Lab 0: Workflow and Python Basics

Due at 11:59pm on Friday, 08/31/2018.

Starter Files

Download Inside the archive, you will find starter files for the questions in this lab, along with a copy of the Ok autograder.


By the end of this lab, you should have submitted the lab with python3 ok --submit. You may submit more than once before the deadline; only the final submission will be graded. Check that you have successfully submitted your code on


This lab explains how to use your own computer to complete assignments for CS 61A, as well as introduce some of the basics of Python. If you are using a lab computer, most of the instructions are the same, except you won't have to install anything.

If you need any help at any time through the lab, please feel free to come to office hours or post on piazza.

This lab looks really long, but it's mostly setup and learning how to use essential tools for this class; these may seem a bit difficult now, but quickly become second nature as we move further into the course.


Register for an account

These accounts allow you to use instructional machines in the CS department, which can be useful if you do not have regular access to a computer. They are not required if you do not plan on using the lab computers or printers.

Go to the EECS account site to register for an instructional account. Login using your Berkeley CalNet ID and click the Get a new account button in the row for CS 61A. Your username will be of the form cs61a-xxx. Write down or download your account form so you don't forget it!

Install a terminal

The terminal is a program that allows you to interact with your computer by entering commands. No matter what operating system you use (Windows, macOS, Linux), the terminal will be an essential tool for CS 61A.


If you're on a Mac or are using a form of Linux (such as Ubuntu), you already have a program called Terminal or something similar on your computer. Open that up and you should be good to go.


For Windows users, we recommend downloading a terminal called Git Bash.

You should be able to install Git Bash with most of the default configuration options, with one exception. In the Configuring the terminal emulator to use with Git Bash step, select the second option: Use Windows' default console window.

This is very important! If you do not select this option, your terminal will not work!

Git Bash configuration options

Install Python 3

Python 3 is the primary programming language used in this course. Use the appropriate download link and additional instructions below to install the Python 3 interpreter.

OS Download
Windows Installer
macOS Installer
Ubuntu sudo apt install python3
Other Python3 Download


Refer to this video for additional help on setting up Python (the video features a slightly older version of Python 3, but the steps are still the same).

You may need to right-click the download icon and select "Open". After installing please close and open your Terminal.


When installing, make sure to check the "Add Python 3.6 to PATH" box, which will allow you to execute the python command from your terminal.

Windows PATH at setup

After installing please close and open your Terminal.

Install a text editor

The Python interpreter that you just installed allows you to run Python code. You will also need a text editor, where you will write Python code.

There are many editors out there, each with its own set of features. We find that Atom and Sublime Text 3 are popular choices among students, but you are free to use other text editors.

Note: Please, please, please do not use word processors such as Microsoft Word to edit programs.

For your reference, we've also written some guides on using popular text editors. After you're done with lab, you can take a look if you're interested:

Using the terminal

Let's check if everything was installed properly! First, open a new terminal window, if you haven't already.

starting the terminal

When you first open your terminal, you will start in the home directory. The home directory is represented by the ~ symbol.

Don't worry if your terminal window doesn't look exactly the same; the important part is that the text on the left-hand side of the $ has a ~ (tilde). That text might also have the name of your computer.

Python Interpreter

We can use the terminal to check if your Python 3 interpreter was installed correctly. Try the following command:


If the installation worked, you should see some text printed out about the interpreter followed by >>> on its own line. This is where you can type in Python code. Try typing some expressions you saw in lecture, or just play around to see what happens! You can type exit() or Ctrl-D to return to your command line.

If you are using Windows and the python3 command doesn't work, try using just python or py. If neither of those work, make sure you set up your PATH correctly as shown above. Ask for help if you get stuck!

Organizing your files

In this section, you will learn how to manage files using terminal commands.


The first command you'll use is ls. Try typing it in the terminal:


The ls command lists all the files and folders in the current directory. A directory is another name for a folder (such as the Documents folder). Since you're in the home directory right now, you should see the contents of your home directory.

Changing directories

To move into another directory, use the cd command. Let's try moving into your Desktop directory. First, make sure you're in your home directory (check for the ~ on your command line) and use ls to see if the Desktop directory is present. Try typing the following command into your terminal, which should move you into that directory:

cd Desktop

If your desktop directory is not located within your home directory and you can't find it, ask a TA or a lab assistant for help.

There are a few ways to return to the home directory:

  • cd .. (two dots). The .. means "the parent directory". In this case, the parent directory of cs61a is your home directory, so you can use cd .. to go up one directory.
  • cd ~ (the tilde). Remember that ~ means home directory, so this command will always change to your home directory.
  • cd (cd on its own). Typing just cd is a shortcut for typing cd ~.

You do not have to keep your files on your Desktop if you prefer otherwise. Where you keep your files locally will not affect your grade. Do whatever is easiest and most convenient for you!

Making new directories

The next command is called mkdir, which makes new directories. Let's make a directory called cs61a on your Desktop to store all of the assignments for this class:

mkdir cs61a

A folder named cs61a will appear on your Desktop. You can verify this by using the ls command again or by simply checking your Desktop.

At this point, let's create some more directories. First, make sure you are in the ~/Desktop/cs61a directory. Then, create folders called projects and lab inside of your cs61a folder:

cd ~/Desktop/cs61a
mkdir projects
mkdir lab

Now if you list the contents of the directory (using ls), you'll see two folders, projects and lab.

cs61a directory

Downloading the assignment

If you haven't already, download the zip archive,, which contains all the files that you'll need for this lab. Once you've done that, let's find the downloaded file. On most computers, is probably located in a directory called Downloads in your home directory. Use the ls command to check:

ls ~/Downloads

If you don't see, ask a TA or lab assistant for help.

Extracting starter files

You must expand the zip archive before you can work on the lab files. Different operating systems and different browsers have different ways of unzipping. If you don't know how, you can search online.

Using a terminal, you can unzip the zip file from the command line. First, cd into the directory that contains the zip file:

cd ~/Downloads

Now, run the unzip command with the name of the zip file:


You might also be able to unzip files without using the terminal by double clicking them in your OS's file explorer.

Once you unzip, you'll have a new folder called lab00 which contains the following files (check it out with cd and ls):

  • The template file you'll be adding your code to
  • ok: A program used to test and submit assignments
  • lab00.ok: A configuration file for ok

Moving files

Move the lab files to the lab folder you created earlier:

mv ~/Downloads/lab00 ~/Desktop/cs61a/lab

The mv command will move the ~/Downloads/lab00 folder into the ~/Desktop/cs61a/lab folder.

Now, go to the lab00 folder that you just moved. Try using cd to navigate your own way! If you get stuck, you can use the following command:

cd ~/Desktop/cs61a/lab/lab00


Here is a summary of the commands we just went over for your reference:

  • ls: lists all files in the current directory
  • cd <path to directory>: change into the specified directory
  • mkdir <directory name>: make a new directory with the given name
  • mv <source path> <destination path>: move the file at the given source to the given destination

Finally, you're ready to start editing the lab files! Don't worry if this seems complicated -- it will get much easier over time. Just keep practicing! You can also take a look at our UNIX tutorial for a more detailed explanation of terminal commands.

Python Basics

Expressions and statements

Programs are made up of expressions and statements. In very simple terms, an expression is a piece of code that evaluates to some value and a statement is one or more lines of code that make something happen in a program.

When you type a Python expression into the Python interpreter, its value will be printed out. As you read through the following examples, try out some similar expressions in your own Python shell, which you can start up by typing this in your terminal:


You'll be learning various types of expressions and statements in this course. For now, let's take a look at the ones you'll need to complete today's lab.

Primitive expressions

Primitive expressions only take one step to evaluate. These include numbers and booleans, which just evaluate to themselves.

>>> 3
>>> 12.5
>>> True

Names are also primitive expressions. Names evaluate to the value that they are bound to in the current environment. One way to bind a name to a value is with an assignment statement.

Assignment statements

An assignment statement consists of a name and an expression. It changes the state of the program by evaluating the expression and binding its value to the name in the current frame.

>>> a = (100 + 50) // 2

Note that the statement itself doesn't evaluate to anything, because it's a statement and not an expression. Now, if we ask for a's value, the interpreter will look it up in the current environment and output its value.

>>> a

Note that the name a is bound to the value 75, not the expression (100 + 50) // 2. Names are bound to values, not expressions.

Doing the assignment

Unlocking tests

One component of lab assignments is unlocking tests. Their purpose is to test your understanding and make sure you have a good enough grasp on the topic to make progress on the assignment.

Enter the following in your terminal to begin this section:

python3 ok -q python_basics -u

You will be prompted to enter the output of various statements/expressions. You must enter them correctly to move on, but there is no penalty for incorrect answers.

>>> 10 + 2
>>> 7 / 2
>>> 7 // 2
>>> 7 % 2 # 7 modulo 2, equivalent to the remainder of 7 // 2
>>> x = 20
>>> x + 2
>>> x
>>> y = 5 >>> y += 3 # Equivalent to y = y + 3 >>> y * 2
>>> y //= 4 # Equivalent to y = y // 4 >>> y + x

Understanding problems

Labs will also consist of function writing problems. Open up in your text editor. You can type open . on MacOS or start . on Windows to open the current directory in your Finder/File Explorer. Then double click or right click to open the file in your text editor. You should see something like this:

def twenty_eighteen():
    """Come up with the most creative expression that evaluates to 2018,
    using only numbers and the +, *, and - operators.

    >>> twenty_eighteen()
return ______
return (((2 + 0) - 1) * 8) * 8 * 8 * 4 - 30

The lines in the triple-quotes """ are called a docstring, which is a description of what the function is supposed to do. When writing code in 61A, you should always read the docstring!

The lines that begin with >>> are called doctests. Recall that when using the Python interpreter, you write Python expressions next to >>> and the output is printed below that line. Doctests explain what the function does by showing actual Python code: "if we input this Python code, what should the expected output be?"

In twenty_eighteen,

  • The docstring tells you to "come up with the most creative expression that evaluates to 2018," but that you can only use numbers and arithmetic operators + (add), * (multiply), and - (subtract).
  • The doctest checks that the function call twenty_eighteen() should return the number 2018.

You should not modify the docstring, unless you want to add your own tests! The only part of your assignments that you'll need to edit is the code.

Writing code

Once you understand what the question is asking, you're ready to start writing code! You should replace the underscores in return ______ with an expression that evaluates to 2018. What's the most creative expression you can come up with?

Don't forget to save your assignment after you edit it! In most text editors, you can save by navigating to File > Save or by pressing Command-S on MacOS or Ctrl-S on Windows.

Running tests

In CS 61A, we will use a program called ok to test our code. ok will be included in every assignment in this class.

Back to the terminal! Make sure you are in the lab00 directory we created earlier (remember, the cd command lets you change directories).

In that directory, you can type ls to verify that there are the following three files:

  • the starter file you just edited
  • ok: our testing program
  • lab00.ok: a configuration file for Ok

Now, let's test our code to make sure it works. You can run ok with this command:

python3 ok

The first time you run Ok, you will be prompted for your bCourses email. Please follow these directions. We use this information to associate your code with you when grading.

Remember, if you are using Windows and the python3 command doesn't work, try using just python or py. See the the install Python 3 section for more info and ask for help if you get stuck!

If you wrote your code correctly, you should see a successful test:

Assignment: Lab 0
Ok, version v1.11.1

Running tests

Test summary
    1 test cases passed! No cases failed.

If you didn't pass the tests, ok will instead show you something like this:

Doctests for twenty_eighteen

>>> from lab00 import *
>>> twenty_eighteen()

# Error: expected
#     2018
# but got
#     2013

Test summary
    0 test cases passed before encountering first failed test case

Fix your code in your text editor until the test passes.

Every time you run Ok, Ok will try to back up your work. Don't worry if it says that the "Connection timed out." We won't use your backups for grading.

While ok is the primary assignment "autograder" in CS 61A, you may find it useful at times to write some of your own tests in the form of doctests. Then, you can try them out using the -m doctest option for Python).

Submitting the assignment

Now that you have completed your first CS 61A assignment, it's time to turn it in. Note that it is not sufficient to receive credit for an assignment simply by running the autograder per the last section. You must follow these steps to submit and get points!

Step 1: Submit with ok

In your terminal, make sure you are in the directory that contains ok. If you aren't there yet, you can use this command:

cd ~/Desktop/cs61a/lab/lab00

Next, use ok with the --submit option:

python3 ok --submit

This will prompt you for an email address if you haven't run Ok before. Please follow these directions. After that, Ok will print out a message like the following:

Submitting... 100% complete
Backup successful for user: ...

Step 2: Verify your submission

You can follow the link that Ok printed out to see your final submission, or you can go to You will be able to view your submission after you log in.

Make sure you log in with the same email you provided when running ok from your terminal!

You should see a successful submission for Lab 0.

Congratulations, you just submitted your first CS 61A assignment!

More information on Ok is available here. You can also use the --help flag:

python3 ok --help

This flag works just like it does for UNIX commands we used earlier.

Appendix: Useful Python command line options

When running a Python file, you can use options on the command line to inspect your code further. Here are a few that will come in handy. If you want to learn more about other Python command-line options, take a look at the documentation.

  • Using no command-line options will run the code in the file you provide and return you to the command line.

  • -i: The -i option runs your Python script, then opens an interactive session. In an interactive session, you run Python code line by line and get immediate feedback instead of running an entire file all at once. To exit, type exit() into the interpreter prompt. You can also use the keyboard shortcut Ctrl-D on Linux/Mac machines or Ctrl-Z Enter on Windows.

    If you edit the Python file while running it interactively, you will need to exit and restart the interpreter in order for those changes to take effect.

    python3 -i
  • -m doctest: Runs doctests in a particular file. Doctests are surrounded by triple quotes (""") within functions. Each test consists of >>> followed by some Python code and the expected output.

     python3 -m doctest