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skills/cpython-code-style/SKILL.md

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+---
+name: cpython-code-style
+description: Use this skill when writing or modifying CPython code to ensure it follows proper coding standards (PEP 7 for C, PEP 8 for Python), formatting rules, linting requirements, and pre-commit checks. It covers whitespace handling, type annotations policy, and documentation formatting.
+---
+
+# CPython Coding Style and Standards
+
+## Style Guidelines
+
+**Python code** (`Lib/` etc): Follow PEP 8. Be consistent with nearby code.
+
+**C code** (`Modules/`, `Objects/`, `Python/`, `Include/`): Follow PEP 7. Be consistent with nearby code.
+
+## Critical Rules
+
+**NEVER leave trailing whitespace on any line.** Pre-commit hooks will catch this.
+
+**ALWAYS preserve the final newline** at the end of files.
+
+**NEVER add type annotations to `Lib/` tree.** Stdlib doesn't use inline annotations (maintained in typeshed separately). Annotations may be OK in `Tools/` or test code if requested.
+
+**No autoformatting by default.** Only use `Doc/venv/bin/ruff format` if explicitly requested.
+
+## Pre-Commit Workflow
+
+Before committing, run from repo root:
+
+```bash
+# 1. Pre-commit hooks (checks whitespace, file endings, syntax)
+pre-commit run --all-files
+
+# 2. Patchcheck (MUST PASS - validates C/Python style, docs, whitespace)
+make -C $BUILD_DIR patchcheck
+
+# 3. If you modified Doc/, verify reStructuredText
+make -C Doc check
+
+# 4. Run relevant tests
+$BUILT_PY -m test test_yourmodule -j $(nproc)
+```
+
+## Documentation
+
+- **Python docstrings**: Follow PEP 257, document params/returns/exceptions
+- **C comments**: Follow PEP 7, document complex algorithms
+- **reStructuredText** (`Doc/`): Follow Sphinx/reST conventions, verify with `make -C Doc check`