Pythonで2つの時点を比較する方法

#字符串转datetime
dt = datetime.datetime.strptime('2017-04-19 00:42:44','%Y-%m-%d %H:%M:%S')

#datetime转字符串
str = dt.strftime("%Y-%m-%d-%H")

>>> dt1 = datetime.datetime.strptime('2017-04-18 00:40:00','%Y-%m-%d %H:%M:%S')
>>> dt2 = datetime.datetime.strptime('2017-04-18 00:20:00','%Y-%m-%d %H:%M:%S')
>>> print(dt1 - dt2)
0:20:00
>>> print(dt2 - dt1)
-1 day, 23:40:00

>>> diff = dt2 - dt1
>>> print(diff)
-1 day, 23:40:00
>>> print(diff.days)
-1
>>> print(diff.seconds)
85200

>>> diff.days * 86400 + diff.seconds
-1200