Python Virtual Environments and pip: Complete Developer Guide

Python Virtual Environments

Virtual environments isolate project dependencies. Without them, packages from one project conflict with another. Every professional Python project uses virtual environments.

python3 -m venv .venv            # create virtual environment
source .venv/bin/activate         # activate (Linux/macOS)
.venv\Scripts\activate          # activate (Windows)

pip install flask sqlalchemy      # installs to .venv, not system Python
pip freeze > requirements.txt     # save all versions
pip install -r requirements.txt   # restore dependencies

deactivate                        # exit virtual environment

Modern: Poetry

# pyproject.toml
[tool.poetry.dependencies]
python  = "^3.11"
flask   = "^3.0"
redis   = "^5.0"

[tool.poetry.group.dev.dependencies]
pytest = "^7.0"
black  = "^23.0"

poetry install           # install all deps
poetry add flask         # add dependency
poetry shell             # activate environment
poetry run pytest        # run in env without activating

Best Practices

echo ".venv/" >> .gitignore        # never commit venv
echo "__pycache__/" >> .gitignore

which python3                      # verify you're using venv python
# Should show: /path/to/project/.venv/bin/python3