CPython Skills Marketplace

A Claude Code marketplace providing specialized skills for working with the CPython repository - the reference implementation of the Python programming language.

Overview

This marketplace hosts Claude Code plugins with AI-agent skills for CPython development. Includes one comprehensive plugin with multiple skills providing deep knowledge about CPython's codebase structure, build system, testing infrastructure, and coding standards.

Skills are now invoked as cpython:dev, cpython:build, cpython:style, cpython:docs, and cpython:jit.

Installation

To use these skills in Claude Code within a CPython repository checkout:

1. Add the marketplace:

   /plugin marketplace add gpshead/cpython-skills
   

2. Install the plugin:

   /plugin install cpython
   

3. In your CPython repository, ask Claude:

   write a CLAUDE.local.md that instructs you to load and use all of the cpython plugin skills in this repo.
   

Available Plugins

CPython Skills

Name: cpython
Description: Skills for working with the CPython repository - helping with building, testing, and contributing to Python's implementation

This plugin provides five specialized skills with a meta-skill that coordinates loading:

1. dev (Entry Point)

When to use: Starting any CPython development task - fixing bugs, adding features, understanding code

Provides:

• Codebase orientation and source code structure
• Workflow coordination - guides you to load other skills as needed
• Recommended tools (rg, gh, jq)
• Engineering notebook management for PRs and branches
• Scratch space usage

2. build

When to use: Compiling CPython, running tests, verifying changes work, debugging test failures

Includes:

• Build directory setup with ccache support
• Platform-specific configuration (Linux, macOS, Windows)
• Argument Clinic code generation
• Build verification and troubleshooting
• unittest-based testing (not pytest!)
• Using --match for test filtering (not -k!)
• Code coverage collection
• Interactive debugging with tmux

3. style

When to use: Preparing commits, running pre-commit hooks, validating changes

Covers:

• PEP 7 (C code) and PEP 8 (Python code) compliance
• Trailing whitespace and file ending rules
• Type annotation policy (no annotations in Lib/!)
• Pre-commit hooks and patchcheck

4. docs

When to use: Editing documentation in Doc/, adding version markers, creating NEWS entries

Covers:

• Documentation in ReST format in Doc/ tree
• Documentation tooling setup and validation
• Version markers (always use next)
• NEWS file entries for bug fixes and features

5. jit

When to use: Working on CPython's experimental JIT compiler - modifying build scripts in Tools/jit/, editing Python/jit.c, changing bytecodes that affect stencils, debugging JIT failures

Covers:

• LLVM 21 toolchain setup
• JIT configure flags (--enable-experimental-jit options)
• Copy-and-patch compilation pipeline (template.c to jit_stencils.h)
• Stencil regeneration workflow
• Runtime JIT debugging (comparing PYTHON_JIT=0 vs PYTHON_JIT=1)
• Supported target platforms and object formats
• Key concepts: stencils, holes, preserve_none, musttail, GOT

Advanced Usage

Preventing Use of System Python

The skills instruct Claude to always use the locally-built interpreter (build/python) rather than python or python3 from $PATH. This is critical when developing CPython — the system Python is a different build and won't reflect your changes.

If you find Claude occasionally still reaches for the system Python (e.g., when running a quick snippet during investigation), you can add a hook as a hard guardrail. In your CPython repo's .claude/settings.local.json:

{
  "hooks": {
    "Bash": {
      "pre": [
        {
          "command": "bash -c 'if echo \"$CLAUDE_TOOL_PARAM_command\" | grep -qP \"(?<![/\\\\w])python3?\\s\" && ! echo \"$CLAUDE_TOOL_PARAM_command\" | grep -qP \"build/python\"; then echo \"ERROR: Use build/python instead of system python. Never use python or python3 from PATH in the CPython repo.\" >&2; exit 1; fi'",
          "description": "Block bare python/python3 commands - must use build/python"
        }
      ]
    }
  }
}

This intercepts Bash tool calls and rejects any that invoke bare python/python3 without going through build/python. Note: you may need to tune the pattern if your workflow legitimately uses the system Python for build tooling (e.g., python3 Tools/jit/build.py).

License

CC0 1.0 Universal

References

• Claude Code Plugin Marketplaces Documentation
• Claude Code Plugins Documentation
• Claude Code Skills Documentation
• Simon Willison on Claude Skills
• CPython Developer Guide