Optional Dependency Groups in pyproject.toml

The [project.optional-dependencies] section in pyproject.toml replaces the common pattern of having multiple requirements-*.txt files. Each group can be installed independently using pip install -e ".[group]".

Traditional multi-file approach:

# requirements.txt (production)
flask>=3.0
sqlalchemy>=2.0
redis>=5.0

# requirements-test.txt
-r requirements.txt
pytest>=7.4
pytest-cov>=4.1
factory-boy>=3.3

# requirements-lint.txt
ruff>=0.1.6
mypy>=1.7
types-redis>=4.6

# requirements-docs.txt
sphinx>=7.2
sphinx-rtd-theme>=2.0

Consolidated pyproject.toml:

[project]
dependencies = [
    "flask>=3.0",
    "sqlalchemy>=2.0",
    "redis>=5.0",
]

[project.optional-dependencies]
test = [
    "pytest>=7.4",
    "pytest-cov>=4.1",
    "factory-boy>=3.3",
]
lint = [
    "ruff>=0.1.6",
    "mypy>=1.7",
    "types-redis>=4.6",
]
docs = [
    "sphinx>=7.2",
    "sphinx-rtd-theme>=2.0",
]
dev = [
    "my-project[test,lint,docs]",
]

Installation commands:

# Install production only
pip install .

# Install with test dependencies
pip install ".[test]"

# Install with all dev dependencies
pip install -e ".[dev]"

# Install specific groups
pip install -e ".[test,lint]"

Advantages of optional groups over multiple files:

1. Single source of truth -- all dependencies in one file
2. Self-referencing groups -- the dev group can reference other groups
3. No -r includes -- TOML doesn't need file includes
4. Build tool integration -- hatch, pdm, and poetry understand groups natively
5. Discoverable -- users can see all available extras in one place