def func():
    for x in range(3):
        yield from (x for x in 'abc')


def func2():
    return ((yield from (x for x in 'abc') for x in range(3))

for y in func():
    print(y, end=',')  # output1
print()
for y in func2():
    print(y, end=',')  # output2

output1:
a,b,c,a,b,c,a,b,c,
output2:
a,b,c,None,a,b,c,None,a,b,c,None,
请教下output2里的None怎么出来的？
原本以为两个输出应该是一样的。