字符串日期格式化为秒数，返回浮点类型【每日一个知识点第348期-Python】

expire_time = "2013-05-21 09:50:35"
d = datetime.datetime.strptime(expire_time,"%Y-%m-%d %H:%M:%S")
time_sec_float = time.mktime(d.timetuple())
print time_sec_float