列表生成式的语法：
#循环for... in...一次，执行一次expression表达式，
#for in 循环，满足condition条件判断，执行左侧expression
[expression for item1 in iterable1 if condition1
for item2 in iterable2 if condition2
...
for itemN in iterableN if conditionN
]
类似于
res=[]
for item1 in iterable1:
if condition1:
for item2 in iterable2:
if condition2
...
for itemN in iterableN:
if conditionN:
res.append(expression)
