cpython/Include/cpython/modsupport.h

#ifndef Py_CPYTHON_MODSUPPORT_H
#  error "this header file must not be included directly"
#endif

// A data structure that can be used to run initialization code once in a
// thread-safe manner. The C++11 equivalent is std::call_once.
_PyOnceFlag;

_PyArg_Parser;

PyAPI_FUNC(int) _PyArg_ParseTupleAndKeywordsFast(PyObject *, PyObject *,
                                                 struct _PyArg_Parser *, ...);