1.生成器
只有在调用时才生成相应的数据，只记录当前位置，只有一个__next__()方法.
def fib(max):
n,a,b = 0,0,1
while n < max:
#print(b)
yield b
a,b = b,a + b
n = n + 1
return ‘done‘
f = fib(6)
while True:
try:
x = next(f)
print(‘f:‘,x)
except StopIteration as e:
print(‘Generator return value:‘,e.value)
break