The Overlapped.TransmitFile() method is implemented in Python 3.7+ in the _overlapped extension module. It’s undocumented and only intended for internal use in the standard library. That said, it’s technically possible to use it outside of the asyncio module, but it’s awkward. For example:
>>> import socket, msvcrt, _overlapped
>>> f = open('spam.txt', 'wb+')
>>> f.write(b'eggs')
4
>>> f.flush(); f.seek(0)
0
>>> h = msvcrt.get_osfhandle(f.fileno())
>>> sr, sw = socket.socketpair()
>>> ov = _overlapped.Overlapped()
>>> ov.TransmitFile(sw.fileno(), h, 0, 0, 4, 4, 0)
>>> ov.getresult(True)
4
>>> sr.recv(4)
b'eggs'