def test_gen():
    v = yield 'a'
    print(v)    #2. None
    v = yield 'b'
    print(v)    #4. None
    v = yield 'c'
    print(v)    #6. e
    v = yield 'd'
    print(v)    #8. None
    
iter = test_gen()

print(iter.__next__())    #1. a
print(next(iter))    #3. b
print(iter.send('e'))    #5. c
for v in iter:
    print(v)    #7. d

注：
iter.__next__()相当于iter.send(None)