A virtual environment (venv) isolates project dependencies.

This prevents situations like:

• Project A needs Flask 2.x
• Project B needs Flask 3.x
• both break if you install packages globally

Recommended: venvvenv (built-in)

Pick a project folder (example: my-flask-app/my-flask-app/), then create a venv:

python3 -m venv .venv

python3 -m venv .venv

Activate it:

• Linux/macOS:

source .venv/bin/activate

source .venv/bin/activate

You’ll usually see your shell prompt change.

Upgrade pip (optional but recommended)

python -m pip install --upgrade pip

python -m pip install --upgrade pip

Install packages into the venv

When the venv is activated, pip install ...pip install ... installs inside that environment.

Freeze dependencies (good habit)

pip freeze > requirements.txt

pip freeze > requirements.txt

Later, someone can reproduce your env:

pip install -r requirements.txt

pip install -r requirements.txt

Common errors

• You forgot to activate the venv → pippip installs globally.
• You activated a different venv in another terminal.
• Your editor uses a different interpreter than the terminal.

If you’re using VS Code, select the Python interpreter pointing to .venv.venv.