Winter 2023
The purpose of this lab is to introduce you to Python and Thonny. This week’s lectures talk about what programming is about and shows a few examples of very short Python programs. In this lab, you’ll expand on upon these examples, and get an introduction to writing and running your own Python programs.
Since there’s a wide variety of backgrounds in this class, everyone will progress at different speeds through this lab - that’s ok! My goal is to design these labs such that you have enough time to complete them during the lab session. If you do not, be sure to upload your submission to Canvas by the due date. If you have questions, be sure to ask the TA. Ask questions often. Labs are your opportunity to get personalized help!
You should have activated your CS account at the following URL prior to coming to lab: http://password.cs.wwu.edu. If you have not done so, please have the TA help you access this address and activate your account.
If for some reason it doesn’t look like your lab computer is on the Windows login screen, please refer to to “Windows and Linux on the Lab Computers” at the bottom of this of lab writeup for instructions on how to log into a specific operating system on a lab computer. Ask your TA for help as needed.
Log in using your CS account credentials. If only an image appears on your screen, click one of the mouse buttons to view the login prompt. The first time that you log into your Windows CS account you may experience a wait time of a few minutes while your account files are created.
There are two basic fonts in use in this document (this is common to
many other help documents and computer books you will use). One is the
font that I have used so far in this document and then there is
this font which is a fixed width font that I will use to
denote exact things you should type, exact names of menus to use,
website addresses and so on. If you have questions about this notation
ask your TA for clarification.
In addition to having office365, every student in a CS class gets an
N drive folder. Your N drive is for you to use as you like. To access
your N drive from a CS lab Windows machine, launch the File Explorer
Desktop app. Under Network Locations, you’ll see your N drive. Double
mouse click to expand the N drive, where you can create folders. I
recommend creating a csci141 folder, and inside that
creating a lab1 folder to keep your files organized. Files
stored on the N drive are available from any other computer in the labs,
and can be accessed from home using VPN. You can find instructions at
https://atus.wwu.edu/remote-computer-lab-access.
We’ll use Thonny as our IDE: integrated Development Environment - a fancy way of saying a program to program in. You can download it for free from http://www.thonny.org. The lab machines all have Thonny installed already. If you want to use it on your own computer, you can download it for free from http://www.thonny.org. Once you’ve logged into Windows on a lab machine, search for and start Thonny using the search box at the bottom left. If there are multiple versions, use the most recent version that is installed. If you’ve launched Thonny correctly, a screen similar to what is shown below should appear. Don’t worry if the version number after ”Python” in the Shell tab differs; as long as the major (first) version number is 3, you’re in good shape.
There are two different ways that Thonny can be used. You can use the interpreter (or shell) directly, which will cause each line of code that you type into the Shell section to be executed after you press return, or you can create and save a python program to a file, and then run (execute) the file in its entirety. Because you’ll be submitting your python program via Canvas, all instructions in labs and homework assignments will ask you to save a python program to a file.
Select New from the File menu, which will begin a new file. Then
select Save As from the file menu, and save your file as
lastName_firstName_lab1.py in your lab1 folder.
As a first step, you’ll recreate the quintessential Hello World
program that we saw in class. An important of programming well is
placing comments throughout your code to document your work. Comment
lines among python code are ignored and not executed when the program is
run. To insert a comment, begin the line with a hash (or pound) symbol,
#. You saw in lecture how to use the print function to
output text to the console.
Type the following into the editor window of python (edit your file)
the following (use your name and the current date):
# Author: Scott Wehrwein
# Date: January 3, 2023
# Description: Code for CSCI 141 Lab 1
print("Hello World")Notice how the IDE colors different parts your code differently. Comments are grey, and the words Hello World, which are enclosed in double quotes, are colored green, to signify that those words are a string literal. You’ll learn more about strings later in the course, but now just think of it as text: the double quotes tell Python that what’s inside them shouldn’t be interpreted as more code, but as a piece of data representing some text.
That’s it - your first python program. It contains comments (which
will be ignored by the python interpreter), and a single call to the
print function, which will print the phrase Hello World to
the console.
Save your program to your lab1 folder (File -> Save as..., or
Ctrl+S), then run the program by either selecting Run Current Script
from the Run menu, clicking on the green circle with the right-pointing
arrow, or press the F5 button on your keyboard. If your code has no
syntax errors, you should see something similar to the following:
One important feature of most programs is that they somehow interact
with the user and allow the user to input data. The input
function in Python is one way to accomplish this: the program to prompts
the user and then waits for some input to be entered using the
keyboard.
We can call input function with no arguments to
effectively pause the program until the user presses enter, as in
input()If we provide a string as an argument to input, it will
print the string as a prompt before waiting for input.
Once the user provides input, you’ll often want the program to to
store whatever the user entered somewhere in memory so that you can
refer to that data when needed. Programming languages rely on variables
as place holders that “remember” where a piece of data is stored in the
computer. The concept of a variable will be discussed extensively in
lecture. For the time being, think of a variable as an easy-to-remember
name for a piece of data. To set the value of a variable use the
assignment operator, =. To remind yourself that
= means the variable on the left gets set to the value on
the right, pronounce = as ‘gets’.
You’ve learned the basic use of two functions so far:
input and print. Here’s another one: the
len function, which is short for length,
calculates the length (number of characters) of its argument (i.e.,
whatever is inside the parentheses to the right of
len).
Add the lines of code below to your python program. Type the text exactly—double quotes, commas, parentheses, line breaks, etc. – all of it:
name = input("What is your name? ")
name_length = len(name)
print("Hello", name)
print("You have", name_length, "characters in your name.")When executed, the four lines of code print to the console What
is your name?, and then wait for the user’s input. Once the user
provides input and presses return (enter), then the user’s input is
placed into the variable name. The len
function is invoked to calculate the length of the data in the variable
name. The output of the len function is placed
into the variable name_length. The second-to-last
print function causes the program to print the phrase
Hello followed by the value that is stored in the variable
name. Lastly, the count of characters in the user’s name
(as calculated by the len function) is also output to the
console.
Run your program. If your code has no syntax errors, you should get
something like this:
Learning how to code means being able to find and fix syntax errors. Moreover, even if your python code has correct syntax, there may be other errors that may cause the program to crash. Knowing how to read error messages is an important skill because they inform you where in the code there is an error.
In this section, you’ll introduce some syntax errors on purpose to
get familiar with what you’ll see when something goes wrong.
Intentionally change print to printt, or
len to LEN, or omit the parentheses around
What is your name in the call to the print
function. Any of these will generate an error when the program is
run.
Save and run your program, and look at the red error message. A sample output is shown below. Assuming you had mistyped your code unintentionally, what information does the error message provide that you can use to troubleshoot your code? Notice that the error message tells you what line of code has the problem. Moreover the python interpreter tells you what exactly on a specific line of code it could not understand.
Play around with small modifications to your code and get a feel for the different error messages. There are many, many rules about what is valid Python syntax. You will see lots of examples of correct Python code (and you’ll encounter your fair share of incorrect code, as well!), but many questions are best answered by trying things out. Get creative about adding or removing spaces and newlines, changing capitalization, and so on.
When you get an error message, pay close attention to how to interpret what the error messages say about where the error is coming from. This will come in very useful later, when you need to locate an error you’ve made unintentionally! You’ll also notice that Thonny provides some helpful suggestions for locating the error in the Assistant panel on the right.
That’s it for this week’s lab—don’t worry if this seemed simple: future labs will be more involved.
Before you submit, undo the errors in your code so that your python
program runs correctly. Save your .py file, and upload it
to Canvas. For this lab, Canvas has been configured to permit only .py
submissions. If you have never used Canvas, log into your canvas
account, proceed to CSCI141, and you should see an option for items that
are open for submission. Click on Lab 1, and select to upload a
file.
The computers in the CS building are dual boot machines. That means that when the computer boots up, you have the option to launch and log into a Windows account, or a Linux account. This lab was done in Windows, while the next one must be done using Linux. After that, you will be able to choose whichever one you are most comfortable with—both the Linux and Windows accounts on the CS computers have Thonny installed.
To switch operating systems, you must reboot the computer. While the computer boots, you will see a screen listing a bunch of choices; one will say Ubuntu (that’s the version of Linux installed on the machines), while another will say Windows. You can choose which you’d like using the arrow keys and press enter to select it.
This lab is graded out of 10 points:
input, len, and
print