求时间精确到秒的数

# a='2016-07-16 09:12:50'
# b='2016-09-09 04:12:59'
# a1=datetime.datetime.strptime(a, "%Y-%m-%d %H:%M:%S")
# b1=datetime.datetime.strptime(b, "%Y-%m-%d %H:%M:%S")
# res =(b1-a1).seconds
#print(res)