cpython/Objects/stringlib/join.h

/* stringlib: bytes joining implementation */

#if STRINGLIB_IS_UNICODE
#error join.h only compatible with byte-wise strings
#endif

Py_LOCAL_INLINE(PyObject *)
STRINGLIB(bytes_join)(PyObject *sep, PyObject *iterable)
{ … }

#undef NB_STATIC_BUFFERS
#undef GIL_THRESHOLD