python generator

Question

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。