Python Development

Python Development

Apply this skill when writing, maintaining, or reviewing Python code.

Supporting Files

• tooling.md — Copy-paste templates for greenfield project setup (pyproject.toml, Makefile, .pre-commit-config.yaml)
• code-style.md — Style rules and conventions (naming, formatting, type hints, imports)
• patterns.md — Architecture patterns (CLI/core separation, exception hierarchy, error handling)

Mode Detection

Detect the mode from the user's request before acting:

WRITE Mode

Step 1: Greenfield or brownfield?

Greenfield (new project or new module in a greenfield project):

• Use templates from tooling.md verbatim — don't approximate
• Apply src/ layout, hatchling build system, full tooling stack
• Apply all rules below from the start

Brownfield (inheriting existing code):

• Read existing code first with Read/Grep before writing anything
• Match existing patterns — don't retrofit greenfield defaults
• Use the brownfield decision tree at the bottom of this file

Step 2: Apply these rules when writing any code

Functions and modules:

• Every function and module gets a docstring summary (one line is enough; add more lines only when behavior is non-obvious)
• Every function gets full type hints using modern syntax (str | None, list[str], not Optional, not Union)
• See code-style.md for all style rules

Error handling:

• Raise domain exceptions in core logic, catch at the CLI layer — never both
• Always chain exceptions: raise AppError("msg") from e
• See patterns.md for exception hierarchy and CLI/core separation

Structure:

• Separate CLI layer (parse args + display output) from core layer (pure logic, no CLI imports)
• Use pathlib.Path for all file operations — never os.path or string concatenation
• Use @dataclass for any domain model with 2+ fields

Output:

• Default to typer.echo — don't add Rich or logging unless the user asks for a --verbose flag

Config:

• Use tomllib (stdlib) + TOML file — not Pydantic, not env vars
• Env vars only for secrets or CI overrides

Tests:

• Class-based: class TestFoo, def test_scenario
• conftest.py for shared fixtures
• tmp_path for filesystem, monkeypatch for env vars
• Test core logic directly; test CLI only for argument parsing and exit codes

MAINTAIN Mode

1. Read before writing — use Read/Grep to understand the existing code
2. Match existing patterns — preserve the style even if it differs from greenfield defaults
3. Run checks after editing — ask the user to run make check to validate
4. Don't change tooling unless the user explicitly asks

For brownfield codebases: apply only the brownfield decision tree below.

REVIEW Mode

Go through the code and report findings in two categories:

Blockers (must fix)

• Bare except: or except Exception without re-raising
• Unused imports or dead code
• Secrets or credentials hardcoded in source

Nits (suggest, don't block)

• Missing type hints on function signatures
• Missing docstrings on functions or modules
• Hardcoded values that should be module-level constants or config
• Exception not chained with from e when re-raising
• os.path or string paths instead of pathlib.Path
• Mutable default arguments (def f(x=[]))
• Wrong exception type (raising base Exception instead of domain-specific)
• Tests touching real filesystem or env vars without monkeypatch/tmp_path

When a blocker is found, grep for the same pattern across the codebase and report all instances.

Brownfield Decision Tree

Build system?

• Exists (Poetry, setup.cfg, etc.) → keep it
• Missing → migrate to hatchling

Python version?

• 3.11 or older → keep it, don't force upgrade
• 3.12+ → align with greenfield defaults

CLI framework?

• Click or Typer → already aligned, keep it
• argparse or custom → refactor only if actively developing
• No CLI → leave as-is

Type hints?

• 80%+ coverage → run ty check --strict
• <80% → run ruff check only, skip ty
• 0% → run tests only

Test count?

• 50+ tests → consider unit/cli split
• <50 → keep flat tests/ directory

Minimal acceptable setup for any brownfield project:

make install   # install deps
make test      # run tests
make lint      # check style
make check     # all of the above

Greenfield Checklist

• .python-version set to 3.12
• pyproject.toml with hatchling, requires-python = ">=3.12", [dependency-groups] dev
• Makefile with: install, test, test-v, lint, format, fix, typecheck, check, clean
• .pre-commit-config.yaml with ruff + ty hooks
• src/my_project/ with cli.py, commands/, core/
• tests/ with conftest.py
• make install && make check passes before first commit
• pre-commit install enabled

See tooling.md for copy-paste templates.