使用两个for中使用字典

Python code

boys=['chris','arnold','bob','davice']girls=['alice','bernice','clarie','acy']litergirls={}g='s'b='a'for girl in girls:    litergirls.setdefault(girl[0],[]).append(girl)print([(b,g) for b in boys])print([(b,g) for g in litergirls[b[0]]])print([(b,g) for b in boys for g in litergirls[b[0]]])>>> ================================ RESTART ================================>>> [('chris', 's'), ('arnold', 's'), ('bob', 's'), ('davice', 's')][('a', 'alice'), ('a', 'acy')]Traceback (most recent call last):  File "D:\Program Files\python\556.py", line 48, in <module>    print([(b,g) for b in boys for g in litergirls[b[0]]])  File "D:\Program Files\python\556.py", line 48, in <listcomp>    print([(b,g) for b in boys for g in litergirls[b[0]]])KeyError: 'd'

print([(b,g) for b in boys for g in litergirls[b[0]]])
是照着书上打的~为什么在这里不能使用多个for的？

[解决办法]
列表生成式是从右往左嵌套的, b是内层， 而你外层却依赖内层， 你说呢。