we invoke the golden_spiral function several times, each new spiral starts from where the last one finished. This reminds us that the pen has “state”, specifically an x and y position and a heading. Controlling that state is a key part of the upcoming lab. How could we control where the drawing starts? That is how could we modify our function to have the following definition and start drawing the spiral at a specific location on the screen?

def golden_spiral(x, y, radius, segments):

We need a way move the pen around the page. If we review the docs, we that there is a function setpos that sounds like it will do exactly what we want. If we try that out in the shell, e.g.

>>> setpos(100, 100)

we notice that every time we move the pen it draws a line. If we think about how we would draw this picture (with pen and paper), we would pick our hand up from the page before moving. Again returning to the docs, we see there is a function penup for picking the pen up and thus not drawing while moving. With those pieces we can now enhance our golden_spiral function to start drawing at an arbitrary location.

def golden_spiral(x, y, radius, segments):
    setpos(x, y)
    pendown() # Need to put the pendown to start drawing again

Recall that pen (turtle) also has a heading that you will need to control in a similar way. I suspect you might find similar code at the beginning of several of your functions. If you we find ourselves with repeated code, we can often DRY it up with a function.

Here are many more turtle examples for you to review in preparation for lab.