how this to make a asyncio thread safe?

[linziyan]

this is the official call_soon_threadsafe

def call_soon_threadsafe(self, callback, *args):
    """Like call_soon(), but thread-safe."""
    self._check_closed()
    if self._debug:
        self._check_callback(callback, 'call_soon_threadsafe')
    handle = self._call_soon(callback, args)
    if handle._source_traceback:
        del handle._source_traceback[-1]
    self._write_to_self()
    return handle

but you cant get any thing that related to thread lock, how to make thread safe ? how this code works?