# CPython source code

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


## Source code layout

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`](../Doc/library/stdtypes.rst)

For builtin functions, the typical layout is:

* [`Python/bltinmodule.c`](../Python/bltinmodule.c)
* [`Lib/test/test_builtin.py`](../Lib/test/test_builtin.py)
* [`Doc/library/functions.rst`](../Doc/library/functions.rst)

Some exceptions to these layouts are:

* built-in type `int` is at [`Objects/longobject.c`](../Objects/longobject.c)
* built-in type `str` is at [`Objects/unicodeobject.c`](../Objects/unicodeobject.c)
* built-in module `sys` is at [`Python/sysmodule.c`](../Python/sysmodule.c)
* built-in module `marshal` is at [`Python/marshal.c`](../Python/marshal.c)
* Windows-only module `winreg` is at [`PC/winreg.c`](../PC/winreg.c)