generator不同於list,沒有list,且只能遊走一次.
下面的code可以發現generator的元素是在被用的時候才產生.

import time

b=[time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()) for i in range(3)]

time.sleep(1)
print("A:",time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()))
time.sleep(1)

for ii in b:
  print("B:",ii)
  time.sleep(1)

###########################################################################

b=(time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()) for i in range(3))

time.sleep(1)
print("C:",time.strftime("%Y-%m-%d %H:%M:%S", time.localtime()))
time.sleep(1)

for ii in b:
  print("D:",ii)
  time.sleep(1)