# CSCI 141 Lab 5: Functions: Turtle Shape Functions solution

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## Introduction

This lab introduces you to the art of writing your own functions, the concept of local variables
and parameters, and the importance of documenting the behavior of the functions you write.
You will write a handful of functions that draw various shapes using a turtle, and use them to
make complex drawings with ease.

1 Functions
This section of the handout reviews the basics of functions. Please read through it before
getting started, and refer back to it if you encounter confusion or unfamiliar terminology when
completing the activity. If you have any questions, your TA is there to help.

Basics
As we’ve seen in lecture, functions provide a way to assign a name to a given procedure, or
a sequence of statements. We’ve been using (calling) functions that have been written for us
since early in the course, such as
print(“Hello, world!”)

A function can take zero or more input arguments, have effects, and optionally return a value.
For example,
print(“Hello world!”)
takes one or more arguments as input, has the effect of printing them to the screen, and does
not return a value.

Writing your own functions is an extremely powerful ability, because it gives you a way to create
customizable pieces of code that you can then use as building blocks for creating more complicated programs. Recall from lecture that the syntax for declaring a function looks something
like this:
def function_name(arg1, arg2):

“””An example function that demonstrates the syntax for
statement_1
statement_2
statement_3
return a_value
Here’s an example program that declares a function that takes two inputs, computes their sum,
and then prints the sum before returning it. The program then declares two variables and calls
the function to print and return their sum.
# example of a function definition:
def calc_sum(num_1, num_2):
“”” Print, then return, the sum of num_1 and num_2
Precondition: num_1 and num_2 are numbers. “””
the_sum = num_1 + num_2
print(the_sum)
return the_sum
a = 4
b = 6
# example call to the function:
ab_sum = calc_sum(a, b)

Notice that printing a value and returning it are two different things: printing is an effect that
causes it to show up on the screen. Returning it means that the expression calc sum(num 1,
num 2) evaluates to the resulting sum. This means when we execute the assignment statement
after the function definition, the returned value pointed at by the sum will get assigned to the
variable ab sum.

## Triple-Quoted Strings

Notice that the lines directly after after the function header (the line with the def keyword)
contain a triple-quoted string. Enclosing a string in triple quotes is like using single or double
quotes, except that newlines are allowed in triple-quoted strings.
my_str = “a normal string
with a newline” # will cause an error
triple_quoted_string = “””A string in
triple-quotes””” # will not cause an error
print(triple_quoted_string)
# would print the following:
# A string in
# triple-quotes

Otherwise, a triple-quoted string behaves just like any other string.

## Specifications

When writing functions in any language, it’s standard practice to write a specification (or
spec, for short): a description of what someone needs to know to use it. In particular, the spec
typically includes a description of any parameters, effects, and the return value, if any, of the
function. This makes it possible for other programmers to make use of your function without
having to read through the function’s code to figure out how it works. This is what you’ve been
doing all along when calling functions like print: you know what it does, but you don’t know
how the code is written to accomplish its behavior.

Docstrings
In Python, the convention is to write function specifications in docstrings: triple-quoted strings
that appear just after the function header (the line with the def keyword). The triple-quoted
string is not technically a comment: it’s actual Python syntax; but an expression (or a value) on
a line by itself has no effect when executed by the Python interpreter, so it’s presence doesn’t
change the behavior of the program. Consequently, it is not a syntactic requirement of the
language to include a docstring in every function. However, it is a requirement of this course

to include a docstring in every function you write, unless you are told otherwise. Take a look
at the docstring in calc sum for an example.
What information should you include in your docstrings? Generally speaking, a programmer
• The meaning of each parameter that the function takes
• Any effects that the function has

• The return value (and its type), if any, of the function
• Any preconditions: Assumptions that your function makes about the state of the program, or about the arguments passed to it.

• Any postconditions: Assumptions that can be made once the function call is finished
about the program state or return value.

Note that it’s a good idea to keep specifications as concise as possible. For example, the
spec for calc sum does not specify as a postcondition that the return value is the sum of the
two inputs, because that’s already stated in the description. However, it is worth including the
precondition that the arguments are numerical, because otherwise an error might result. A user
of your function is now aware that they shouldn’t call calc sum with non-numeric arguments.
If they do and an error occurs, it’s their mistake, not yours!

Local Variables
Notice that in the definition of calc sum, we created a new variable called the sum1
. Because it
was created inside a function definition, the sum is what is known as a local variable, meaning
that it doesn’t exist (or isn’t “visible”) anywhere in the program except inside the calc sum
function definition. A variable’s scope refers to the set of statements in a program where it is
accessible; in this case, the sum’s scope is within the calc sum function. If we tried to refer to
the sum outside that indented code block, we’d get an error.

Variables such as a, b, and ab sum are called global variables because once they are defined
using an assignment statement, they are visible for the entire remainder of the program.

I didn’t call it sum because that’s already the name of a builtin function; it’s syntactically valid to create a
variable with that name, but it “hides” the builtin function so you can’t use it anymore because sum now refers
to a varaible.

Parameters Are Local Variables
When defining a function, we need a way to refer to the arguments that are passed in when
it’s called. Parameters serve this purpose: in the calc sum function definition, num 1 and
num 2 are the function’s parameters.

When the function is being executed, num 1 points to the
value of the first argument and num 2 points to the value of the second one. Consequently,
parameters are simply special local variables that are automatically assigned the values passed
to the function as arguments. Like any other local variable, their scope is limited to the function
definition to which they belong. Referring to num 1 or num 2 outside of the function definition
will result in an error for the same reason that trying to refer to the sum will cause an error.

## 2 Shape Functions for Turtles

You’ll now write some functions that will make it easier to make complicated drawings using
Python’s turtle module. For now, you’ll be provided with the function header and specification, and it is your job to make sure that the function implements (or adheres to) the spec
exactly and in all cases.

2.1 Setup
Create a lab5 directory in your lab environment of choice. Fire up Thonny, create a new
file, and save it as turtleshape.py. Write a comment at the top with author, date, and a
description of the program. Download turtleshape test.py from the course webpage and
save it in your lab5 directory alongside turtleshape.py.

2.2 Testing
One of the many benefits of writing small, self-contained functions is the ability to test them
independently of other code that uses them. Once you’ve tested a function thoroughly, you can
safely use it without fear of a bug lurking somewhere inside. It’s a good idea to test functions
one at a time, as you write them. Start by making calls to your function in the interactive shell
(the bottom pane in Thonny) to see if they’re working correctly.

Once you believe a function behaves as specified, open up turtleshape test.py, and look
toward the bottom for a commented-out call to a function named test function name. For
example, for draw square, the corresponding function is called test draw square. Remove

Figure 1: The correct output of turtleshape test.py after all functions are completed.
the # from the beginning of the line to enable the test function, then hit the green Run button
to run turtleshape test.py. Each of the functions draws one piece of the drawing shown in
Figure 1. For example, the black squares of increasing size in the bottom left should appear
exactly as in the figure once your draw square function works correctly.

To make sure that everything’s set up correctly, first run the test program unmodified: you
should see the drawing with only the green dot in the middle.

2.3 draw square
In last week’s lab, you wrote a loop to make a turtle draw a square. Now, let’s do the same
thing, but wrap it in a draw square function so we can draw a square with a simple function
call.

def draw_square(t, side_length):
“”” Use the turtle t to draw a square with side_length.
Precondition: t’s pen is down
Postcondition: t’s position and orientation are the same as before
“””

Type the function header and specification (docstring) into turtleshape.py, then write code
in the function body to make the turtle draw a square.
Try out your draw square function in the interactive pane, something like this:
>>> import turtle
>>> scott = turtle.Turtle()
>>> draw_square(scott, 100)

2.4 draw rectangle
Next, write a more general function to draw a rectangle of any size. Notice that once we have a
rectangle function, we can use it to draw a square by calling the rectangle function with equal
side lengths.
def draw_rectangle(t, width, height):
“”” Draw a rectangle using turtle t with size width x height

Precondition: t’s pen is down
Postcondition: t’s position and orientation are the same as before
“””
After uncommenting test draw rectangle in turtleshape test.py, the orange spiral of rectangles should appear as in Figure 1.

2.5 draw triangle
Another way to generalize the square function would be to draw different equilateral polygons (i.e., shapes with different numbers of equal side lengths). To get started, implement a
draw triangle function that draws a triangle with equal length sides:
def draw_triangle(t, side_length):
“”” Draw an equilateral triangle using turtle t with side_length

Precondition: t’s pen is down
Postcondition: t’s position and orientation are the same as before
“””
When completed and the corresponding test function is uncommented, the purple bowtie-like
figure should appear as in Figure 1.

2.6 draw polygon
You’ve now figured out how to draw a square (4-sided polygon) and a triangle (3-sided polygon).
Now, write a function that draws an n-sided polygon:
def draw_polygon(t, side_length, num_sides):
“”” Draw a polygon with num_sides sides, each with length side_length
using turtle t
Precondition: t’s pen is down; num_sides > 2
Postcondition: t’s position and orientation are the same as before
“””
The red pattern with nested n-gons should appear as in the Figure when this function works
correctly.

2.7 draw snowflake
One of the reasons that functions are so powerful is that we can compose them; in other words,
one function can call another. Now that we have a function that draws polygons, it’s pretty
simple to make a function that uses it to create variations on the snowflake-like pattern from
last week’s lab:

def draw_snowflake(t, side_length, num_sides):
“”” Use t to draw a snowflake made of ngon-sided polygons. The snowflake
contains 10 copies of a polygon with num_sides and side_length, each
drawn at a 36-degree angle from the previous one.
Postcondition: t’s position and orientation are the same as before
“””
The teal, green, and blue snowflakes in the bottom right corner should appear as in the Figure
once this function is working correctly.

2.8 teleport
Finally, write a function that implements a convenient ability: teleport the turtle to a given
location without drawing, but leaving the pen state unchanged afterwards. This is similar to
the turtle object’s goto method, except it never draws. To accomplish this, you’ll need to
pick up the pen first, then put it down only if it started out down. You may find it helpful to
look at the turtle documentation for methods that could be useful here.

def teleport(t, x, y):
“”” Move the turtle to (x, y), ensuring that nothing is drawn along the
way. Postcondition: the turtle’s orientation and pen up/down state is
the same as before.
“””
When basic movement is working, the gradient-colored grid of dots should appear. When the
pen is correctly restored to its previous state, the vertical red lines should appear as in the
Figure.

2.9 Screenshot
Take a screenshot of the Turtle window when you have everything working, and name the file
turtleshape.png.

## 3 Drawing

Your final task will be to write a short program that uses your drawing functions to make some
interesting drawing.
Importing Local Files; Writing “Main” Functions

You may have noticed that the code in turtleshape test.py calls functions from turtleshape.py.
To make this possible, turtleshape test.py had to execute import turtleshape; this is just
like importing a module (like math or turtle, except the module is located right in the same
directory. Python looks first in the local directory for a module with name turtleshape.py,
then it imports the code if it is found.

What happens when importing code? Basically, it’s like pasting the imported module’s code
into your file: it all gets run. So far, your turtleshape.py contains only function definitions,
so importing it simply defines those functions. But if you wrote code outside the functions, it
would get executed when you import turtleshape. Often, we want to separate the behavior
of a file as a program versus as a module, so importing it causes functions to be defined but
doesn’t run the program, but you can still run the program, e.g., with python turtleshape.py
or by clicking the Run button.

The way to do this is to use a so-called “main function” or “main guard”. turtleshape test.py
contains an example of this. Basically, any code you want to run as a program but don’t want
to execute when you import your file as a module, you can put inside the following if statement:
if __name__ == “__main__”:
# code here will not run when this file is imported
# but will run if the file is run as a program
print(“Hello, world!”)

You don’t need to worry about the details of why this happens, but it’s a good thing to
remember how to do. You can always google it if you forget the syntax.
A common way to use this is to have one function that contains your program code, usually
called main(), and place a single call to that function inside the main guard:

def other_function():
“”” Return the number 4 “””
return 4
def main():
“”” Main program: print hello, world! “””
print(“Hello, world!”)
if __name__ == “__main__”:
main()

If this were in a file called prog.py, then running python prog.py would print ”Hello, world!”,
whereas executing import prog would make other function and main available for use, but
wouldn’t execute the call to main().

Make a Drawing
Below all your function definitions in turtleshape.py, write some code that creates a drawing.
Put your code in a main function and call it inside a main guard as illustrated above, so that
running the test program (which imports your code) does not cause your main function to be
called.

Your code should use at least one loop and make use of at least two of the functions you wrote
in this lab. Feel free to also use other functions from the turtle module. Take a screenshot
of your drawing and save it as drawing.png. Your drawing should complete in under a few
seconds (use turtle.tracer(0,0) and turtle.update() as in Lab 4), and should match your
screenshot.

Submission
At this point, show the output of the test program and your drawing to your TA
so you can be immediately awarded points for it. Unless you do not finish during the
lab period, do not leave until your TA has verified that your output is correct.

Create a zip file called lab5.zip containing turtleshape.py, turtleshape.png, and drawing.png.
Upload your zip file to the Lab 5 assignment on Canvas. Even if your TA has checked you off,
you still need to submit to Canvas.

Rubric
You submitted a single zip file called lab5.zip, containing the correct files 1