Dev

CPython Development

You are working in the CPython repository - the implementation of the Python language runtime and standard library itself.

Load Specialized Skills As Needed

This skill provides orientation. Load additional skills when your task requires them:

• Load build skill when: compiling CPython, running tests, verifying changes work, debugging test failures, or checking if your fix is correct
• Load style skill when: preparing commits, running pre-commit hooks, checking code style, or validating changes before pushing
• Load docs skill when: editing files in Doc/, adding version markers, creating NEWS entries, or updating documentation
• Load jit skill when: working on the JIT compiler, modifying Tools/jit/ or Python/jit.c, debugging JIT-specific failures, or changing bytecodes that affect stencil generation

Recommended Tools

Prefer these tools when available: rg, gh, jq

Source Code Structure

Lib/ - Python standard library (pure Python). Example: Lib/zipfile.py

Modules/ - C extension modules for performance/low-level access. Example: Modules/_csv.c

Objects/ and Python/ - Core types (list, dict, int), builtins, runtime, interpreter loop

Include/ - C header files for public and internal C APIs

Lib/test/ - All unittests

• Test naming: test_{module_name}.py or test_{module_name}/
• Examples: Lib/zipfile.py → Lib/test/test_zipfile**, Modules/_csv.c → Lib/test/test_csv.py
• Test packages require load_tests() in test_package/__init__.py to work with python -m test

Doc/ - Documentation in .rst format (source for python.org docs), builds to Doc/build/

InternalDocs/ - Maintainer documentation (InternalDocs/README.md is the starting point)

Tools/ - Build tools like Argument Clinic, development utilities

Argument Clinic

**/clinic/** subdirectories are auto-generated - never edit these directly. Load the build skill for regeneration commands.

Engineering Notebooks

ALWAYS load and maintain notebooks when working on features or PRs:

• For PRs: .claude/pr-{PR_NUMBER}.md
• For branches: .claude/branch-{branch_name_without_slashes}.md (when not on main)

Keep notebooks updated with learnings and project state as you work and after commits. Include: problem statement, key findings, file locations, design decisions, testing strategy, and status.

Scratch Space

NEVER create throwaway files in repo root. Use .claude/sandbox/ for exploration files, test scripts, and prototypes.

Optional Developer Resources

• Developer Guide: If REPO_ROOT/../devguide/ exists, see developer-workflow/ and documentation/ subdirectories
• PEPs: May exist in REPO_ROOT/../peps/ tree - reference relevant PEPs when working on changes

Typical Workflow

1. Understand the task - explore code, read relevant files
2. Make changes - edit source files
3. Build and test - load build skill, compile, run tests
4. Validate style - load style skill, run pre-commit and patchcheck
5. Update docs - if needed, load docs skill for documentation changes

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