Python on Your Own Machine

This is the one lesson with nothing to run on the page — the commands below belong in a terminal on your own computer. Everything else in the course works without it, but sooner or later you will want a Python that can reach the network, read your real files, and install other people’s libraries.

Getting Python

Check whether you already have it. macOS and most Linux distributions ship a Python, though sometimes an old one:

python3 --version

If that prints Python 3.11 or newer, you are set. Otherwise:

• Windows — install from the Microsoft Store, or python.org. Tick “Add Python to PATH” in the installer.
• macOS — brew install python if you use Homebrew, otherwise the python.org installer.
• Linux — sudo apt install python3 python3-venv python3-pip on Debian/Ubuntu.

Running a file

Put your code in a file ending in .py and run it:

python3 hello.py

Running python3 with no file gives you the REPL — an interactive prompt where each line runs as you press Enter. It is the closest thing to the playgrounds on this site, and it is where experienced Python programmers test small ideas. exit() or Ctrl-D leaves it.

Virtual environments

A virtual environment is a private folder of packages belonging to one project. Without one, every pip install goes into your system Python, and two projects that need different versions of the same library cannot both work.

Make one per project, in the project folder:

python3 -m venv .venv

Then activate it:

source .venv/bin/activate

On Windows PowerShell, that line is .venv\Scripts\Activate.ps1 instead. Your prompt gains a (.venv) prefix, and python now means this project’s Python. deactivate leaves it.

Installing packages

With the environment active, pip installs from the Python Package Index:

pip install requests

Record what a project needs so someone else — or you, on another machine — can reproduce it:

pip freeze > requirements.txt

And install from that record:

pip install -r requirements.txt

Three packages worth knowing about early:

An editor

Any text editor works, but a real Python editor will show you errors as you type. VS Code with the Python extension is the common choice and free; PyCharm is the heavyweight alternative.

Point your editor at the interpreter inside .venv so it sees the same packages your program will.

A sensible project layout

myproject/
├── .venv/               # virtual environment, not committed
├── .gitignore
├── requirements.txt
├── README.md
├── src/
│   └── myproject/
│       ├── __init__.py
│       └── main.py
└── tests/
    └── test_main.py

You do not need all of this for a fifty-line script — but when a script grows into a program, this is the shape it grows into. Day 7 builds a small version.

What you learned

• python3 --version checks what you have; python3 file.py runs a program.
• python3 alone opens the REPL for quick experiments.
• python3 -m venv .venv plus source .venv/bin/activate isolates a project.
• pip install adds packages; pip freeze > requirements.txt records them.
• Keep .venv/ out of version control.