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Running Python Code & Virtual Environments

Run your first Python program, experiment in the REPL, and keep each project isolated with a virtual environment so packages never clash.

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Running a file

Why: Python is a program that runs the code in a text file. You save your code in a file ending in .py and run it by typing "python" followed by the file name in your terminal. (On some systems the command is "python3".)

Create a file called hello.py with one line:

print('Hello, Python!')

Then run it from the same folder:

python hello.py

The REPL (interactive playground)

Why: REPL stands for Read-Eval-Print Loop — a live prompt where you type Python and instantly see the result. It is perfect for quick experiments. Start it by typing "python" with no file name. Type exit() or press Ctrl+Z then Enter (Windows) / Ctrl+D (Mac, Linux) to leave.

python

>>> 2 + 2

>>> name = 'Ada'

>>> name.upper()

>>> exit()

Comments

Why: a comment is a note for humans that Python ignores when it runs. Anything after a # on a line is a comment. Use them to explain why the code does something.

# this whole line is a comment
price = 100  # you can also comment at the end of a line

# Python has no /* ... */ block comments — just start each line with #

Why a virtual environment

Why: a virtual environment is a private copy of Python for one project. Each project keeps its own packages in its own folder, so upgrading a package for project A can never break project B. venv is built into Python — nothing to install.

Create one (the second "venv" is just the folder name people use by convention):

python -m venv venv

Activating it

When: activate the environment in every new terminal before you work. Your prompt changes to show (venv) so you know it is on. "deactivate.bat" turns it back off.

Windows (PowerShell):

venv\Scripts\Activate.ps1

Mac / Linux:

source venv/bin/activate