CPython Source Code Navigation

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

Generates C boilerplate for argument parsing from specially formatted comments in .c files. Output goes to **/clinic/** subdirectories.

NEVER edit files in **/clinic/** subdirectories - they're auto-generated! Edit the source .c file, then run make -C BUILD_DIR clinic.

Scratch Space

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