Run and Explain Your First Python Program

A Python file is plain text containing instructions. Saving it stores the instructions; it does not run them. Python normally follows the instructions from top to bottom. You use three tools:

• The editor is where you type and save the code.
• The terminal is a text window where you type commands.
• The interpreter is the Python program that reads and runs the code.

Think of writing a recipe and then asking someone to follow it. Saving the recipe is not the same as cooking.

Install Python 3.11 or newer from python.org. On Windows, tick "Add python.exe to PATH" on the first installer screen. You will check the installation from the terminal in a moment.

Open a terminal. On Windows, press the Windows key, type Terminal (or PowerShell on older versions) and press Enter. On a Mac, press Command and Space, type Terminal and press Enter. On Linux, look for Terminal in your applications menu.

See where you are. The terminal always works inside one folder, called the current folder. Type pwd and press Enter on Mac, Linux or Windows PowerShell; it prints the full path of the current folder.

List what is here. Type ls (Mac, Linux, PowerShell) to list the files and folders inside the current folder.

Move into a folder. cd Documents moves into a folder called Documents. cd .. moves up one level. If a folder name contains spaces, put it in quotes: cd "My Projects".

Make a folder. mkdir python-study-practice creates a new folder in the current one.

Try them now:

cd Documents
mkdir python-study-practice
cd python-study-practice
pwd

The last line should print a path ending in python-study-practice. If cd Documents fails, run ls to see which folders exist and pick one.

Check Python. In the same terminal, type python3 --version on Mac or Linux, or py -3 --version on Windows. You should see a version such as Python 3.13.1. If you see "command not found," reinstall Python and, on Windows, tick the PATH box.

A virtual environment is a project folder with its own Python entry point and package location. It keeps one project's installed packages apart from another's. This course uses only the standard library, the tools included with Python, so you will not install extra packages. Create the environment now to practice keeping projects separate.

On Mac or Linux, inside python-study-practice, run:

python3 -m venv .venv

On Windows, run py -3 -m venv .venv. Run ls again: you should now see a .venv folder (you may need ls -a on Mac or Linux, because names starting with a dot are hidden). Never put your own code inside .venv.

Open your editor, create a new file in python-study-practice, and save it as hello.py:

student = "Casey"
goal = "organize a fictional study log"
print("Student:", student)
print("Goal:", goal)
print("First practice session:", 20, "minutes")

Back in the terminal, run it with the environment's interpreter. On Mac or Linux: .venv/bin/python hello.py. On Windows: .venv\Scripts\python.exe hello.py.

Expected output:

Student: Casey
Goal: organize a fictional study log
First practice session: 20 minutes

student and goal are variable names. Each refers to a string, a text value. Quotes mark a string's beginning and end in the code. print displays the text without those quotes. Several values passed to print are separated by spaces in the output.

20 has no quotes, so it is a number. Compare the three output lines with the three print instructions. Casey is invented; use no personal details for this practice.

1. Change the goal and add a second session line. Predict the exact output before running it.
2. In a copy, remove one closing quote. Read the SyntaxError and its line number, then put the quote back.
3. Set session_minutes = 25 and print it in a sentence.

If the file is not found, use pwd and ls to check the folder and filename. It should be hello.py, not hello.py.txt. On Windows, use the environment's backslash path shown above.

Keep hello.py outside .venv. If you use Git, add .venv to .gitignore. Write down the exact run command that works on your computer; you will use it in every lesson.

Next, you will turn text a person types into numbers your program can add.