使用小括号包裹推导式会生成生成器对象，而不是元组。
>>> a =(2*x for x in range(2))
>>> type(a)
<class'generator'>
>>>next(a)
0
>>>next(a)
2
>>>next(a)
Traceback(most recent call last):
File"", line 1,in
next(a)
StopIteration
使用小括号包裹推导式会生成生成器对象，而不是元组。
>>> a =(2*x for x in range(2))
>>> type(a)
<class'generator'>
>>>next(a)
0
>>>next(a)
2
>>>next(a)
Traceback(most recent call last):
File"", line 1,in
next(a)
StopIteration
