字符串转日期
from datetime import datetime
text = '2012-09-20'
y = datetime.strptime(text, '%Y-%m-%d')
print(y)
from datetime import datetime
def parse_ymd(s): #这种方式会快很多
year_s, mon_s, day_s = s.split('-')
return datetime(int(year_s), int(mon_s), int(day_s))
