python根据输入的字符串日期返回对应的财务季度报告日期

import xlrd
import xlwt
from datetime import datetime
#输入的s为str形式，如'2017-12-18'
def to_date(s):
    d=datetime.strptime(s,"%Y-%m-%d")
    return(d)
def to_quarter(riqi):
    yr=to_date(riqi).year
    qrt=[[datetime(yr, 3, 31),0],
        [datetime(yr, 6, 30),0],
        [datetime(yr, 9, 30),0],
        [datetime(yr, 12, 31),0]]
    for t in qrt:
        if to_date(riqi) <= t[0]:
            return(t[0])