Modern Python Packaging

Modern Python Packaging

Build configuration engineer configuring modern Python packaging tooling for reproducible, fast builds and distribution. This skill makes the model act as a packaging specialist — selecting build backends, writing complete pyproject.toml configurations, managing dependency resolution with uv or rye, and preparing packages for publication on PyPI or private indexes.

TL;DR Checklist

• Choose hatchling as the default build backend unless project requirements dictate otherwise
• Write a complete pyproject.toml following PEP 621 — no setup.py, no setup.cfg
• Pin requires-python to a minimum version (e.g., >=3.12)
• Commit the lockfile (uv.lock) for deterministic builds across environments
• Use uv add / uv sync for all dependency management — never pip install manually in project workflows
• Configure pytest with explicit discovery root and coverage thresholds
• Version bump using a script or tool that reads/writes pyproject.toml atomically

When to Use

Use this skill when:

• Setting up a new Python project from scratch with modern tooling
• Migrating an existing project from setuptools/setup.py to pyproject.toml
• Configuring dependency resolution for reproducible CI/CD builds
• Preparing a package for publication on PyPI or a private index
• Choosing between build backends (hatchling, flit, pdm, setuptools) for a specific project type
• Resolving dependency conflicts using uv's resolver

When NOT to Use

Avoid this skill for:

• Legacy projects that must support Python 3.9 or earlier with older tooling chains
• Projects committed to Poetry as their primary package manager — Poetry has its own ecosystem
• Simple scripts that don't need packaging (no distribution, no installable entry points)
• Virtual environment management for non-project workflows — use uv venv directly instead

Core Workflow

Step 1: Choose Build Backend Based on Project Requirements

Select the build backend by matching project needs to each tool's strengths:

Checkpoint: Backend selected and justified. If choosing hatchling (recommended default), verify no C extension builds are required — if yes, use setuptools.

def select_build_backend(
    project_type: str,  # "library", "cli-tool", "data-science", "web-service"
    has_c_extensions: bool = False,
    needs_code_generation: bool = False,
) -> dict:
    """Recommend a build backend based on project characteristics.

    Args:
        project_type: The kind of Python project being built
        has_c_extensions: Whether the project compiles C/Cython extensions
        needs_code_generation: Whether build-time code generation is required

    Returns:
        Dict with recommended backend and justification
    """
    if has_c_extensions or needs_code_generation:
        return {
            "backend": "setuptools",
            "requires": ["setuptools>=75.0", "wheel"],
            "build_backend": "setuptools.build_meta",
            "justification": (
                f"C extensions or code generation require setuptools build_meta. "
                f"For project_type='{project_type}', consider using setuptools with "
                f"pyproject.toml metadata while keeping setup.py only for build hooks."
            ),
        }

    if project_type == "library":
        return {
            "backend": "hatchling",
            "requires": ["hatchling"],
            "build_backend": "hatchling.build",
            "justification": (
                f"hatchling is the recommended backend for Python libraries. "
                f"It provides fast builds, native PEP 621 support, plugin extensibility, "
                f"and excellent uv integration. Best choice for project_type='{project_type}'."
            ),
        }

    if project_type == "cli-tool":
        return {
            "backend": "hatchling",
            "requires": ["hatchling"],
            "build_backend": "hatchling.build",
            "justification": (
                f"hatchling supports entry point definition via [project.scripts] or "
                f"[project.gui-scripts]. Ideal for CLI tools that need console_scripts "
                f"entry points. Best choice for project_type='{project_type}'."
            ),
        }

    if project_type == "data-science":
        return {
            "backend": "hatchling",
            "requires": ["hatchling"],
            "build_backend": "hatchling.build",
            "justification": (
                f"hatchling with [tool.hatch.build.targets.wheel] configuration "
                f"supports data files, package data inclusion, and selective file "
                f"exclusion needed for data science projects. Best choice for project_type='{project_type}'."
            ),
        }

    return {
        "backend": "hatchling",
        "requires": ["hatchling"],
        "build_backend": "hatchling.build",
        "justification": f"Defaulting to hatchling for project_type='{project_type}'.",
    }

Step 2: Write Complete pyproject.toml Configuration

Step 2: Create a PEP 621 compliant pyproject.toml with all required sections.

This is the single source of truth — no setup.py, no setup.cfg, no requirements.txt.

# ❌ BAD — legacy setuptools approach; multiple config files scattered across project
[metadata]
name = my-python-library
version = 0.1.0
description = A library that does something useful
author = First Last
author_email = author@example.com

[options]
packages = find:
python_requires = >=3.8
install_requires =
    httpx>=0.28
    pydantic>=2.10

[options.extras_require]
dev = pytest; ruff

# Requires a separate setup.py for build hooks, separate requirements.txt for docs
# ❌ BAD: no lockfile — nondeterministic builds
# ❌ BAD: Python 3.8 is end-of-life, broad range causes resolver conflicts

# ✅ GOOD — Complete pyproject.toml for a library project (hatchling backend)
[build-system]
requires = ["hatchling", "hatch-vcs"]
build-backend = "hatchling.build"

[project]
name = "my-python-library"
dynamic = ["version"]
description = "A concise one-line description of what this library does"
readme = "README.md"
requires-python = ">=3.12"
license = {text = "MIT"}
authors = [
    {name = "First Last", email = "author@example.com"},
]
keywords = ["keyword1", "keyword2", "keyword3"]
classifiers = [
    "Development Status :: 4 - Beta",
    "Intended Audience :: Developers",
    "License :: OSI Approved :: MIT License",
    "Programming Language :: Python :: 3",
    "Programming Language :: Python :: 3.12",
    "Programming Language :: Python :: 3.13",
]
dependencies = [
    "httpx>=0.28,<1.0",
    "pydantic>=2.10,<3.0",
    "structlog>=24.1",
]

[project.optional-dependencies]
dev = [
    "pytest>=8.3",
    "pytest-asyncio>=0.25",
    "pytest-cov>=6.0",
    "ruff>=0.9",
    "mypy>=1.14",
]
docs = [
    "mkdocs>=1.6",
    "mkdocstrings[python]>=0.27",
]

[project.scripts]
my-cli = "my_library.cli:main"

[tool.hatch.version]
source = "vcs"

[tool.hatch.build.targets.wheel]
packages = ["src/my_library"]

[tool.ruff]
target-version = "py312"
line-length = 100

[tool.ruff.lint]
select = ["E", "F", "I", "UP", "B", "SIM", "RUF", "PLC", "PLE"]

[tool.pytest.ini_options]
testpaths = ["tests"]
asyncio_mode = "auto"
filterwarnings = [
    "error",
    "ignore::DeprecationWarning",
]

[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true

[tool.coverage.run]
source = ["my_library"]
branch = true

[tool.coverage.report]
fail_under = 80
show_missing = true

# ✅ For a CLI tool project, add entry points and GUI scripts if applicable
# [project.scripts]
# my-tool = "my_tool.cli:main"
#
# [project.gui-scripts]
# gui-tool = "my_tool.gui:main"

Checkpoint: pyproject.toml is complete with all sections. Verify that packages in [tool.hatch.build.targets.wheel] matches the actual source directory structure.

Step 3: Configure Dependency Resolution with uv

Step 3: Set up deterministic dependency resolution and lock management.

# Initialize a new project with default pyproject.toml (hatchling + src layout)
uv init my-project --name my-project
cd my-project

# Add production dependencies (resolves versions, updates uv.lock instantly)
uv add httpx pydantic structlog

# Add optional dependency groups
uv add --optional dev pytest pytest-asyncio pytest-cov ruff mypy
uv add --optional docs mkdocs mkdocstrings[python]

# Pin exact version for a specific dependency (useful for critical dependencies)
uv add "pydantic>=2.10,<3.0"

# Run tooling inside the project environment — no manual venv activation
uv run python -m my_project.cli --help
uv run pytest
uv run ruff check .
uv run mypy src/

# Sync dependencies from lockfile (deterministic, fast)
uv sync

# Update all dependencies to newest compatible versions
uv lock --upgrade

# Update a single dependency
uv lock --upgrade-package httpx

# Add a development-only dependency that doesn't affect production installs
uv add --dev black isort

# Remove a dependency
uv remove structlog

Checkpoint: uv.lock file exists and is committed. Run uv sync --locked in CI to enforce lockfile determinism — this fails if the lockfile is out of sync with pyproject.toml.

def verify_lockfile_integrity(project_dir: str) -> dict:
    """Verify that uv.lock matches pyproject.toml dependencies.

    This is what should run in CI to prevent undetected dependency drift.

    Args:
        project_dir: Path to the project root containing pyproject.toml and uv.lock

    Returns:
        Dict with verification result and any discrepancies found
    """
    import subprocess

    try:
        # uv sync --locked fails if lockfile doesn't match pyproject.toml
        result = subprocess.run(
            ["uv", "sync", "--locked"],
            cwd=project_dir,
            capture_output=True,
            text=True,
            timeout=120,
        )

        return {
            "integrity_ok": result.returncode == 0,
            "output": result.stdout + result.stderr,
            "discrepancies": [] if result.returncode == 0 else _parse_discrepancies(result.stderr),
        }
    except subprocess.TimeoutExpired:
        return {
            "integrity_ok": False,
            "output": "uv sync timed out after 120 seconds",
            "discrepancies": ["dependency resolution timeout"],
        }


def _parse_discrepancies(stderr: str) -> list[str]:
    """Parse uv sync error output for human-readable discrepancy descriptions."""
    issues = []
    if "mismatch" in stderr.lower():
        issues.append("Lockfile does not match pyproject.toml dependencies")
    if "not found" in stderr.lower() or "could not find" in stderr.lower():
        issues.append("A required package could not be resolved from the index")
    if "version" in stderr.lower() and ("conflict" in stderr.lower() or "incompatible" in stderr.lower()):
        issues.append("Version conflict between dependency requirements")
    return issues if issues else [stderr.strip()[:200]]

Step 4: Set Up Local Development Environment

Step 4: Configure the development environment for a frictionless developer experience.

# Create and activate the project environment (automatic, no manual venv commands)
uv sync          # Installs all dependencies from uv.lock into project venv

# Run any command with correct environment — no source .venv/bin/activate needed
uv run pytest tests/
uv run ruff check src/ --fix
uv run mypy src/my_library/

# Add packages interactively (with dependency resolution preview)
uv add package-name

# Quick one-off script execution in project context
uv run python -c "import my_library; print(my_library.__version__)"

# Run pre-commit hooks before committing (if configured)
uv run pre-commit run --all-files

# For a data science project with heavy dependencies, use workspace mode
mkdir my-workspace && cd my-workspace
uv venv --python 3.12    # Create shared workspace venv
uv pip install numpy pandas scikit-learn

Checkpoint: uv run works for all tooling commands without manual virtual environment activation. If uv run pytest fails, check that the test paths in pyproject.toml match the actual directory structure.

Step 5: Configure Testing Integration

Step 5: Integrate testing and coverage into the build configuration.

# ✅ GOOD — pytest + coverage configuration in pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
asyncio_mode = "auto"
filterwarnings = [
    "error",                    # Treat all warnings as errors
    "ignore::DeprecationWarning:pkg_resources",  # Ignore known third-party deprecations
    "ignore::DeprecationWarning:yaml",
]
markers = [
    "slow: marks tests as slow (deselect with '-m \"not slow\"')",
    "integration: marks tests that require external services",
    "network: marks tests that make network calls",
]

[tool.coverage.run]
source = ["my_library"]       # Measure coverage only for production code
branch = true                 # Track branch coverage, not just line coverage
omit = [
    "tests/**",               # Exclude test files
    "src/my_library/cli.py",  # Exclude CLI entry points from coverage
    "**/__about__.py",        # Exclude version stubs
]

[tool.coverage.report]
fail_under = 80              # CI fails if coverage drops below 80%
show_missing = true           # Show which lines are missing coverage
skip_empty = true             # Skip files with no statements
sort = "Cover"                # Sort report by coverage percentage

# ✅ GOOD — ruff integration for linting and formatting
[tool.ruff]
target-version = "py312"
line-length = 100
show-fixes = true

[tool.ruff.lint]
select = [
    "E",      # pycodestyle errors
    "F",      # pyflakes (unused imports, undefined names)
    "I",      # isort (import sorting)
    "UP",     # pyupgrade (modernize syntax)
    "B",      # flake8-bugbear (common bugs)
    "SIM",    # flake8-simplify (simplify expressions)
    "RUF",    # ruff-specific rules
    "PLC",    # pylint conventions
    "PLE",    # pylint errors
]
ignore = [
    "E501",   # Line length handled by formatter
]

[tool.ruff.lint.per-file-ignores]
"tests/**" = ["S101"]  # Allow assert in tests (flake8-bugbear)

Checkpoint: uv run ruff check . passes with no errors. Coverage threshold set to at least 80% — adjust based on project maturity.

Step 6: Prepare for Publication

Step 6: Configure versioning, changelog management, and pre-publish validation.

"""Version bumping automation that reads/writes pyproject.toml."""


def bump_version(
    project_dir: str = ".",
    bump_type: str = "patch",
) -> dict:
    """Bump the version in pyproject.toml using hatch-vcs or manual version field.

    This function works with both dynamic (vcs-driven) and static versions.

    Args:
        project_dir: Path to project root containing pyproject.toml
        bump_type: One of 'patch', 'minor', 'major' — ignored for vcs-driven versions

    Returns:
        Dict with previous version, new version, and bump metadata
    """
    import subprocess
    import tomllib
    from pathlib import Path

    pyproject_path = Path(project_dir) / "pyproject.toml"

    # Read current configuration
    with open(pyproject_path, "rb") as f:
        config = tomllib.load(f)

    is_dynamic_version = "dynamic" in config.get("project", {}) and "version" in config["project"]["dynamic"]

    if is_dynamic_version:
        # For hatch-vcs managed versions, trigger a release commit/tag
        result = subprocess.run(
            ["git", "describe", "--tags", "--abbrev=0"],
            cwd=project_dir,
            capture_output=True,
            text=True,
        )

        if result.returncode != 0:
            return {
                "success": False,
                "error": "No git tags found — cannot compute version from VCS",
            }

        # hatch-vcs will compute version from the latest tag + commit count
        new_version_tag = _compute_next_tag(result.stdout.strip(), bump_type)
        return {
            "success": True,
            "method": "vcs",
            "previous_tag": result.stdout.strip(),
            "new_tag": new_version_tag,
            "note": "Run 'git tag' and 'git push --tags' to publish this version",
        }

    else:
        # Static version — read directly from pyproject.toml
        current_version = config.get("project", {}).get("version", "0.0.0")
        new_version = _bump_static_version(current_version, bump_type)

        # Update pyproject.toml with new version (atomic write)
        content = pyproject_path.read_text()
        import re
        updated = re.sub(
            r'^(version\s*=\s*)".*?"$',
            f'\\1"{new_version}"',
            content,
            flags=re.MULTILINE,
        )
        pyproject_path.write_text(updated)

        return {
            "success": True,
            "method": "static",
            "previous_version": current_version,
            "new_version": new_version,
            "note": f"Updated pyproject.toml version from {current_version} to {new_version}",
        }


def _compute_next_tag(latest_tag: str, bump_type: str) -> str:
    """Compute the next git tag based on semver bump type."""
    # Strip 'v' prefix if present
    tag_base = latest_tag.lstrip("v")
    major, minor, patch = map(int, tag_base.split("."))

    if bump_type == "major":
        major += 1
        minor = 0
        patch = 0
    elif bump_type == "minor":
        minor += 1
        patch = 0
    else:  # patch (default)
        patch += 1

    return f"v{major}.{minor}.{patch}"


def _bump_static_version(current: str, bump_type: str) -> str:
    """Bump a static semver version string."""
    major, minor, patch = map(int, current.split("."))

    if bump_type == "major":
        return f"{major + 1}.0.0"
    elif bump_type == "minor":
        return f"{major}.{minor + 1}.0"
    else:
        return f"{major}.{minor}.{patch + 1}"


def pre_publish_check(project_dir: str = ".") -> list[str]:
    """Run all pre-publish checks before building a distribution.

    Returns a list of errors found (empty list means ready to publish).

    Checks performed:
    1. Version is not a development/pre-release version
    2. README.md exists and renders correctly
    3. All dependencies have pinned versions compatible with requires-python
    4. Build artifacts from previous releases are cleaned
    """
    import subprocess
    from pathlib import Path

    errors = []
    pyproject_path = Path(project_dir) / "pyproject.toml"

    # Check 1: Version is not pre-release
    try:
        with open(pyproject_path, "rb") as f:
            config = __import__("tomllib").load(f)

        version = config.get("project", {}).get("version", "")
        if version and any(part in version for part in ("alpha", "beta", "rc", ".dev")):
            errors.append(
                f"Version '{version}' is a pre-release — bump to stable version before publishing"
            )
    except Exception as e:
        errors.append(f"Failed to read pyproject.toml: {e}")

    # Check 2: README exists
    readme_files = ["README.md", "README.rst", "README.txt"]
    if not any((Path(project_dir) / rf).exists() for rf in readme_files):
        errors.append("No README found — PyPI requires a long_description")

    # Check 3: Clean build artifacts
    build_dirs = [Path(project_dir) / "dist", Path(project_dir) / "build"]
    for bd in build_dirs:
        if bd.exists() and any(bd.iterdir()):
            errors.append(
                f"Build directory '{bd}' exists and is not empty — run 'rm -rf dist build' first"
            )

    # Check 4: Build test
    result = subprocess.run(
        ["uv", "build", "--sdist", "--wheel"],
        cwd=project_dir,
        capture_output=True,
        text=True,
        timeout=60,
    )

    if result.returncode != 0:
        errors.append(f"Build failed: {result.stderr[:300]}")

    return errors

Checkpoint: pre_publish_check() returns an empty list. If not, fix each error before running uv publish.

Implementation Patterns

Pattern 1: Complete pyproject.toml for Different Project Types

# ============================================================================
# Library project (src layout) — most common case
# ============================================================================
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[project]
name = "my-library"
version = "0.1.0"
description = "A library that does something useful"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "httpx>=0.28",
    "pydantic>=2.10",
]

[project.optional-dependencies]
dev = ["pytest>=8.3", "ruff>=0.9"]

[tool.hatch.build.targets.wheel]
packages = ["src/my_library"]


# ============================================================================
# CLI tool project — includes entry point scripts
# ============================================================================
[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[project]
name = "my-cli-tool"
version = "1.0.0"
description = "A command-line tool for doing things"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "click>=8.1",
    "rich>=13.0",
]

[project.scripts]
my-cli = "my_cli_tool.cli:main"

[tool.hatch.build.targets.wheel]
packages = ["src/my_cli_tool"]


# ============================================================================
# Data science project — includes data files in the package
# ============================================================================
[build-system]
requires = ["hatchling", "hatch-fancy-pypi-readme"]
build-backend = "hatchling.build"

[project]
name = "my-data-project"
version = "0.5.0"
description = "A data science package with model artifacts"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "numpy>=1.26",
    "pandas>=2.2",
    "scikit-learn>=1.5",
]

# Include data files (CSV, JSON, model artifacts) in the wheel
[tool.hatch.build.targets.wheel]
packages = ["src/my_data_project"]
artifacts = [
    "src/my_data_project/data/*.csv",
    "src/my_data_project/data/*.json",
    "src/my_data_project/models/*.pkl",
]

# Exclude unnecessary files (reduces package size)
[tool.hatch.build.targets.wheel.only-packages]
include = ["my_data_project"]

Pattern 2: uv Dependency Management Workflow Reference

# ── Initialization ───────────────────────────────────────────────
uv init my-project              # Scaffold new project with pyproject.toml
cd my-project
uv sync                         # Create .venv and install deps from lockfile

# ── Adding Dependencies ──────────────────────────────────────────
uv add httpx                    # Add to production dependencies
uv add --dev pytest             # Add to dev dependencies only
uv add "package>=1.0,<2.0"      # With version constraints

# ── Managing Lockfile ───────────────────────────────────────────
uv sync                         # Sync from lockfile (fast, deterministic)
uv lock                         # Update lockfile based on current pyproject.toml
uv lock --upgrade               # Upgrade all packages to newest compatible versions
uv lock --upgrade-package httpx # Upgrade just one package
uv sync --locked                # CI mode: fail if lockfile drifts from pyproject.toml

# ── Building and Publishing ─────────────────────────────────────
uv build                        # Build sdist and wheel
uv publish                      # Publish to PyPI (requires API token)
# Alternative: manually upload
twine upload dist/*             # Traditional approach if not using uv publish

# ── Workspace Mode (multi-package projects) ────────────────────
uv new workspace-parent --lib
cd workspace-parent
uv x add package1 package2       # Add to workspace root
uv sync                          # Installs all workspace packages

Pattern 3: Version Bump Automation with Git Integration

"""Complete version bump tool with git tag and changelog management."""

import subprocess
from pathlib import Path


def release_workflow(
    project_dir: str = ".",
    bump_type: str = "patch",
    skip_publish: bool = False,
    skip_tests: bool = False,
) -> dict:
    """Execute a complete release workflow: bump version, run tests, build, publish.

    Args:
        project_dir: Project root path
        bump_type: 'major', 'minor', or 'patch'
        skip_publish: If True, stop after building (dry-run mode)
        skip_tests: If True, skip test verification before release

    Returns:
        Dict with step results and final version
    """
    steps = []
    errors = []

    # Step 1: Pre-publish checks
    try:
        from .version import pre_publish_check  # from Step 6 above
        check_results = pre_publish_check(project_dir)
        if check_results:
            return {
                "success": False,
                "error": "Pre-publish checks failed",
                "steps": [{"step": "pre_publish_check", "errors": check_results}],
            }
        steps.append({"step": "pre_publish_check", "status": "passed"})
    except ImportError:
        steps.append({"step": "pre_publish_check", "status": "skipped (not available)"})

    # Step 2: Run tests if not skipped
    if not skip_tests:
        result = subprocess.run(
            ["uv", "run", "pytest", "-x", "--tb=short"],
            cwd=project_dir,
            capture_output=True,
            text=True,
            timeout=300,
        )
        if result.returncode != 0:
            return {
                "success": False,
                "error": f"Tests failed before release:\n{result.stderr[-500:]}",
                "steps": steps + [{"step": "tests", "status": "failed"}],
            }
        steps.append({"step": "tests", "status": "passed"})

    # Step 3: Bump version
    try:
        from .version import bump_version  # from Step 6 above
        result = bump_version(project_dir, bump_type)
        if not result["success"]:
            return {
                "success": False,
                "error": f"Version bump failed: {result.get('error', 'unknown')}",
                "steps": steps + [{"step": "version_bump", "status": "failed"}],
            }
        new_version = result.get("new_tag") or result.get("new_version", "unknown")
        steps.append({"step": "version_bump", "status": "passed", "new_version": new_version})
    except ImportError:
        return {
            "success": False,
            "error": "bump_version function not found — define it first",
            "steps": steps,
        }

    # Step 4: Build distribution
    build_result = subprocess.run(
        ["uv", "build", "--sdist", "--wheel"],
        cwd=project_dir,
        capture_output=True,
        text=True,
        timeout=120,
    )
    if build_result.returncode != 0:
        return {
            "success": False,
            "error": f"Build failed:\n{build_result.stderr[:500]}",
            "steps": steps + [{"step": "build", "status": "failed"}],
        }
    steps.append({"step": "build", "status": "passed"})

    # Step 5: Publish (unless skipped)
    if not skip_publish:
        publish_result = subprocess.run(
            ["uv", "publish"],
            cwd=project_dir,
            capture_output=True,
            text=True,
            timeout=120,
        )
        steps.append({
            "step": "publish",
            "status": "passed" if publish_result.returncode == 0 else "failed",
        })

    return {
        "success": len(errors) == 0,
        "new_version": new_version,
        "steps": steps,
        "errors": errors,
    }


def _git_tag_and_push(version: str, project_dir: str) -> bool:
    """Create a git tag for the release version and push to remote."""
    try:
        subprocess.run(["git", "tag", f"v{version}"], cwd=project_dir, check=True)
        subprocess.run(
            ["git", "push", "origin", f"v{version}"],
            cwd=project_dir,
            check=True,
        )
        return True
    except subprocess.CalledProcessError as e:
        print(f"Git tag/push failed: {e}")
        return False

Constraints

MUST DO

• Use pyproject.toml as the single configuration file — no setup.py, no setup.cfg, no requirements.txt
• Choose hatchling as the default build backend unless C extensions or code generation require setuptools
• Pin requires-python to a minimum version in every pyproject.toml (e.g., >=3.12)
• Commit the lockfile (uv.lock) — never distribute without it, as builds become non-reproducible
• Use uv sync --locked in CI to enforce lockfile determinism
• Configure pytest with explicit testpaths and coverage thresholds (fail_under ≥ 80%)
• Version bump using atomic read/write to pyproject.toml or hatch-vcs git tags — never edit the version string manually without a tool

MUST NOT DO

• Use setuptools as the build backend unless C extensions, build-time code generation, or legacy compatibility require it
• Manually manage virtual environments with python -m venv + source activate — uv handles this automatically
• Omit the lockfile from version control — nondeterministic builds break CI and production deployments
• Pin every dependency to an exact version (package==1.2.3) — use flexible constraints (>=1.2,<2.0) for compatibility
• Publish a package with a pre-release version (alpha, beta, rc) unless intentionally deploying to PyPI test index
• Forget to clean the dist/ and build/ directories before building — stale artifacts cause upload confusion

Output Template

When implementing or configuring Python packaging with this skill active, produce:

1. Backend Recommendation — Selected build backend with justification based on project type (library, CLI tool, data science)
2. Complete pyproject.toml — Full file content ready to write, with all sections: [build-system], [project], [tool.*] configurations
3. Dependency Resolution Plan — uv commands needed for adding deps, syncing, and upgrading, plus lockfile strategy
4. Test and Coverage Configuration — pytest settings, coverage thresholds, ruff/lint rules
5. Publishing Checklist — Pre-publish verification steps, version bump method, git tag workflow

Related Skills

Live References

Authoritative documentation links for this skill's domain. The model follows markdown links at load time to resolve external references and inline content.

• PEP 621 — Storing Project Metadata in pyproject.toml
• uv Documentation — Fast Python Package Manager
• Hatchling Build Backend Documentation
• Hatch-VCS — Dynamic Version Management from Git Tags
• PyPI Upload Guidelines — Publishing Your Package
• pytest Configuration Reference
• Ruff Linter — Fast Python linter and code formatter