由range()（或xrange()在Python2.x中）返回的对象被称为惰性迭代。
[0,1,2,..,9]生成器没有将整个范围存储在内存中，而是存储的定义(i=0; i<10; i+=1)并仅在需要时才计算下一个值（又称惰性求值）。
本质上，生成器允许您返回类似于结构的列表，但是这里有一些区别：
（1）与清单理解非常相似：
# this is a list, create all 5000000 x/2 values immediately, uses []
lis = [x/2 for x in range(5000000)]
# this is a generator, creates each x/2 value only when it is needed, uses ()
gen = (x/2 for x in range(5000000))
# this is also a generator, it will run until a yield occurs, and return that result.
# on the next call it picks up where it left off and continues until a yield occurs...
def divby2(n):
num = 0
while num < n:
yield num/2
num += 1
# same as (x/2 for x in range(5000000))
print divby2(5000000)
即使range(5000000)在python3.x中[x/2 for x in range(5000000)]是生成器，仍然是列表。range(...)它的工作并一次生成x一个，但是在x/2创建此列表时将计算整个值列表。
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