CPython Codebase

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

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. See cpython-build-and-test 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