This section gives an overview of CPython's code structure and provides a summary of file locations for modules and built-ins.

For a Python module, the typical layout is:

• Lib/<module>.py
• Modules/_<module>.c (if there's also a C accelerator module)
• Lib/test/test_<module>.py
• Doc/library/<module>.rst

For an extension module, the typical layout is:

• Modules/<module>module.c
• Lib/test/test_<module>.py
• Doc/library/<module>.rst

For builtin types, the typical layout is:

• Objects/<builtin>object.c
• Lib/test/test_<builtin>.py
• Doc/library/stdtypes.rst

For builtin functions, the typical layout is:

• Python/bltinmodule.c
• Lib/test/test_builtin.py
• Doc/library/functions.rst

Some exceptions to these layouts are:

• built-in type int is at Objects/longobject.c
• built-in type str is at Objects/unicodeobject.c
• built-in module sys is at Python/sysmodule.c
• built-in module marshal is at Python/marshal.c
• Windows-only module winreg is at PC/winreg.c