python中yield from怎么用？

def g(x):

    yield from range(x , 0, -1)

    yield from range(x)

print(list(g(5)))

def writer():

    while True:

        w = (yield)

        print('>>', w)

def writer_wrapper(coro1):

coro1.send(None)

    while True:

        try:

            x = (yield)

            coro1.send(x)

        except StopIteration:

            pass

def writer_wrapper(coro2):

yield from coro2