Virtual environment (venv)

When you will meet it

Your first pip install on macOS or Linux is quite likely to be refused outright with error: externally-managed-environment. Not recognising that line leads you to assume you mistyped, or to find a 'one-line fix' online that damages your system Python. The real answer is a venv, and it costs two commands.

Minimal example

python3 -m venv .venv                # 在目前資料夾建一個叫 .venv 的環境
source .venv/bin/activate            # macOS / Linux：啟動它
.venv\Scripts\Activate.ps1           # Windows PowerShell：啟動它
python3 -m pip install requests      # 之後裝的東西都只進到 .venv 裡
deactivate                           # 用完離開

After activation your prompt gains a (.venv) prefix. Seeing that prefix is how you know the python and pip you are about to type mean this environment, not the system one.

What people get wrong

• Creating a venv and then running pip install without activating it. The package still goes to the system Python and you get the identical error again.
• Reading externally-managed-environment as 'not enough permission' and re-running with sudo. It has nothing to do with permissions — your distribution blocked it deliberately, and sudo only makes the aftermath harder to clean up.