Why Dependencies Need Isolation

Your first projects used only the standard library — nothing to install. Real projects lean on third-party packages (requests, rich, pytest...), and that creates a problem: two projects on one machine often need different versions of the same package.

The problem

project-a needs requests==2.25
project-b needs requests==2.31

Install globally and one project silently breaks the other.

The fix: virtual environments

A venv is a private, disposable Python setup for one project:

python3 -m venv .venv        # create (once per project)
source .venv/bin/activate    # activate (every shell) — Windows: .venv\Scripts\activate

With the venv active, pip install affects ONLY that project. Deactivate with deactivate. The venv is regenerable — delete it, recreate it, nothing of yours is lost.