python類之間變數的傳輸的時候變數是字串和變數是列表或者字典是完全不同的：

試著把self.time的格式分別用字串，列表，和字典，看輸出的結果：

import time 

class Book:
    def __init__(self):
        self.time=[None]
    def update_time(self):
        self.time[0]=time.strftime("%Y-%m-%d %H:%M:%S", time.localtime())
        
class Strategy:
    def __init__(self,book):
        self.time=book.time
book=Book()
strategy=Strategy(book)     
for i in range(10):
    time.sleep(1)
    book.update_time()
    print( strategy.time)
 

self.time是字串的形式：

None
None
None
None
None
None
None
None
None
None

self.time是列表的形式：

['2018-11-21 19:49:04']
['2018-11-21 19:49:05']
['2018-11-21 19:49:06']
['2018-11-21 19:49:07']
['2018-11-21 19:49:08']
['2018-11-21 19:49:09']
['2018-11-21 19:49:10']
['2018-11-21 19:49:11']
['2018-11-21 19:49:12']
['2018-11-21 19:49:13']

self.time是字典形式的：

{'datetime': '2018-11-21 19:51:40'}
{'datetime': '2018-11-21 19:51:41'}
{'datetime': '2018-11-21 19:51:42'}
{'datetime': '2018-11-21 19:51:43'}
{'datetime': '2018-11-21 19:51:44'}
{'datetime': '2018-11-21 19:51:45'}
{'datetime': '2018-11-21 19:51:46'}
{'datetime': '2018-11-21 19:51:47'}
{'datetime': '2018-11-21 19:51:48'}
{'datetime': '2018-11-21 19:51:49'}